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Prostate specific antigen (PSA, also known as human kallikrein 3) is an important diagnostic indicator of prostatic disease. PSA exhibits low protease activity (>10(4)-fold less than chymotrypsin) under the usual in vitro assay conditions. In addition, PSA does not react readily with prototypical serine protease inactivators. We expressed human PSA (rh-PSA) in Escherichia coli and have demonstrated that rh-PSA has properties similar to those of native PSA isolated from human seminal fluid. Both PSA and rh-PSA are >10(3)-fold more active in the presence of 1.3 M Na(2)SO(4). This activation is anion-dependent, following the Hofmeister series when normality is considered: SO(4)(2)(-) approximately citrate > Ac(-) > Cl(-) > Br(-) > I(-). The nature of the cation has little effect on salt activation. The rate of inactivation of rh-PSA by DFP is 30-fold faster in the presence of 0.9 M Na(2)SO(4), and the rate of inactivation by Suc-Ala-Ala-Pro-Phe-CK is >20-fold faster under these conditions. Azapeptides containing Phe or Tyr at position P(1) also inactivate rh-PSA in the presence of high salt concentrations. These compounds represent the first described inhibitors designed to utilize the substrate binding subsites of PSA. CD spectroscopy demonstrates that the conformation of rh-PSA changes in the presence of high salt concentrations. Analytical ultracentifugation and dynamic light scattering indicate that PSA remains monomeric under high-salt conditions. Interestingly, human prostatic fluid contains as much as 150 micro mol citrate/g wet weight, which suggests that salt concentrations may regulate PSA activity in vivo.

Two dioxygenases (ARD and ARD') were cloned from Klebsiella pneumoniae that catalyze different oxidative decomposition reactions of an advanced aci-reductone intermediate, CH(3)SCH(2)CH(2)COCH(OH)=CH(OH) (I), in the methionine salvage pathway. The two enzymes are remarkable in that they have the same polypeptide sequence but bind different metal ions (Ni(2+) and Fe(2+), respectively). ARD converts I to CH(3)SCH(2)CH(2)COOH, CO, and HCOOH. ARD' converts I to CH(3)SCH(2)CH(2)COCOOH and HCOOH. Kinetic analyses suggest that both ARD and ARD' have ordered sequential mechanisms. A model substrate (II), a dethio analogue of I, binds to the enzyme first as evidenced by its lambda(max) red shift upon binding. The dianion formation from II causes the same lambda(max) red shift, suggesting that II bind to the enzyme as a dianion. The electron-rich II dianion likely reacts with O(2) to form a peroxide anion intermediate. Previous (18)O(2) and (14)C tracer experiments established that ARD incorporates (18)O(2) into C(1) and C(3) of II and C(2) is released as CO. ARD' incorporates (18)O(2) into C(1) and C(2) of II. The product distribution seems to necessitate the formation of a five-membered cyclic peroxide intermediate for ARD and a four-membered cyclic peroxide intermediate for ARD'. A model chemical reaction demonstrates the chemical and kinetic competency of the proposed five-membered cyclic peroxide intermediate. The breakdown of the four-membered and five-membered cyclic peroxide intermediates gives the ARD' and ARD products, respectively. The nature of the metal ion appears to dictate the attack site of the peroxide anion and, consequently, the different cyclic peroxide intermediates and the different oxidative cleavages of II. A cyclopropyl substrate analogue inactivates both enzymes after multiple turnovers, providing evidence that a radical mechanism may be involved in the formation of the peroxide anion intermediate.

We have synthesized (acyloxy)methyl ketone inactivators of papain, cathepsin B, and interleukin-1beta conversion enzyme (ICE) that interact with both the S and S' subsites. The value of k(inact)/K(i) for these inactivators is strongly dependent on the leaving group. For example, Z-Phe-Gly-CH(2)-X is a poor inactivator of papain when X is OCOCH(3) (k(inact)/K(i) = 2.5 M(-)(1) s(-)(1)) but becomes a potent inactivator when X is OCO-L-Leu-Z (k(inact)/K(i) = 11 000 M(-)(1) s(-)(1)). Since these leaving groups have similar chemical reactivities, the difference in potency must be attributed to interactions with the S' sites. The potency of the leaving group correlates with the P' specificity of papain. Similar results are also observed for the inactivation of cathepsin B by these compounds. A series of inactivators with the general structure Fmoc-L-Asp-CH(2)-X were designed to inactivate ICE. No inhibition was observed when X was OCOCH(3). In contrast, ICE is inactivated when X is OCO-D-Pro-Z (k(inact)/K(i) = 131 M(-)(1) s(-)(1)). These results demonstrate that S'-P' interactions can be utilized to increase the efficacy and selectivity of (acyloxy)methyl ketone inactivators.

Two enzymes, designated, E-2 and E-2', catalyze different oxidation reactions of an aci-reductone intermediate in the methionine salvage pathway. E-2 and E-2', overproduced in Escherichia coli from the same gene, have the same protein component. E-2 and E-2' are separable on an anion exchange column or a hydrophobic column. Their distinct catalytic and chromatographic properties result from binding different metals. The apo-enzyme, obtained after metal is removed from either enzyme, is catalytically inactive. Addition of Ni2+ or Co2+ to the apo-protein yields E-2 activity. E-2' activity is obtained when Fe2+ is added. Production in intact E. coli of E-2 and E-2' depends on the availability of the corresponding metals. These observations suggest that the metal component dictates reaction specificity.

The cysteine proteases papain and cathepsin B are inactivated by a Michael acceptor, a peptidyl-beta-chloro-alpha, beta-unsaturated ester (N-Ac-L-Phe-NHCH2-CCl=CH-COOMe). Inactivation occurred concomitant with chloride release which was stoichiometric with the amount of enzyme. This result is consistent with nucleophilic attack of the active site cysteine on the beta-carbon of the inhibitor, followed by expulsion of chloride ion. Inactivation by this class of compounds requires the carbon skeleton about the double bond to be in the trans configuration. The cis isomer was a competitive inhibitor. The difference in the mode of inhibition between the isomers is probably due to non-productive binding of the cis isomer due to bulky chlorine substituent in the beta-position.

We have synthesized two sets of noncleavable peptide-inhibitor libraries to map the S and S' subsites of human heart chymase. Human heart chymase is a chymotrypsin-like enzyme that converts angiotensin I to angiotensin II. The first library consists of peptides with 3-fluorobenzylpyruvamides in the P1 position. (Amino acid residues of substrates numbered P1, P2, etc., are toward the N-terminal direction, and P'1, P'2, etc., are toward the C-terminal direction from the scissile bond.) The P'1 and P'2 positions were varied to contain each one of the 20 naturally occurring amino acids and P'3 was kept constant as an arginine. The second library consists of peptides with phenylalanine keto-amides at P1, glycine in P'1, and benzyloxycarbonyl (Z)-isoleucine in P4. The P2 and P3 positions were varied to contain each of the naturally occurring amino acids, except for cysteine and methionine. The peptides of both libraries are attached to a solid support (pins). The peptides are evaluated by immersing the pins in a solution of the target enzyme and evaluating the amount of enzyme absorbed. The pins with the best inhibitors will absorb most enzyme. The libraries select the best and worst inhibitors within each group of peptides and provide an approximate ranking of the remaining peptides according to Ki. Through this library, we determined that Z-Ile-Glu-Pro-Phe-CO2Me and (F)-Phe-CO-Glu-Asp-ArgOMe should be the best inhibitors of chymase in this collection of peptide inhibitors. We synthesized the peptides and found Ki values were 1 nM and 1 microM, respectively. The corresponding Ki values for chymotrypsin were 10 nM and 100 microM. The use of libraries of inhibitors has advantages over the classical method of synthesis of potential inhibitors in solution: the libraries are reusable, the same libraries can be used with a variety of different serine proteases, and the method allows the screening of hundreds of compounds in short periods of time.

The 5-methylthio-D-ribose moiety of 5'-(methylthio)-adenosine is converted to methionine in a wide variety of organisms. 1,2-Dihydroxy-3-keto-5-methylthiopentene anion (an aci-reductone) is an advanced intermediate in the methionine salvage pathway present in the Gram-negative bacterium Klebsiella pneumoniae and rat liver. This metabolite is oxidized spontaneously in air to formate and 2-keto-4-methylthiobutyric acid (the alpha-keto acid precursor of methionine). Previously, we had purified an enzyme (E2) from Klebsiella which catalyzes the oxidative degradation of the aci-reductone to formate, CO, and methylthiopropionic acid. To further characterize the reactions of the aci-reductone we used its desthio analog, 1-2-dihydroxy-3-ketohexene anion (III), which was described previously. This molecule undergoes the analogous enzymatic and non-enzymatic reactions of the natural substrate, namely the formation of formate, CO, and butyrate from III. Experiments with 18O2 show that E2 is a dioxygenase which incorporates one molecule of 18O into formate and butyric acid. No cofactor has been identified. We were unable to find an enzyme which catalyzes the conversion of 1,2-dihydroxy-3-keto-5-methylthiopentane to a keto acid precursor of methionine. The keto acid is probably produced non-enzymically in Klebsiella. We have, however, identified and purified an enzyme (E3) from rat liver, which catalyzes the formation of formate and 2-oxopentanoic acid from III. This enzyme has a monomeric molecular mass of 28,000 daltons, and no chromophoric cofactor has been identified. Experiments with 18O2 show that E3 is a dioxygenase which incorporates an 18O molecule into formate and the alpha-keto acid. In rat liver CO formation was not detected.

The reaction of enantiomerically pure (2S)-N-acetyl-L-alanyl-L-phenylalanyl alpha-chloroethane with gamma-chymotrypsin was studied as a probe of the mechanism of inactivation of serine proteases by peptidyl chloroalkanes. It was determined crystallographically that the peptidyl chloroethane alkylates His57 with retention of configuration at the chiral center, indicating a double displacement mechanism. We think it likely that a Ser195-epoxy ether adduct is an intermediate on the inactivation pathway, although other possibilities have not been disproven. Kinetic data reported by others [Angliker et al. (1988) Biochem. J. 256, 481-486] indicate that the epoxy ether intermediate is not an irreversibly inactivated form of enzyme [a conclusion confirmed experimentally (Prorok et al. (1994) Biochemistry 33, 9784-9790)] and that both ring closure of the tetrahedral intermediate to form the epoxy ether and ring opening by His57 partially limit the first-order rate constant for inactivation, ki. The peptidyl chloroethyl derivative adopts a very different active site conformation from that assumed by serine proteases inactivated by peptidyl chloromethanes. Positioning the chloroethyl derivative into the conformation adopted by chloromethyl derivatives would cause the extra methyl group to make a bad van der Waals contact with the inactivator P2 carbonyl carbon, thereby preventing the formation of the invariant hydrogen bond between the inactivator P1 amide nitrogen and the carbonyl group of Ser214. We conclude that the unusual conformation displayed by the chloroethyl derivative is caused by steric hindrance between the extra methyl group and the rest of the inactivator chain.

Mevalonate pyrophosphate decarboxylase catalyzes the decarboxylation of mevalonate pyrophosphate to isopentyl pyrophosphate. The mechanism of action of this enzyme was investigated to elucidate the mechanism of inhibition by 3-hydroxy-3-(fluoromethyl)-5-pyrophosphopentanoic acid (II). It was previously found that II is a competitive inhibitor (Ki = 0.01 microM) of the enzyme reaction [Reardon, J.E., & Abeles, R.H. (1987) Biochemistry 26, 4717-4722; Nave, J.F., d'Orchymont, H., Ducep, J.B., Piriou, F., & Jung, M.J. (1985) Biochem. J. 227, 247-254]. We have now observed that II is decarboxylated 2500-fold more slowly than mevalonate pyrophosphate (3-hydroxy-3-methyl-5-pyrophosphopentanoic acid, I). The enzyme was exposed to saturating concentrations of II and ATP and then passed through a Penefsky column to remove excess substrate. The enzyme was denatured immediately upon emerging from the Penefsky column. Nearly 1 equiv of both 3-phospho-3-(fluoromethyl)-5-pyrophosphopentanoic acid and ADP was bound to the enzyme. 3-Hydroxy-5-pyrophosphopentanoic acid (III) is phosphorylated at the secondary hydroxyl group and released from the enzyme without decarboxylation. This reaction is 30-fold slower than the rate of decarboxylation of I. The introduction of the 3-fluoromethyl group as well as the removal of the 3-methyl group results in low rates of decarboxylation. These substrate analogs have decreased electron density relative to the tertiary carbon of the substrate. Therefore, the transition state of the decarboxylation step has considerable carbocationic character. Further support for the carbocationic transition state is provided by the finding that N-methyl-N-carboxymethyl-2-pyrophosphoethanolamine (IV) inhibits the enzyme reaction with Ki = 0.75 microM. IV is probably a transition-state analog in which the positively charged nitrogen atom is analogous to the carbocation.

We have examined the reaction of N-(benzyloxycarbonyl)-L-alanyl-L-glycyl-L-phenylalanyl chloromethyl ketone (ZAGFCMK) with chymotrypsin (Cht) and have found that, in addition to irreversible alkylation of the enzyme, some of the corresponding hydroxymethyl ketone is produced. For each molecule of hydroxy ketone formed, 3.6 molecules of chymotrypsin are inactivated. Chloroketone hydrolysis is also observed with chymotrypsin methylated at N-3 of the active site histidine (MeCht). The hydrolysis proceeds slowly (k = 0.14 min-1). Alkylation of the modified enzyme was not observed. An initial burst of free chloride is detected during the MeCht-catalyzed hydrolysis. The magnitude of the chloride burst is proportional to the enzyme concentration in an approximate 1:1 stoichiometry and indicates a relatively rapid chloride-releasing step which gives rise to an intermediate which is more slowly converted to hydroxy ketone. We have also investigated both the solution and MeCht-mediated hydrolysis of the S isomer of N-acetyl-L-alanyl-L-phenylalanyl chloroethyl ketone (S-AcAFCEK). We have concluded that the nonenzymatic hydrolysis proceeds with inversion of configuration at the stereocenter, while the enzymatic process occurs with retention of configuration. The two nucleophilic displacements attending the MeCht-mediated hydrolysis of S-AcAFCEK imply the formation of an intermediate, possibly of an epoxy ether, formed by internal displacement of the chloride by the oxyanion of the initially generated enzyme-chloroketone hemiketal adduct.

D-Proline is converted to 5-amino valeric acid by D-proline reductase. This conversion involves the reductive cleavage of the alpha-carbon-nitrogen bond. We have examined the fate of the carboxyl oxygen atoms during conversion of D-proline to delta-NH2-valeric acid. 18O atoms from the carboxyl group of D-proline are not lost during conversion to product. In contrast, in the conversion of glycine to acetyl phosphate by glycine reductase a carboxyl oxygen atom is lost to solvent. An intermediate acyl-enzyme is found during the reduction of glycine. We conclude that the reduction of proline proceeds without the formation of an acyl enzyme intermediate.

The 5-methylthio-D-ribose moiety of 5'-(methylthio)-adenosine is converted to methionine in a wide variety of organisms. 2,3-Diketo-5-methylthio-1-phosphopentane is an advanced intermediate in the methionine recycling pathway present in the Gram-negative bacterium Klebsiella pneumoniae. This unusual metabolite is oxidatively cleaved to yield formate (from C-1), 2-keto-4-methylthiobutyrate (the transamination product of methionine), and 3-methylthiopropionate. To further characterize this oxidative conversion, the desthio analog of the naturally occurring diketone, namely 2,3-diketo-1-phosphohexane I, was synthesized. If the metabolism of I is analogous to that of 2,3-diketo-5-methylthio-1-phosphopentane it should be converted to formate, 2-ketopentanoate, and butyrate. An enzyme (E-1), which mediates the oxidative conversion of I to formate and 2-ketopentanoate, was isolated from extracts of K. pneumoniae. E-1 was purified 100-fold to homogeneity in 10% yield. The native enzyme is a monomeric protein of M(r) 27,000. The activity of E-1 requires magnesium ion as a cofactor. No other prosthetic groups were detected. Incubation of the enzyme with I, under anaerobic conditions, led to the discovery of two intermediates. These species have been identified by 1H and 13C NMR, UV-visible spectroscopy, and model chemistry studies as 2-hydroxy-3-keto-1-phospho-1-hexene II, generated by enolization of I; and 1,2-dihydroxy-3-keto-1-hexene III, generated by enzymatic dephosphorylation of II. Intermediates II and III are released from the active site of the enzyme; III accumulates under anaerobic conditions. Under aerobic conditions, III is non-enzymically oxidized to 2-ketopentanoate, formate, and other products. Compound II was also generated by heating I at pH 7.5 for 7 min. Action of alkaline phosphatase on II produces III.

We have isolated and purified an enzyme (E-2) from Klebsiella pneumoniae, which catalyzes the formation of CO from CH3-S-CH2-CH2-CO-C(OH) = CH-O- (III). This compound is an intermediate in the conversion of 5'-methylthioadenosine to methionine. Concomitant with CO formation, methylthiopropionic acid and formate are produced and O2 is consumed. E-2 also catalyzes the formation of CO, formate, and butyrate from CH3-CH2-CH2-CO-C(OH) = CH-O- (IIIa), the desthio analog of III. Experiments with isotopic IIIa have shown that formate is derived from 1-C, and CO from 2-C. E-2 has a M(r) = 18,500 and requires Mg2+, and no chromophoric cofactor has been detected.

Tryptophan synthase (alpha 2 beta 2 complex) from Salmonella typhimurium catalyzes the formation of tryptophan from serine and indole. The enzyme is inactivated by (1-fluorovinyl)glycine. Concomitant with enzyme inactivation, the absorbance at 485 nm increases, indicating covalent modification of pyridoxal 5'-phosphate. It is proposed that inactivation involves elimination of HF to form an allene, which reacts with a nucleophile at the active site. The inactivation reaction involves an alpha,beta-elimination, as does the formation of tryptophan from indole and serine. The inactivation occurs with k(in) > 1.3 s-1, which is very close to k(cat) (6.4 s-1) for the formation of tryptophan from indole and serine. The inactive enzyme (alpha 2 beta 2) regains activity with k(off) = 0.005 min-1. Aminoacetone is formed during reaction, and pyridoxal 5'-phosphate is regenerated. Tryptophan synthase also catalyzes the dehydration of serine, or 3-fluoroalanine, to pyruvate in the absence of indole. This reaction involves an alpha,beta-elimination and the intermediate formation of an aminoacrylate adduct with pyridoxal 5'-phosphate, as does the formation of tryptophan. Pyruvate formation proceeds at less than 5% the rate of tryptophan formation. With [2-2H]serine an isotope effect (DVmax = 1.5) is observed. We propose that pyruvate formation is limited by the rate of hydration of the aminoacrylate intermediate and the rate of the abstraction of the serine alpha-hydrogen.

The function of a hydrogen bond network, comprised of the hydroxyl groups of Tyr 171 and Ser 214, in the hydrophobic S2 subsite of alpha-lytic protease, was investigated by mutagenesis and the kinetics of a substrate analog series. To study the catalytic role of the Tyr 171 and Ser 214 hydroxyl groups, Tyr 171 was converted to phenylalanine (Y171F) and Ser 214 to alanine (S214A). The double mutant (Y171F: S214A) also was generated. The single S214A and double Y171F:S214A mutations cause differential effects on catalysis and proenzyme processing. For S214A, kcat/Km is (4.9 x 10(3))-fold lower than that of wild type and proenzyme processing is blocked. For the double mutant (Y171F:S214A), kcat/Km is 82-fold lower than that of wild type and proenzyme processing occurs. In Y171F, kcat/Km is 34-fold lower than that of wild type, and the proenzyme is processed. The data indicate that Ser 214, although conserved among serine proteases and hydrogen bonded to the catalytic triad [Brayer, G. D., Delbaere, L. T. J., & James, M. N. G. (1979) J. Mol. Biol. 131, 743], is not essential for catalytic function in alpha-lytic protease. A substrate series (in which peptide length is varied) established that the mutations (Y171F and Y171F:S214A) do not alter enzyme-substrate interactions in subsites other than S2. The pH dependence of kcat/Km for Y171F and Y171F:S214A has changed less than 0.5 unit from that of wild type; this suggests the catalytic triad is unperturbed. In wild type, hydrophobic interactions at S2 increase kcat/Km by up to (1.2 x 10(3))-fold with no effect on Km.(ABSTRACT TRUNCATED AT 250 WORDS)

Papain, a prototype cysteine protease, was inhibited in a time-dependent manner by azapeptide esters designed to deliver an azaglycine group to the active-site thiol. For example, the rate of inhibition was 18 M-1s-1 for Ac-L-PheAglyOiBu (2) and > 11,000 M-1s-1 for Ac-L-PheAglyOPh (7). The rate of inhibition was slowed in the presence of substrate, and there was no reactivation of the inhibited enzyme after dialysis and incubation in the assay buffer. The inhibited enzyme was completely reactivated after the addition of valine methyl ester. The inhibited form of the enzyme is presumed to be acylated on the active-site thiol. An azaalanine-based peptide inhibited papain much more slowly. Azapeptide alkyl esters are unreactive with serine proteases; therefore, these inhibitors are selective for cysteine proteases.

A series of fluorinated alpha-keto acid derivatives [PhCHFCOCO2R,PhCH2CHFCOCO2R,PhCF2-COCO2R, and PhCH2CF2COCO2R (R = H, Me, and Et)] was synthesized. They were inhibitors of chymotrypsin, with Ki values ranging from 4700 to 15 microM. Benzylpyruvic derivatives were generally more potent than the corresponding phenylpyruvic analogs. Esters of the first series were also more potent than their corresponding acids, and potency increased with the number of fluorine atoms. By replacing the ethoxy group of PhCH2CF2COCO2Et (15b) with an amino acid chain (i.e., alanyl-leucyl-arginine methyl ester hydrochloride and alanyl-leucyl-valine ethyl ester), the resultant peptides PhCH2CF2COCO-Ala-Leu-Arg-OMe.HCl.H2O (20) and PhCH2CF2COCO-Ala-Leu-Val-OEt.H2O (23) were found to be slow-binding inhibitors of chymotrypsin with considerably lower Ki values (0.19 and 3.6 microM, respectively). 19F NMR studies indicate, in the case of 20, the presence of an enzyme-inhibitor complex with a hemiketal structure similar to those observed between trifluoromethyl ketones and chymotrypsin. The results illustrate that effective protease inhibitors can be designed by enhancing the electrophilic character of the reactive carbonyl group (with an electron-withdrawing group placed on each side of the carbonyl group). Their potency and/or selectivity can also be improved by taking advantage of binding interactions at S' subsites of the protease.

Clostridial glycine reductase consists of proteins A, B, and C and catalyzes the reaction glycine + Pi + 2e(-)----acetyl phosphate + NH4+. Evidence was previously obtained that is consistent with the involvement of an acyl enzyme intermediate in this reaction. We now demonstrate that protein C catalyzes exchange of [32P]Pi into acetyl phosphate, providing additional support for an acetyl enzyme intermediate on protein C. Furthermore, we have isolated acetyl protein C and shown that it is qualitatively catalytically competent. Acetyl protein C can be obtained through the forward reaction from protein C and Se-(carboxymethyl)selenocysteine-protein A, which is generated by the reaction of glycine with proteins A and B [Arkowitz, R. A., & Abeles, R. H. (1990) J. Am. Chem. Soc. 112, 870-872]. Acetyl protein C can also be generated through the reverse reaction by the addition of acetyl phosphate to protein C. Both procedures lead to the same acetyl enzyme. The acetyl enzyme reacts with Pi to give acetyl phosphate. When [14C]acetyl protein C is denaturated with TCA and redissolved with urea, radioactivity remained associated with the protein. At pH 11.5 radioactivity was released with t1/2 = 57 min, comparable to the hydrolysis rate of thioesters. Exposure of 4 N neutralized NH2OH resulted in the complete release of radioactivity. Treatment with KBH4 removes all the radioactivity associated with protein C, resulting in the formation of [14C]ethanol. We conclude that a thiol group on protein C is acetylated. Proteins A and C together catalyze the exchange of tritium atoms from [3H]H2O into acetyl phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracts of Klebsiella pneumoniae convert 5-S-methyl-5-thio-D-ribose (methylthioribose) to methionine and formate. To probe the terminal steps of this biotransformation, [1-13C]methylthioribose has been synthesized and its metabolism examined. When supplemented with Mg2+, ATP, L-glutamine, and dioxygen, cell-free extracts of K. pneumoniae converted 50% of the [1-13C]methylthioribose to [13C]formate. The formation of [13C]formate was established by 13C and 1H NMR spectroscopy studies of the purified formate, and by 13C and 1H NMR spectroscopy and mass spectrometry studies of its p-phenylphenacyl derivative. By contrast, no incorporation of label from [1-13C]methylthioribose into the biosynthesized methionine was detected by either mass spectrometry or 13C and 1H NMR spectroscopy. The most reasonable interpretation of these results is that C-1 of methylthioribose is converted directly to formate concomitant with the conversion of carbon atoms 2-5 to methionine. The penultimate step in the conversion of methylthioribose to methionine and formate is an oxidative carbon-carbon bond cleavage reaction in which an equivalent of dioxygen is consumed. To investigate the fate of the dioxygen utilized in this reaction, the metabolism of [1-13C]methylthioribose in the presence of 18O2 was also examined. Mass spectrometry revealed the biosynthesis of substantial amounts of both [18O1]methionine and [13C, 18O1]formate under these conditions. These results suggest that the oxidative transformation in the conversion of methylthioribose to methionine and formate may be catalyzed by a novel intramolecular dioxygenase. A mechanism for this dioxygenase is proposed.

A series of peptidyl alpha-keto esters, alpha-keto amides, alpha-keto acids, and alpha-diketones were synthesized which reversibly inhibit papain and cathepsin B. Methyl 3-(N-benzyloxycarbonyl-L-phenylalanyl)amino-2-oxopropionate (a dicarbonyl compound) inhibits papain with a Ki value of 1 microM, whereas the Ki of 3-(N-acetyl-L-phenylalanyl)aminopropanone (a monocarbonyl compound) is 1.5 mM (M. R. Bendall et al., 1979. Eur. J. Biochem. 79, 201-209). Both carbonyl groups are required for effective inhibition. Extension of these inhibitors by addition of P substituents (e.g., hexyl) does not affect the Ki for papain, but reduces Ki for cathepsin B 33-fold. For these two enzymes slow binding inhibition was observed with slow on rates (kappa on, 5.2 X 10(2) M-1 s-1 for papain, and 2.7 X 10(3) M- s-1 for cathepsin B). Addition of a P3 substituent (glycine) has no effect on Ki. We propose that the mechanism of inhibition involves the formation of a hemithioketal by addition of the active-site thiol to the carbonyl group of the inhibitor closer to the N-terminus. The hemithioketal intermediate is most likely stabilized by the electron withdrawing effect of the second carbonyl group.

The peptidyl trifluoromethyl ketones Ac-Phe-CF3 (1) and Ac-Leu-Phe-CF3 (2) are inhibitors of chymotrypsin. They differ in Ki (20 and 2 microM, respectively) as well as in their kinetics of association with chymotrypsin in that 1 is rapidly equilibrating, with an association rate too fast to be observed by steady-state techniques, while 2 is "slow binding", as defined by Morrison and Walsh [Morrison, J. F., & Walsh, C. T. (1988) Adv. Enzymol. Relat. Areas Mol. Biol. 61, 202], with a second-order association rate constant of 750 M-1 s-1 at pH 7.0 [Imperiali, B., & Abeles, R. (1986) Biochemistry 25, 3760]. The crystallographic structures of the complexes of gamma-chymotrypsin with inhibitors 1 and 2 have been determined in order to establish whether structural or conformational differences can be found which account for different kinetic and thermodynamic properties of the two inhibitors. In both complexes, the active-site Ser 195 hydroxyl forms a covalent hemiketal adduct with the trifluoromethyl ketone moiety of the inhibitor. In both complexes, the trifluoromethyl group is partially immobilized, but differences are observed in the degree of interaction of fluorine atoms with the active-site His 57 imidazole ring, with amide nitrogen NH 193, and with other portions of the inhibitor molecule. The enhanced potency of Ac-Leu-Phe-CF3 relative to Ac-Phe-CF3 is accounted for by van der Waals interactions of the leucine side chain of the inhibitor with His 57 and Ile 99 side chains and by a hydrogen bond of the acetyl terminus with amide NH 216 of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

A series of seven peptidyl trifluoromethyl ketone (TFK) inhibitors of chymotrypsin have been prepared which differ at the P1 and P2 subsites. Inhibition equilibria and kinetics of association and dissociation with chymotrypsin have been measured. The association rate of Ac-Phe-CF3 was measured at enzyme concentrations between 8 nM and 117 microM in order to examine the relation between the ketone/hydrate equilibrium of trifluoromethyl ketones and the "slow binding" by these inhibitors. The association rate decreases at high enzyme concentrations, indicating that TFK ketone is the reactive species and that conversion of TFK hydrate to ketone becomes rate limiting under these conditions. Inhibitors with hydrophobic side chains at P2 bind more tightly but more slowly to chymotrypsin, indicating that formation of van der Waals contacts between the P2 side chain and the His 57 and Ile 99 side chains of chymotrypsin is a relatively slow process. Inhibitor properties were compared to the Michaelis-Menten kinetic constants of a homologous series of peptide methyl ester and peptide amide substrates. Plots of log Ki vs log (kcat/Km) are linear with slopes of 0.65 +/- 0.2, indicating that these inhibitors are able to utilize 65% of the total binding energy between chymotrypsin and its hydrolytic transition state.

We have synthesized a series of peptidyl fluoroketones that reversibly inhibit the serine proteases human leukocyte elastase (HLE) and alpha-lytic protease (alpha-LP). Ac-ambo-AlaCF3 (1) inhibits HLE and alpha-LP with Ki's of 2.4 and 15 mM, respectively. The effects of structural variations on this parent compound on Ki and the kinetics of inhibition were studied. The acetyl group was replaced by the tripeptide Z-L-Ala-L-Ala-L-Pro to yield the tetrapeptide trifluoroketone (TFK) Z-L-Ala-L-Ala-L-Pro-ambo-AlaCF3 (2). This extension reduced Ki 3500-fold for HLE and 3000-fold for alpha-LP. Removal of a fluorine atom from a TFK decreases Ki about 15- to 30-fold with both enzymes. Replacement of one fluorine atom of 2 by a residue (-CH2-CH2-COLeuOMe) (6) which can interact with the S'1 and S'2 subsites decreased Ki 30-fold for HLE and 150-fold for alpha-LP compared to Z-L-Ala-L-Ala-L-Pro-ambo-AlaCF2H (3). The Ki of 6 for HLE is approximately equal to that of trifluoroketone 2. For alpha-LP Ki of 6 is 10-fold lower than that for the trifluoroketone 2. Inhibitors with Ki values less than 10(-7) M exhibit slow binding kinetics. By analogy to cholinesterases and chymotrypsin, it is likely that these enzymes combine with the keto form of the inhibitor to form the enzyme-inhibitor complex. Therefore, kon and Ki were corrected for the ketone concentration. The corrected kon values for the slow binding inhibitors are in most cases less than diffusion controlled, ranging between 8.2 X 10(4) and 4.68 X 10(6) M-1 s-1. An exception is Z-L-Ala-L-Ala-L-Pro-ambo-ValCF3 (8) where kon = 9 X 10(7) M-1 s-1, which is nearly diffusion controlled.

The inactivation of chymotrypsin by 5-benzyl-6-chloro-2-pyrone has been studied. Chloride analysis of the inactivated enzyme suggests that chlorine is no longer present in the complex. 13C NMR spectroscopy of chymotrypsin inactivated with 5-benzyl-6-chloro-2-pyrone-2,6-13 C2 shows the presence of two new resonances from the protein-bound inactivator. The chemical shift values of these resonances are consistent with an intact pyrone ring on the enzyme as well as the replacement of the C-6 chlorine by a different heteroatom. X-ray diffraction analysis at 1.5-A resolution of the inactivator-enzyme complex demonstrates that the gamma-oxygen of the active site serine residue (serine 195) is covalently attached to C-6 of the inactivator and that the pyrone ring is intact. The 5-benzyl group of the inactivator is bound to the enzyme in the hydrophobic specificity pocket. The conformational changes that occur in the protein as a result of complexation with the inactivator are discussed.

DL-threo-beta-Fluoroasparagine (DL-threo-beta-F-Asn) inhibited the growth of murine leukemia L1210 cells and three human leukemia cell lines in culture. Fifty % inhibiting dose values ranged between 30 and 50 microM DL-threo-beta-F-Asn. L1210 cells were not sensitive to DL-erythro-beta-fluoroasparagine, DL-threo-beta-fluoroaspartic acid, or DL-erythro-beta-fluoroaspartic acid at 300 microM, the highest dose studied. The antileukemia activity of DL-threo-beta-F-Asn was studied in further detail using the L1210 model system. Inhibition of growth in culture was prevented by L-asparagine but not by D-asparagine. Inhibition of growth of L1210 cells incubated for 40 hr in the presence of 300 microM DL-threo-beta-F-Asn was reversed after DL-threo-beta-F-Asn removal. Treatment for longer periods of time resulted in cell lysis. DL-threo-beta-F-Asn at doses of 250 mg/kg increased life span in mice bearing L1210 tumors by 60%. These results demonstrate the chemotherapeutic potential of the amino acid analogue DL-threo-beta-F-Asn.

beta,beta-Dichloro- and beta,beta,beta-trifluoroalanine irreversibly inactivate a number of pyridoxal phosphate dependent enzymes which catalyze beta- or gamma-elimination reactions. The inactivation is time dependent and the rate of inactivation is first order in enzyme concentration. This suggests that inactivation is due to covalent modification of the enzyme by a species generated at the active site from the polyhaloalanine (i.e., suicide inactivation). Monohaloalanines are substrates and do not inactivate. For gamma-cystathionase, covalent and stoichiometric attachment of [1-14C]beta,beta,beta-trifluoroalanine was shown. It is proposed that the mechanism of inactivation involves Schiff base formation between inactivator and enzyme-bound pyridoxal and subsequent elimination of HC1 from dichloroalanine or HF from trifluoroalanine. This results in the formation of a beta-halo-alpha,beta unsaturated imine, an activated Michael acceptor. Michael addition of a nucleophile at the active site leads to covalent labeling of the enzyme and inactivation. Alanine racemase is also inactivated by the two polyhaloalanines. Glutamate-pyruvate and gultamate-oxaloacetate transaminase are inactivated by monohaloalanines but not by polyhaloalanines.

Bacterial methoxatin-dependent methanol dehydrogenase requires an electron acceptor, e.g., phenazine methosulfate (PMS), for activity. Oxidation of methanol shows a deuterium isotope effect of 4.3 in the presence of high concentrations of PMS; the effect is reduced at low PMS concentrations. This suggests that the catalytic reaction comprises at least two steps, one involving substrate and the other PMS. The UV-visible spectrum of methanol dehydrogenase undergoes a series of characteristic changes when PMS is added to the enzyme. It is proposed that these changes correspond to the formation of an enzyme intermediate, resulting from the reaction of PMS with the enzyme, which can subsequently react with substrate. Preincubation of the enzyme with PMS followed by dilution into a solution containing [14C]methanol results in an additional enzyme turnover compared to the control performed without preincubation with PMS. This confirms the proposal that the enzyme reacts with PMS before reacting with substrate. The reaction between the enzyme and PMS is not a redox reaction since no oxygen is consumed during the reaction nor is the PMS reduced. Methanol dehydrogenase is inactivated by cyclopropanol in the presence of PMS. The intermediate formed by reaction of the enzyme with PMS is also an intermediate in the cyclopropanol inactivation reaction. Preincubation of the enzyme with PMS gives an increase in the rate of inactivation by cyclopropanol. Cyclopropanol stoichiometrically equivalent to 13% of the enzyme is sufficient to completely inactivate the enzyme. The inactivation reaction shows neither a deuterium nor a tritium isotope effect.(ABSTRACT TRUNCATED AT 250 WORDS)

4'-Phosphopantothenoylcysteine decarboxylase was purified 900-fold from Escherichia coli B with an overall yield of 6%. The enzyme migrates as a single band with a molecular weight of 35,000 +/- 3000 in 10% polyacrylamide gel electrophoresis under denaturing conditions. The native enzyme has an apparent molecular weight of 146,000 +/- 9000 as determined by a gel exclusion column. At pH 7.6 and 25 degrees C, Km = 0.9 mM and Vmax = 600 nmol/(min X mg of protein). The pH optimum for Vmax is between 7.5 and 7.7. Hydroxylamine, phenylhydrazine, potassium cyanide, and sodium borohydride as well as pyridoxal phosphate and pyridoxal inactivated the enzyme. The enzyme contains covalently bound pyruvate as suggested by the isolation of [3H]lactate and pyruvate from [3H]NaBH4-reduced enzyme and native enzyme, respectively. One mole of [3H]lactate was isolated per 39,000 g of [3H]NaBH4-reduced and completely inactivated enzyme, and 1 mol of pyruvate was isolated per 31,000 +/- 4000 g of native enzyme. Mild base treatment released lactate and pyruvate from the reduced and the native enzymes, respectively, suggesting the pyruvate is attached to the enzyme by an ester bond. These findings are in accord with similar results obtained with the horse liver enzyme (R. Scandurra, personal communication). The presence of covalently bound pyruvate in the bacterial and mammalian enzymes suggests that pyruvate plays a major role in the mechanism of action.

The inactivation of chymotrypsin by 3-benzyl-6-chloro-2-pyrone has been studied. A covalent adduct is formed that deacylates slowly with a half-life of 23 h. X-ray diffraction analysis at 1.9-A resolution of the inactivator-enzyme complex shows that the gamma-oxygen of the active-site serine (serine-195) is covalently attached to C-1 of (Z)-2-benzylpentenedioic acid, the benzyl group of the inactivator is held in the hydrophobic specificity pocket of the enzyme, and the free carboxylate forms a salt bridge with the active-site histidine (histidine-57). The conformational changes that occur in the protein as a result of complexation are described. It is proposed that formation of the salt bridge prevents access of water and, therefore, hydrolysis of the acyl-enzyme.

We have synthesized peptidyl fluoromethyl ketones that are specific inhibitors of the serine proteases alpha-chymotrypsin and porcine pancreatic elastase. By analogy with the corresponding aldehydes it is assumed that the fluoromethyl ketones react with the gamma-OH group of the active site serine to form a stable hemiacetal [Lowe, G., & Nurse, D. (1977) J. Chem. Soc., Chem. Commun., 815; Chen, R., Gorenstein, D.G., Kennedy, W.P., Lowe, G., Nurse, D., & Schultz, R.M. (1979) Biochemistry 18, 921; Shah, D.O., Lai, K., & Gorenstein, D.G. (1984) J. Am. Chem. Soc. 106, 4272]. 19F NMR studies of the chymotrypsin-bound trifluoromethyl ketone inhibitors Ac-Leu-ambo-Phe-CF3 and Ac-ambo-Phe-CF3 clearly indicate that the carbonyl carbon is tetrahedral at the active site of the enzyme. The inhibitor is bound as either the stable hydrate or the hemiacetal, involving the active site serine. The effect of varying the number of amino acid residues in the peptidyl portion of the inhibitor and the number of fluorines in the fluoromethyl ketone moiety is examined. In the series of trifluoromethyl ketone elastase inhibitors, the lowering of Ki concomitant with the change from a dipeptide analogue to a tetrapeptide analogue (Ac-Pro-ambo-Ala-CF3, Ki = 3 X 10(-3) M; Ac-Ala-Ala-Pro-ambo-Ala-CF3, Ki = 0.34 X 10(-6) M) correlates well with the variation in V/K for hydrolysis of the corresponding amide substrates. This trend is indicative of the inhibitors acting as transition-state analogues [Bartlett, P.A., & Marlowe, C.K. (1983) Biochemistry 22, 4618; Thompson, R.C. (1973) Biochemistry 12, 47].(ABSTRACT TRUNCATED AT 250 WORDS)

Methanol oxidase from Hansenula polymorpha contains five "red" flavin semiquinones and two oxidized flavins per octamer. Addition of substrate results in the reduction of the two oxidized flavins but does not affect the flavin semiquinones. Enhanced water proton relaxation rates indicate that the unpaired electron of the flavin semiquinones is accessible to the solvent and this accessibility is significantly decreased upon binding of the suicide inhibitor cyclopropanol. In the native enzyme, the semiquinones are not oxidizable by air. All flavins were resolved from the enzyme, and holoenzyme was reconstituted by addition of oxidized flavin. The reconstituted enzyme was catalytically active. The specific activity was 50% that of the original enzyme. It was concluded that the semiquinone is not required for the oxidation of methanol, although it may be present at an otherwise intact site.

We propose that adverse effects of the antibiotic metronidazole may be due, wholly or in part, to its conversion to a thiamine analog and consequent vitamin B1 antagonism. Consistent with this hypothesis, the drug is accepted as a substrate for the thiaminase (EC 2.5.1.2) elaborated as an exoenzyme by the human gut flora constituent Bacillus thiaminolyticus and is also a substrate for the intracellular thiaminase of the mollusk Venus mercenaria. The product, identified as the 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-hydroxyethyl)-2-methyl-4 - nitroimidazolium cation, is a close structural analog of thiamine and is an effective inhibitor of thiamine pyrophosphokinase in vitro. Due to its susceptibility to nucleophilic attack, the analog is unstable, releasing inorganic nitrite under mild conditions. Enzymatic alkylation reactions such as that effected by thiaminase may have general pharmacological significance as a route of increasing the electrophilicity and/or reduction potential of drugs which are heterocyclic weak bases.

We have synthesized inhibitors of chymotrypsin, based on fluoromethyl ketones, that bind at S and S' subsites. "Small" inhibitors of serine proteases, which have previously been synthesized, only interact with S subsites. The parent compound is Ac-Leu-ambo-Phe-CF2H (1) (Ki = 25 X 10(-6) M). This inhibitor was modified by successively replacing H of the -CF2H group by -CH2CH2CONHCH3, (4), -CH2CH2CONH-Leu-NHMe (5), -CH2CH2CONH-Leu-Val-OEt (6), and -CH2CH2CONH-Leu-Arg-OMe (7). Corresponding Ki values are 7.8 (4), 0.23 (5), 0.21 (6), and 0.014 (7) microM. Extending 5 to 6 by addition of Val-OEt at P3' does not decrease Ki. In contrast, extension of 5 to 7 by incorporating Arg-OMe at P3' decreases Ki approximately 15-fold, suggesting interaction between Arg and the S3' subsite but no corresponding interaction at that subsite with Val. These results are in accordance with results obtained with the homologous family of avian ovomucoid third domain proteins. Proteins with Arg at the P3' position show highly favorable interactions with the protease at the S3' subsite [Park, S. J. (1985) Ph.D. Thesis, Purdue University; M. Laskowski, Jr., personal communication]. These results establish that incorporation of residues which interact with S' subsites significantly increases the efficacy of inhibitors and that valuable information concerning the most effective amino acid composition of small inhibitors can be obtained from the amino acid sequence of protein inhibitors.

2-Hydroxy-3-butynoate is both a substrate and an irreversible inactivator of the flavoenzyme L-lactate oxidase. The partitioning between catalytic oxidation of 2-hydroxy-3-butynoate and inactivation of the enzyme is determined by the concentration of the second substrate, O2. Rapid reaction studies show the formation of an intermediate which is common to both the oxidation and inactivation pathways. This intermediate appears to be a charge-transfer complex between enzyme-reduced flavin and 2-keto-3-butynoate. It is characterized by a long-wavelength absorbing band (gamma(max) 600 nm) and lack of fluorescence, making it easily distinguished from the subsequently formed inactivated enzyme, which has no long wavelength absorption (gamma(max) 318, 368 nm) and which is strongly fluorescent. Inactivation is also accomplished by reaction of the reduced enzyme with 2-keto-3-butynoate. The absorbance and fluorescence characteristics of the inactivated enzyme are similar to those of a model compound, C(4a), N(5)-propano-bridged FMN bound to apolactate oxidase. That the modified chromophore of the inactivated enzyme is an adduct involving both the C(4a) and N5 positions is further supported by the spectral and fluorescence changes resulting from treatment of the inactivated enzyme with borohydride.

5-Butyl-3H-1,3-oxazine-2,6-dione (1) inactivates chymotrypsin. The extent of inactivation is dependent upon the concentration of 1. Upon dilution of the inactivated enzyme, catalytic activity is partially restored. Reactivation is a biphasic process. An initial relatively rapid phase (k = 1.8 X 10(-2) min), whose amplitude is dependent upon the extent of dilution, is observed. Maximally, 60-65% of the catalytic activity can be recovered. The rapid phase is followed by a slow phase (k approximately 1 X 10(-3) min-1). With 1 labeled with 14C at C-2, it was shown that two forms of inactive enzyme are formed, E.1 and E.1'. 14C label is retained in E.1 but is no longer present in E.1'. Presumably, C-2 is lost as CO2. The following reaction sequence is proposed for the inactivation of chymotrypsin: E + 1 in equilibrium E.1 CO2----E.1'----E + 1''. The probable structures of E.1, E . 1', and 1'' are shown in Scheme I in the text.

S-Adenosylhomocysteinase (SAHase), a tetrameric enzyme, is inactivated by 2'-deoxyadenosine (2'dAdo) in a time-dependent process [Hirshfield, M. S. (1979) J. Biol. Chem. 254, 22-25]. It has been proposed that inactivation involves oxidation of 2'dAdo at C-3' by enzyme-bound nicotinamide adenine dinucleotide (NAD), subsequent proton abstraction at C-2', and elimination of adenine. This results in irreversible formation of enzyme-bound NADH and of adenine (Ade) and inactivation [Abeles, R. H., TAshjian, A. H., Jr., & Fish, S (1980) Biochem. Biophys. Res. Commun. 95, 612-617]. It has now been established that upon inactivation of SAHase with deoxy[2'(R)-3H]adenosine, 3H2O is formed. This is consistent with the proposed mechanism and of 3H2O release shows that maximally two of the four subunits participate in the reaction that results in 3H2O release. Reaction of SAHase with 2'dAdo results in reduction of two of the enzyme-bound NAD molecules. However, all four NAD molecules can be reduced by NaBH4, but only two are reduced to C-4 NADH. When the enzyme is inactivated with adenine-labeled 2'dAdo, radioactivity corresponding to 0.5-1.0 mumol of 2'dAdo binds tightly per micromole of subunit. This radioactive material is not removed from the enzyme by extensive dialysis but can be displaced by unlabeled 2'dAdo or Ade. After denaturation of the complex, radioactive material is released. Of this material 80-90% is adenine and less than 1% 2'dAdo. 2'dAdo also binds tightly to the enzyme reduced with NaBH4. Upon denaturation mostly adenine (80-90%) is released. Reaction of [2'-3H]2'dAdo with enzyme reduced with NaBH4 does not result in 3H2O formation. We conclude that the enzyme catalyzes the release of adenine from 2'dAdo by two mechanisms: One involves formation of 3'keto-2'dAdo and subsequent elimination of adenine. The other does not involve oxidation of 2'dAdo and probably is a hydrolytic process. It is proposed that the ability of the enzyme to carry out the hydrolytic process is a direct consequence of the manner in which 2'dAdo as well as the normal substrate binds to the enzyme, i.e., hydrogen-bond interaction of the protein with the adenine moiety and distortion of the ribose ring. When adenine-labeled adenosine is added to the enzyme, radioactivity corresponding to 0.5 mumol/mumol of subunit is associated with the protein after gel filtration. Of the radioactive material bound to the protein, 20% is adenine, 15% is adenosine, and the remaining radioactivity is present in unidentified compounds. The adenine bound to the enzyme does not participate in the catalytic process, and we conclude that it is bound to two of the subunits that do not participate in catalysis. Possible, these two subunits have a regulatory function. SAHase probably consists of two nonequivalent pairs of subunits. Only one pair participates in catalysis, but all four subunits probably bind Ado and 2'dAdo. We have confirmed the fact that the carbocyclic analogue of adenosine inactivates SAHase [Guranowski, A., Montgomery, J. A., 110-115]...

Previously, we reported that incorporation of threo-beta-fluoroasparagine into cellular protein inhibits N-linked glycosylation. We now show that short synthetic peptides which contain N-acetyl-threo-beta-fluoroasparagine fail to undergo glycosylation in a cell-free system except at extremely high substrate concentrations. An N-benzoyl-threo-beta-fluoroasparagine-containing peptide has a 100-fold lower Vmax/Km than the analogous N-benzoyl-asparagine-containing peptide. Substitution of a fluorine for a hydrogen on the beta-carbon of asparagine weakens the ability of the peptide to bind the oligosaccharyltransferase. A 100-fold excess of acetyl-threo-beta-fluoroasparaginyl-leucyl-threonine methylamide over acetyl-asparaginyl-leucyl-threonine methylamide inhibited glycosylation of the latter peptide by less than 10%. Both threo-beta-fluoroasparagine and erythro-beta-fluoroasparagine-containing peptides are glycosylated at the same rate. Glycofluoropeptides generated from beta-fluoroasparagine-containing peptides were N-glycosylated. These cell-free studies with synthetic fluoropeptides suggest that incorporation of beta-fluoroasparagine into cellular protein inhibits N-linked glycosylation by rendering protein substrates ineffective for glycosylation. In the course of this work, we also demonstrate that the N-linked glycosylating enzyme acts only on L-asparagine-containing peptides and not on D-asparagine peptides.

Transcarboxylase and propionyl-CoA carboxylase catalyze the elimination of HF from beta-fluoropropionyl-CoA to form acrylyl-CoA. No carboxylation products of fluoropropionyl-CoA could be detected. The elimination proceeds at approximately the same rate as the normal carboxylation reactions. With propionyl-CoA carboxylase, ATP was required (HCO3- was also present) and was hydrolyzed to ADP and Pi. The rate of ADP formation was equal to that of acrylyl-CoA formation. A previous report (Stubbe, J. A., and Abeles, R. H. (1977) J. Biol. Chem. 252, 8338--8340) that acrylyl-CoA formation is faster than ADP formation is in error. With transcarboxylase, oxalacetate was required for acrylyl-CoA formation, and pyruvate was produced. The rate of pyruvate formation was equal to that of acrylyl-CoA formation. We conclude that the ability of the enzyme to catalyze the elimination of HF from beta-fluoropropionyl-CoA indicates that the enzyme can catalyze the abstraction of the substrate alpha-proton without concomitant carboxylation of the substrate. We also conclude that the normal catalytic reaction, therefore, probably involves a carbanion intermediate and does not proceed through a concerted process as has frequently been proposed.

The benzoxazinones 2-ethoxy-4H-3,1- benzoxazin -4-one (1a) and 2-(trifluoromethyl)-4H-3,1- benzoxazin -4-one (1d) inactivate chymotrypsin. The inactivation is stoichiometric and proceeds with rate constants of 7 X 10(5) M-1 min-1 and greater than 4 X 10(6) M-1 min-1, respectively. The inactivated enzyme recovers catalytic activity slowly, k = 2.3 X 10(-3) min-1 and 3.7 X 10(-2) min-1 (pH 7.1). When the enzyme regains catalytic activity, 2-[N-(ethoxycarbonyl)amino]benzoic acid is released from enzyme inactivated with 1a and N-(trifluoroacetyl)anthranilic acid from enzyme inactivated with 1d. The mechanism of inactivation involves attack of the active site serine on the C-4 carbonyl of the inactivator which leads to ring opening and formation of an ortho-substituted benzoylchymotrypsin , which hydrolyzes slowly due to electron releasing ability of the substituents. The rate of hydrolysis of the benzoylchymotrypsin from 1a or 1d is in close agreement with those predicted from the Hammett parameters (sigma, rho) for hydrolysis of their para-substituted analogues [ Caplow , M., & Jencks , W. P. (1962) Biochemistry 1, 883-893]. The inactivation of chymotrypsin by 2-methyl-4H-3,1- benzoxazin -4-one (1b) is an equilibrium process (kinact = 1 X 10(4) M-1 min-1 and Keq = 2 X 10(6) M-1). Formation of a benzoylchymotrypsin is demonstrated by spectral changes and methanol trapping. The benzoylchymotrypsin can also decay by direct hydrolysis to N- acetylanthranilic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

5-S-Ethyl-5-thio-D-ribose (ethylthioribose) exhibits antiprotozoal activity against Plasmodium falciparum, Giardia lamblia, and Ochromonas malhamensis, but is nontoxic to cultured human and murine bone marrow cells (Riscoe, M. K., Ferro, A. J., and Fitchen, J. H. (1988) Antimicrob. Agents Chemother. 32, 1904-1906). We propose the following mechanism to account for the observed selective toxicity of ethylthioribose. 1) The cytocidal action of ethylthioribose against protozoa is a result of its conversion to ethionine, a well-known cytotoxic agent. 2) This transformation occurs through the pathway which normally converts 5-S-methyl-5-thio-D-ribose (methylthioribose) to methionine. 3) Conversion of ethylthioribose to ethionine cannot occur in mammalian cells since these cells cannot phosphorylate methylthioribose (ethylthioribose), a first step in the pathway to methionine (ethionine). To test this hypothesis, [5-3H]ethylthioribose has been synthesized and its metabolism by cell-free extracts of Klebsiella pneumoniae and rat liver was examined. The pathway by which methylthioribose is converted to methionine in K. pneumoniae is well characterized. When supplemented with ATP and L-glutamine, the bacterial extract efficiently converted [5-3H]ethylthioribose to [3H]ethionine. By contrast, ethionine was not produced upon incubation of [5-3H]ethylthioribose, ATP, and L-glutamine with rat liver homogenate. The mammalian cell extract lacks a kinase activity capable of converting ethylthioribose to 1-phospho-5-S-ethyl-5-thio-alpha-D-ribofuranoside, an obligate intermediate in the biosynthesis of ethionine from ethylthioribose in K. pneumoniae. These results support our hypothesis and provide a basis for understanding the apparently selective toxicity of ethylthioribose.

The actions of DL-threo-beta-fluoroasparagine (DL-beta-F-Asn) on the glycosylation of proteins were examined in AtT-20/D16v cells which synthesize several forms of the glycoprotein prohormone, pro-opiomelanocortin (POMC). Treatment with threo-beta-F-Asn(5-10 mM) resulted in: 1) a reduction in the amount of the more highly glycosylated form of POMC (Mr = 32,000) relative to the less glycosylated form (Mr = 29,000) and 2) the appearance of a new species of POMC (Mr = 27,000). 35S]Methionine-labeled tryptic peptides prepared from 27,000 POMC were identical to those from 29,000 and 32,000 POMC; however, 27,000 POMC was found to contain 10% as much [3H]glucosamine relative to [35S]methionine as the 32,000 molecule. Furthermore, 27,000 POMC comigrated with a previously characterized unglycosylated form of this prohormone produced by treatment of cells with tunicamycin. These findings indicate that treatment of cells with threo-beta-F-Asn results in the production of a species of POMC which contains little or no carbohydrate. The effects of beta-F-Asn were specific for the threo diastereomer, were reversible by equimolar concentrations of Asn, but not Asp, and were dose-dependent. Evidence that threo-beta-F-Asn can replace Asn in proteins was obtained by showing that an identified Asn-containing tryptic peptide from threo-beta-F-Asn-treated cells displayed an altered mobility during electrophoresis consistent with threo-beta-F-Asn substitution within this peptide. We conclude that threo-beta-F-Asn can inhibit the glycosylation of proteins in intact cells and that this effect is due to its ability to replace Asn at glycosylation sites.

5'-Deoxy-5'-methylthioadenosine (methylthioadenosine) is cleaved to adenine and 5-methylthioribose-1-phosphate (methylthioribose-1-P). Methylthioribose-1-P is converted to 2-keto-4-methylthiobutyrate ( ketomethylthiobutyrate ) which is transaminated to methionine. We report that one subline of a heterogeneous human colon carcinoma, DLD-1 Clone D, only forms methylthioribose-1-P from methylthioadenosine or 5'-deoxy-5'-methylthioinosine (methylthioinosine), a deaminated derivative of methylthioadenosine, whereas Clone A converts methylthioadenosine and methylthioinosine to methionine, as shown by growth studies in culture of Clone A and Clone D cells and radioactive studies utilizing [methyl-14C]methylthioadenosine or [methyl-14C]methylthioinosine in the presence of extracts of these cells lines. To characterize this defect, we utilized three protein fractions isolated from rat liver which together convert methylthioribose-1-P to ketomethylthiobutyrate . Addition of only Fraction A to Clone D sonicates restores its ability to convert methylthioadenosine to methionine. This fraction is responsible for converting methylthioribose-1-P to 5- methylthioribulose -1-phosphate; radioactive studies confirm this observation. Thus, Clone D is deficient in an enzyme contained in Fraction A; this represents a qualitative biochemical difference between the two clones derived from a single human tumor.

Methanol oxidase isolated from Hansenula polymorpha contains two distinct flavin cofactors in approximately equal amounts. One has been identified as authentic FAD and the other as a modified form of FAD differing only in the ribityl portion of the ribityldiphosphoadenosine side chain. The significance of this finding is as yet unknown. Previous studies have shown that cyclopropanol irreversibly inactivates methanol oxidase [Mincey, T., Tayrien, G., Mildvan, A. S., & Abeles, R. H. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 7099-7101]. We have now established that inactivation is accompanied by covalent modification of the flavin cofactor. The stoichiometry of this reaction is 1 mol of cyclopropanol/mol of active flavin. The structure of the covalent adduct was determined by NMR, IR, and UV spectral studies to be an N5,C4a-cyclic 4a,5-dihydroflavin. Reduction of the covalent adduct with NaBH4 at pH 9.0 before removal from the enzyme converted it to the 1-(ribityldiphosphoadenosine)-substituted 4-(3-hydroxypropyl)-2,3-dioxoquinoxaline. Cyclopropyl ring cleavage accompanies inactivation, and covalent bond formation occurs between a methylene carbon of cyclopropanol and N5 of flavin. Methanol oxidase was also reconstituted with 5-deazaflavin adenine dinucleotide (dFAD). Reconstituted enzyme did not catalyze the oxidation of alcohols to the corresponding aldehydes, nor did reduced reconstituted enzyme catalyze the reverse reaction. Incubation of reconstituted enzyme with cyclopropanol resulted in an absorbance decrease at 399 nm, but no irreversible covalent modification of the deazaflavin cofactor. A reversible addition complex between cyclopropanol and dFAD is formed. The structure of that complex was not definitively established, but it is likely that it is formed through the addition of cyclopropoxide to C5 of dFAD. The failure of dFAD-reconstituted methanol oxidase to catalyze the oxidation of substrate, as well as the lack of reaction with cyclopropanol, supports a radical mechanism for alcohol oxidation and cyclopropanol inactivation. Methanol oxidase catalyzes the oxidation of cyclopropylcarbinol to the corresponding aldehyde. No ring-opened products were detected. The failure to form ring-opened products has been used as an argument against radical processes [MacInnes, I., Nonhebel, D. C., Orsculik, S. T., & Suckling, C. J. (1982) J. Chem. Soc., Chem. Commun., 121-122]. We present arguments against this interpretation.