Search

Eleven polycyclic aromatic hydrocarbons (PAHs) and 14 acetylenic PAHs and biphenyls were used to analyze interactions with cytochrome P450 (P450) 1B1 in inhibiting catalytic activity, using 7-ethoxyresorufin O-deethylation (EROD) as a model reaction. Most of the chemicals examined were direct inhibitors of P450 1B1 except for 4-(1-propynyl)biphenyl, a mechanism-based inhibitor. In the case of direct inhibition of EROD activity {15 of 24 chemicals, e.g., benzo[a]pyrene, 1-(1-propynyl)pyrene, and 3-(1-propynyl)phenanthrene}, restoration of the EROD activity occurred with increasing incubation time, and kinetic analysis showed that EROD K(m) values were higher with these inhibitors at initial stages of incubation but became lower with increasing incubation time. With the other nine chemicals, the K(m) values for P450 1B1-mediated EROD increased during the incubations. Acetylenic inhibitors, but not the 11 PAHs, induced reverse type I spectral changes with P450 1B1, and the low dissociation constants (K(s)) suggested a role for such interaction in the inhibition of catalytic activity. Studies of quenching of P450 1B1-derived fluorescence with inhibitors demonstrated that acetylenic inhibitors and PAHs interacted rapidly with P450 1B1, with K(d) values < 10 microM. However, studies of quenching of inhibitor-derived fluorescence with P450 1B1 showed these interactions to be different, that is, B[a]P interacted with P450 1B1 more slowly. Molecular docking of P450 1B1, based on P450 1A2 crystal structure, suggested that there are clear differences in the interaction of PAH inhibitors with P450 1B1 and 1A2 and that these differences may explain why PAH inhibitors inhibit P450 1 enzymes by different mechanisms. The results suggest that P450 1B1 interacts with synthetic polycyclic aromatic acetylenes and PAHs in different ways, depending on the chemicals, and that these differences in interactions may explain how these chemicals inhibit P450 activities by different mechanisms.

Previously, a high percentage of Wilms tumors was found to be hypomethylated in the unusually long region of pericentromeric satellite DNA on chromosome 1. We now show that these pediatric cancers are also frequently hypomethylated in centromeric satellite DNA throughout the genome and compare satellite DNA hypomethylation with chromosome rearrangements. Relative to normal somatic tissues, 83% of the tumors were hypomethylated in centromeric satellite alpha DNA. This was assessed by blot hybridization under low-stringency conditions after digestion with CpG methylation-sensitive restriction endonucleases. Similar results were obtained with different enzymes, indicating generalized hypomethylation of centromeric DNA. Hypomethylation of another heterochromatic sequence, juxtacentromeric satellite 2 DNA of chromosome 1, was observed in 51% of the tumors. By cytogenetic analysis, rearrangements in the centromeric or juxtacentromeric heterochromatin of chromosome 1 were the most frequent structural aberration and were seen in 14% of the tumors. Tumors with such rearrangements had hypomethylation of satellite DNA in the pericentromeric region. These results show a high degree of targeting of DNA hypomethylation to centromeric and juxtacentromeric satellite DNA sequences in cancer and are consistent with satellite DNA hypomethylation contributing to, but not sufficing for, karyotypic instability in cancer and possibly playing other roles in carcinogenesis.

Previously we have reported the induction of CYP102 in Bacillus megaterium by 17beta-estradiol (E2) and 4-sec-butylphenol (4-sBP). Electrophoretic mobility shift assay analyses demonstrated that E2 and 4-sBP both cause a dose-dependent disassociation of the Bm3R1 repressor protein from its binding site on the operator sequence of the CYP102 gene. Equimolar combinations of E2 and 4-sBP demonstrated additive induction of CYP102 compared to equivalent samples of E2 and 4-sBP added alone. Two gene constructs were used in this investigation. One construct designated BMC143 contained the entire regulatory region of CYP102. The other gene construct, designated BMA45, had the "Barbie box" sequence deleted. While the induction of CYP102 by 4-sBP was much higher in the BMC 143 construct, E2 induced CYP102 in both constructs to the same extent. This difference in induction of CYP102 by these two inducers indicates that they act at different sites, either on the Bm3R1 repressor protein or on positive regulatory sites, or that they act, in part, through different mechanisms.

Facioscapulohumeral muscular dystrophy (FSHD) has an unusual molecular etiology. In a putatively heterochromatic subtelomeric region of each chromosome 4 homologue (4q35), unaffected individuals have 11 to about 95 tandem copies of a complex 3.3-kb repeat (D4Z4). Most FSHD patients have less than 10 copies at one allelic 4q35. This has been proposed to lead to the loss of heterochromatinization and, thereby, inappropriate gene expression by position effects, explaining the dominant nature of FSHD and the role of a decreased number of copies of D4Z4 at 4q35 but not at 10q26. Consistent with the proposed heterochromatinization of this repeat, by Southern blot analysis, we found that SmaI, MluI, SacII, and EagI sites in D4Z4 are highly methylated in normal and FSHD cell lines and somatic tissues, including skeletal muscle. Like repeated DNA sequences in the juxtacentromeric heterochromatin of chromosomes 1, 9, and 16, D4Z4 was hypomethylated at numerous CpGs in sperm and in cell lines from patients with an unrelated DNA methyltransferase deficiency syndrome (ICF; immunodeficiency, centromeric region instability, facial anomalies) in contrast to its hypermethylation in non-ICF postnatal somatic tissues. Our data on FSHD samples suggest that the disease-associated 4q35 D4Z4 repeats, which constitute a small percentage of the total D4Z4 repeats, are not generally hypomethylated relative to the other repeats of this sequence. However, in individuals not affected with FSHD, the hypermethylation of tandem, high-copy-number D4Z4 repeats might help stabilize heterochromatinization at allelic 4q35 regions just as hypermethylation elsewhere in the genome has been linked to chromatin compaction., (Copyright 2001 Academic Press.)

Cytochrome P450 102 (CYP102 or Cytochrome P450(BM)(-)(3)) is induced in Bacillus megaterium by barbiturates, perioxisome proliferators, estrogen, and nonsteroidal antiinflammatory drugs. We have previously demonstrated that a CYP102 construct (BMC 143) coupled with a luciferase reporter gene can be used to identify the inducers of CYP102. We now describe the effect of added phytochemicals on the induction of CYP102 by phenobarbital (PB) in B. megaterium. The isoflavones genistein, biochanin A, coumestrol, and equol, the green tea flavanoid epicatechin, and the fungal toxin zearalenone inhibit the induction of CYP102 by PB in a dose-dependent manner. However, the isoflavone daidzein, the phytoalexin glyceollin, and catechin, an epimer of epicatechin, failed to exhibit a similar inhibitory effect on PB-mediated CYP102 induction.

Epidemiologic and experimental studies support the hypothesis that dietary estrogens from plant sources (phytoestrogens) may play a role in the prevention of breast and prostate cancer. The molecular mechanisms for such chemopreventive effect are still unclear. We investigated the possibility that phytoestrogens may bind differentially to estrogen receptor proteins (ER[alpha] and ERss) and affect the interactions of the ligand-ER complexes with different estrogen response element (ERE) sequences. We used fluorescence polarization to measure the binding affinities of genistein, coumestrol, daidzein, glyceollin, and zearalenone for human ER[alpha] and ERss. Competition binding experiments revealed higher affinity of the phytoestrogens for ERss than for ER[alpha]. Genistein [median inhibitory concentration 12nM] is the most potent and has the same relative binding affinity for ERss as 17ss-estradiol. We also studied the effect of these phytoestrogens on the ability of ER[alpha] and ERss to associate with specific DNA sequences (EREs). The direct binding of human recombinant estrogen receptors to fluorescein-labeled EREs indicates that phytoestrogens can cause conformational changes in both human ERs, which results in altered affinities of the complexes for the ERE from the Xenopus vitellogenin A2 gene and an ERE from the human pS2 gene.

Introduction: Chronic constipation is a common symptom in pediatrics, and physicians often use mineral oil to treat chronic constipation in children. Mineral oil, a hydrocarbon, may not elicit a normal protective cough reflex and may impair mucociliary transport. These effects can increase the likelihood of its aspiration and subsequent impaired clearance from the respiratory tract. We report a case of a child with neurodevelopmental delay with chronic constipation and a history of chronic mineral oil ingestion presenting as asymptomatic exogenous lipoid pneumonia (ELP)., Case History: A 6-year-old white boy with a history of developmental delay was found to have an infiltrate in his right upper lobe on a chest radiograph obtained during evaluation for thoracic scoliosis. The patient had a long history of constipation with daily use of mineral oil. He was fed by mouth and had occasional episodes of coughing and choking during feeding. He was asymptomatic at presentation and physical examination was unremarkable. The patient was advised to stop administration of the mineral oil and was treated empirically with antibiotics during a 3-month period. At follow-up examination the patient continued to be asymptomatic, with the radiologic persistence of the infiltrate. Diagnosis of lipoid pneumonia was made by diagnostic bronchoscopy with bronchoalveolar lavage (BAL). The exogenous origin of the lipid in the BAL fluid was confirmed by gas chromatography/mass spectrometry., Discussion: The clinical presentation of ELP is nonspecific and ranges from the totally asymptomatic patient with incidental radiologic finding, like our patient, to the patient with acute or chronic symptoms attributable to pneumonia, pulmonary fibrosis, or cor pulmonale. Bronchoscopy with BAL can be successful in establishing the diagnosis of ELP by demonstration of a high lipid-laden macrophage index. Treatment of ELP in children is generally supportive, with the symptoms and roentgenographic abnormalities resolving within months after stopping the use of mineral oil., Conclusion: Lipoid pneumonia as a result of mineral oil aspiration still occurs in the pediatric population. It can mimic other diseases because of its nonspecific clinical presentation and radiographic signs. In patients with swallowing dysfunction and pneumonia, a history of mineral oil use should be obtained and a diagnosis of ELP should be considered in the differential diagnoses if mineral oil use has occurred. Our case points to the need for increased awareness by the general pediatricians of the potential hazards of mineral oil use for chronic constipation.

Cytochrome P450 2B1 clones were isolated from a phenobarbital-induced Wistar-Kyoto (WKY) hepatic cDNA library and were found to contain a Glu-322 --> Val substitution, compared with wild-type 2B1 from Sprague-Dawley rats. After heterologous expression in Escherichia coli and purification, activities of this 2B1 E322V variant were determined for ethoxycoumarin and androstenedione. The total activities and metabolite profiles did not differ between 2B1 E322V and wild-type 2B1 for these substrates. In addition, similar rate constants of inactivation were observed with the mechanism-based inactivators chloramphenicol, N-(2-p-nitrophenethyl)chlorofluoroacetamide, and 9-ethynylphenanthrene. These results suggest that the Glu-322 --> Val alteration in the 2B1 WKY variant does not significantly affect 2B1 activity. However, another clone obtained from the cDNA library contained two additional substitutions: Val-103 --> Ala and Glu-424 --> Lys. As residue 103 is within a predicted substrate recognition site (SRS-1), it was of interest to determine whether the Val --> Ala substitution conferred any unique catalytic activities on 2B1. No differences in the metabolism of ethoxycoumarin or androstenedione were observed. However, the Val-103 --> Ala alteration caused an approximately threefold decrease in the rate constant of inactivation for 9-ethynylphenanthrene in comparison with either 2B1 E322V or wild-type 2B1. Based on computer modeling, residue 103 is predicted to be near the active site but at a distance greater than 5A from 9-ethynylphenanthrene. Our results suggest that the Val-103 --> Ala alteration may have an indirect influence on the susceptibility of P450 2B1 to mechanism-based inactivators.

Human cytochrome P450s 1A1, 1A2, and 1B1 are known to have overlapping substrate specificities. All are regulated in part by the Ah locus; P450 1A2 is expressed essentially only in liver, but P450s 1A1 and 1B1 are both expressed in many extrahepatic tissues. Twenty-five polycyclic hydrocarbons, many containing acetylenic side chains, were examined as inhibitors of the three enzymes using 7-ethoxyresorufin O-deethylation as the enzyme assay in all cases. Several compounds were inhibitory at low nanomolar concentrations. 1-(1-Propynyl)pyrene and 2-(1-propynyl)phenanthrene nearly completely inhibited P450 1A1 at concentrations at which no P450 1B1 inhibition was observed. 2-Ethynylpyrene and alpha-naphthoflavone (7, 8-benzoflavone) nearly completely inhibited P450 1B1 at concentrations at which no P450 1A1 inhibition was noted. All four of the above compounds also inhibited P450 1A2. Several polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s. Some of the acetylenic compounds examined showed enhanced inhibition following preincubation with the P450s in the presence of cofactors NADPH and O2. However, of seven compounds (five acetylenes) tested with P450 1B1, only two [2-ethynylpyrene and 4-(1-propynyl)biphenyl] showed such evidence for mechanism-based inactivation. We conclude that (i) several polycyclic hydrocarbons and their oxidation products are very inhibitory with respect to human P450s 1A1, 1A2, and 1B1; (ii) of these inhibitors only some are mechanism-based inactivators; and (iii) some of the inhibitors are potentially useful for distinguishing between human P450s 1A1 and 1B1.

CYP102 (Cytochrome P450BM-3) is induced in Bacillus megaterium by barbiturates, peroxisome proliferators, and nonsteroidal anti-inflammatory drugs. We now describe the induction of CYP102 in B. megaterium by 17 beta-estradiol and by 4-sec-butylphenol. These estrogens interact with the repressor protein Bm3R1, causing it to dissociate with the operator of the CYP102 gene and allowing transcription to occur. We have developed a stable transfection of a construct into B. megaterium of a truncated CYP102 gene coupled with the luciferase gene in a promoterless plasmid and have used this construct to test the induction of CYP102 by these estrogens. Estradiol demonstrated a dose-dependent induction of CYP102 which saturated at a 2-fold increase at 150 microM 4 hr post-addition. 4-sec-Butylphenol produced a dose-dependent and time-dependent induction up to 300 microM and 6 hr post-induction.

The time-dependent loss of the 7-ethoxy-4-trifluoromethylcoumarin (EFC) O-deethylase activity of rat P450 2B1, rabbit P450 2B4, or dog P450 2B11 by 1-ethynylnaphthalene (1EN), 2-ethynylnaphthalene (2EN), 2-(1-propynyl)naphthalene (2PN), 1-ethynylanthracene (1EA), 2-ethynylanthracene, 2-ethynylphenanthrene, 3-ethynylphenanthrene, 9-ethynylphenanthrene (9EPh), 9-(1-propynyl)phenanthrene (9PPh), 4-ethynylpyrene (4EP), and 4-(1-propynyl)biphenyl (4PbP) was investigated. The rate constants for inactivation by the arylalkynes in descending order of effectiveness for the top five compounds were 9EPh>9PPh>1EN, 2EN, 2PN for 2B1, 9EPh>2EN>4EP>1EN, 1EA for 2B4, and 9EPh>1EA>4EP, 9PPh>2EN for 2B11. The size and the shape of the aromatic ring system and the placement of the alkyne functional group were important for inactivation. The most effective inactivator with all the isozymes was 9EPh. This compound also inactivated the EFC activity in microsomes from human lymphoblastoid cells expressing human P450 2B6. The specificity of 9EPh for the inhibition or inactivation of different P450 activities in microsomes from rats treated with various inducing agents was determined by measuring lidocaine, testosterone, p-nitrophenol, or erythromycin metabolism. The greatest effect was observed with the 2B-specific products from lidocaine and testosterone, whereas no effect was seen with p-nitrophenol or erythromycin. When the covalent binding of [3H]2EN to microsomal protein was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, a radiolabeled protein band that corresponds to 2B1 was observed in the lanes containing microsomes from rats treated with phenobarbital and, to a lesser extent, pyridine and isosafrole after incubation with NADPH. When these microsomes were incubated with [3H]9EPh or [3H]1EP, two NADPH-dependent bands were radiolabeled. One corresponded to 2B1/2 and the other to a protein of approximately 59 kDa, which was observed in the lanes from phenobarbital-treated male and female rats and pyridine-treated male rats. No radiolabeled bands were observed with [3H,14C]4PbP with any of the microsomes.

The mechanism of inactivation of the major phenobarbital-inducible cytochrome P450 of rat liver, P450 2B1, by 9-ethynylphenanthrene (9EPh) has been investigated. Matrix-assisted laser desorption ionization-mass spectrometry analysis of the cyanogen bromide-generated peptides from 9EPh-inactivated P450 2B1 confirmed the addition of a phenanthrylacetyl group to the peptide corresponding to residues 290 to 314. When this peptide was further digested with pepsin, the site of attachment could be assigned to one of the amino acids in the peptide Phe297 to Leu307 [Roberts, E. S., Hopkins, N. E., Zaluzec, E. J., Gage, D. A., Alworth, W. L., and Hollenberg, P. F. (1995) Arch. Biochem. Biophys. 323, 000-000]. The inactivation by 9EPh resulted in a 90-95% loss in the NADPH-supported deethylation of 7-ethoxy-4-trifluoromethylcoumarin (EFC), but had no effect on the iodosobenzene- or cumene hydroperoxide-supported metabolism of EFC. The loss of NADPH-supported activity was not affected by the addition of cytochrome b5 or the presence of excess levels of reductase. The magnitude of the Type 1 spectral change upon the addition of benzphetamine was decreased with the 9EPh-modified protein. There was no decrease in the ability of modified 2B1 to form the steady-state level of the CO-reduced complex either enzymatically with NADPH and reductase or chemically with sodium dithionite, but the rate of reduction by reductase under anaerobic conditions was 57% that of native protein in the absence of substrate and 35% that of native protein in the presence of substrate. The 9EPh-modified 2B1 had an overall slower rate of NADPH oxidation, H2O2 formation, and formaldehyde formation during metabolism of benzphetamine compared to native 2B1. The ratio of H2O2 to HCHO was 1.0:1.0 for the native and 1.6:1.0 for the modified protein. The ability of the modified protein to form the steady-state level of the oxygen-iron complex in the presence of cyclohexane was decreased. These results are consistent with the idea that the covalent modification of one of the residues in the peptide Phe297 to Leu307 by the phenanthrylacetyl group impairs the reduction of P450 2B1 by reductase and also causes the uncoupling of NADPH utilization and oxygen consumption from product formation.

The 7-ethoxycoumarin O-deethylase activity of rat cytochrome P450 (P450) 2B1 was inactivated by 9-ethynylphenanthrene (9EPh) in a time- and NADPH-dependent manner, and the loss of activity followed pseudo-first-order kinetics. At 20 degrees C, the extrapolated maximal rate constant of inactivation (kinactivation) was 0.45 min-1 and the inactivator concentration required for half-maximal inactivation (KI) was 138 nM. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and HPLC analysis demonstrated that [2'-3H]-9EPh was irreversibly bound to the protein moiety of P450 2B1 and the stoichiometry of binding was determined to be 0.82 mol of inactivator bound per mole of P450 2B1. A radiolabeled peptide of approximately 3.0 kDa was identified by autoradiography after Tricine SDS-PAGE analysis of the peptides generated from a cyanogen bromide cleavage of [2'-3H]9EPh-inactivated P450 2B1. After HPLC separation of these peptides, the fraction containing the most radioactivity was analyzed by matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) and peaks at m/z 2720.9 and 2939.9 were detected. The lower mass peak represents the molecular ion (MH+) for the peptide Ile290 to Met314 (theoretical 2722.2), while the higher mass peak corresponds to the MH+ of the modified peptide (theoretical 2940.5). The difference in mass (approximately 219) would correspond to the addition of a phenanthrylacetyl group to the peptide. When the fraction containing the modified and unmodified peptides was further digested with pepsin and reanalyzed by MALDI-MS, the site of attachment could be assigned to one of the amino acids contained in the peptide Phe297 to Leu307.

Objective: To determine whether the urinary excretion of 6-hydroxychlorzoxazone is an index of CYP2E1 activity in vivo., Methods: Male volunteers (n = 27; age range, 17 to 36 years) who were abstinent from alcohol were studied. Chlorzoxazone, 500 mg, was given orally and plasma was collected at 31/2, 41/2, 51/2, and 61/2 hours after dosing. Urine was collected for 8 hours. Ten volunteers participated in full kinetic studies to define the absorption phase and plasma area under the concentration-time curve of chlorzoxazone and the urinary kinetics of the 6-hydroxy metabolite. Chlorzoxazone and the 6-hydroxy metabolite were measured by high-performance liquid chromatography. CYP2E1 activity was expressed as a hydroxylation index (HI = mmole oral chlorzoxazone dose/mmole 6-hydroxychlorzoxazone in 8-hour urine)., Results: There was a significant positive correlation between plasma elimination rate constant for chlorzoxazone (Ke) and urinary excretion of the metabolite (n = 27, r = 0.42, p < 0.03) and a significant negative correlation between plasma Ke and HI (n = 27, r = -0.41, p < 0.04). The mean absorption rate constant for chlorzoxazone of 3.11 +/- 4.67 hr-1 was fivefold greater than the plasma Ke of 0.57 +/- 0.17 hr-1 for the full kinetic studies. The formation clearance of the 6-hydroxy metabolite was negative between plasma Ke of the parent compound and disposition rate constant for urinary excretion of the 6-hydroxy metabolite (n = 15, r = 0.85, p < 0.0001)., Conclusions: The urinary excretion of 6-hydroxychlorzoxazone is limited by formation rate and may be useful as an in vivo probe of CYP2E1 activity.

2-Ethynylnaphthalene (2EN) is a mechanism-based inactivator of rat cytochrome P450 (P450) 2B1 with 1.3 mol of adduct bound per mole of P450 inactivated [Roberts, E.S., Hopkins, N.E., Alworth, W.L., & Hollenberg, P.F. (1993) Chem. Res. Toxicol. 6, 470-479]. Further studies have shown that 2EN is also an efficient mechanism-based inactivator of the 7-ethoxycoumarin O-deethylase activity of rabbit P450 2B4 with 0.83 mol of adduct bound per mole of P450. Cleavage of [3H]2EN-inactivated 2B1 with cyanogen bromide, separation of the peptides by HPLC, and further purification of the radiolabeled fraction by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) led to the identification by autoradiography of a radiolabeled peptide (M(r) approximately 3000). Amino acid sequence analysis of the first 12 N-terminal residues revealed the sequence ISLLSLFFAGTE corresponding to positions 290-301 in the protein. When the radiolabeled fraction from the HPLC separation was analyzed by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), peaks at m/z 2722.5 and 2890.6 were detected. The lower mass peak corresponds to the molecular ion (average mass) of the cyanogen bromide peptide Ile290 to Met314 (theoretical 2722.2), while the higher mass peak corresponds to the same peptide with a bound 2-naphthylacetyl group (theoretical 2890.4). When [3H]2EN-inactivated 2B4 was treated with cyanogen bromide, the peptides were separated by HPLC, and the fractions were analyzed by Tricine-SDS-PAGE, two radiolabeled peptides (M(r) = 5000 and 8000) were identified by autoradiography. Amino acid sequence analysis of the first 11 residues revealed identical N-termini with the sequence EKDKSDPSSEF corresponding to positions 273-283.(ABSTRACT TRUNCATED AT 250 WORDS)

The 7-ethoxycoumarin O-deethylase activity of rat liver cytochrome P450 2B1 reconstituted with NADPH-cytochrome P450 reductase and lipid was inactivated by 2-ethynylnaphthalene (2EN) in a time- and NADPH-dependent manner, and the loss of activity followed pseudo-first-order kinetics. The extrapolated KI and kinactivation were 0.08 microM and 0.83 min-1, respectively. The loss of 7-ethoxycoumarin O-deethylation activity displayed a number of characteristics consistent with mechanism-based inactivation, including irreversibility, saturability, protection by an alternate substrate, and the lack of an effect of exogenous nucleophiles on the inactivation. The inactivation was not accompanied by a concomitant loss of spectrally detectable cytochrome P450. HPLC analysis showed that [3H]2EN was irreversibly bound to the protein moiety of cytochrome P450 and the stoichiometry of inactivation was approximately 1.3 mol of 2EN bound per mole of cytochrome P450. Liquid chromatographic and GC-MS analyses of the organic extracts from these incubations showed that the major metabolite was 2-naphthylacetic acid, and a partition ratio of 4-5 mol of acid produced per mole of cytochrome P450 2B1 inactivated was determined. A radiolabeled peptide, approximately 6.5 kDa when analyzed by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), was isolated by HPLC from a tryptic digest of the [3H]2EN-inactivated cytochrome P450 and NADPH-cytochrome P450 reductase. Sequence data were obtained after cyanogen bromide cleavage of this amino-terminally blocked peptide. These results in conjunction with the results from the cleavage of the intact [3H]2EN-inactivated cytochrome P450 by cyanogen bromide and separation of the peptides either by HPLC or by Tricine-SDS-PAGE followed by transfer of the peptides to a poly(vinylidene difluoride) membrane and sequencing of the labeled peptides from both experiments, led to the identification of a 2EN-modified active-site peptide with the sequence ISLLSLFFAGTETSSTTLRYGFLLM. This corresponds to positions 290-314 in cytochrome P450 2B1. Sequence alignments of cytochrome P450 2B1 with cytochrome P450 2B1 with cytochrome P450 101 predict that this region might correspond to helix I of the bacterial protein [Poulos, T.L. (1988) Pharm. Res. 5, 67-75] that contains a highly conserved threonine residue involved in oxygen binding.

2-Ethynylnaphthalene (2EN) had previously been demonstrated to be a mechanism-based inactivator of rat cytochrome P450 (P450) 1A2 [Hammons, G.J., Alworth, W.L., Hopkins, N.E., Guengerich, F. P., & Kadlubar, F. F. (1989) Chem. Res. Toxicol. 2, 367-374]. In this work 2EN was also demonstrated to be a useful inactivator of rabbit P450 1A2 (k(inactivation) 0.094 min-1, K(i) 11 microM) but it did not inactivate human P450 1A2, although the sequences of the three proteins are approximately 80% identical. Rat and rabbit P450 1A2 were modified by incubation with NADPH-P450 reductase, NADPH, and [3H]2EN to levels of 0.35 and 0.47 nmol of adduct (nmol of P450)-1, respectively. In each case only a single tryptic peptide was labeled; recovery of labeled peptides was low under the acidic HPLC conditions. The rabbit P450 1A2 peptide FQELMAAVGR (positions 175-184) and the rat P450 1A2 peptide L(S)QQYGDVLQIR (positions 67-78) were identified. 4-Azidobiphenyl (4-N3BP) was developed as a photoaffinity label for P-450 1A2 proteins because of its similarity to 4-aminobiphenyl, a known substrate for the enzymes. 4-N3BP was shown to be photolyzed with 350-nm light and radioactive label could be incorporated into rat P450 1A2. Labeling of the protein was found to be saturable with increasing concentrations of 4-N3BP and up to 0.59 nmol of label could be incorporated (nmol P450 1A2)-1. The substrate 4-aminobiphenyl and the competitive inhibitor 7,8-benzoflavone blocked photolabeling of P450 1A2 with 4-N3BP, and 4-N3BP inhibited N-hydroxylation of 4-aminobiphenyl by P450 1A2 in the usual enzyme assay.(ABSTRACT TRUNCATED AT 250 WORDS)

The inhibition of the P450 1A1 dependent de-ethylation of 7-ethoxyphenoxazone (7EPO) and the P450 2B1 dependent de-pentylation of 7-pentoxyphenoxazone (7PPO) by 1-ethynylnaphthalene (1EN), 2-ethynylnaphthalene (2EN), 1-ethynylanthracene (1EA), 2-ethynylanthracene (2EA), 9-ethynylanthracene (9EA), 2-ethynylphenathrene (2EPh), 3-ethynylphenanthrene (3EPh), 9-ethynylphenanthrene (9EPh), 1-ethynylpyrene (1EP) and 2-ethynylpyrene (2EP) was studied in hepatic microsomal preparations from rats. Although all of the polycyclic aromatic acetylenes studied inhibited the dealkylation of 7EPO or 7PPO, only some of the acetylenes produced a mechanism-based irreversible inactivation (suicide inhibition) of the P450 dependent dealkylation of 7EPO or 7PPO. Of the molecules tested, only 1EP, 1EN, 2EN, 2EPh and 3EPh were effective suicide inhibitors of the P450 1A1 dependent de-ethylation of 7EPO and only 1EN, 2EN, 1EA and 9EPh were effective suicide inhibitors of the P450 2B1 dependent de-pentylation of 7PPO. In addition to the size and shape of the polycyclic aromatic ring system, placement of the carbon--carbon triple bond on the ring system was critical for suicide inhibition. In contrast to 1EP, 2EP was not a mechanism-based inhibitor of P450 1A1; 9EPh, but not 2EPh or 3EPh, was a suicide inhibitor of P450 2B1. None of the aryl acetylenes tested produced heme destruction under assay conditions that produced the suicide inhibition of the P450 dependent 7EPO or 7PPO dealkylation activities. Because a precise orientation of the terminal acetylene is required to produce suicide inhibition without heme destruction, acetylenic suicide inhibitors can potentially be used to differentiate between P450 isozymes and to establish some distinguishing geometric features of the active site of these isozymes.

The effects of 1-ethynylpyrene (EP), 1-vinylpyrene (VP) and 2-ethynlnaphthalene (EN) on the covalent binding of 7,12-dimethylbenz[a]anthracene (DMBA) and of benzo[a]-pyrene (B[a]P) to the epidermal DNA in mouse skin were investigated. When applied topically, 5 min before an initiating dose of 10 nmol DMBA or of 200 nmol B[a]P, EP was an effective inhibitor of the formation of the covalent complexes of these procarcinogenic polycyclic aromatic hydrocarbons (PAHs) with the epidermal DNA. VP, applied under the same conditions, was a significantly less effective inhibitor of the binding of DMBA to DNA and showed even weaker inhibition of the binding of B[a]P. EN was ineffective as an inhibitor of the binding of either DMBA or B[a]P. These results establish that both the pyrene nucleus and the ethynyl substituent of EP contribute to the effective inhibition of the binding of DMBA and B[a]P to the epidermal DNA of mouse skin. No significant changes in the ratios of the anti- to the syndiol epoxide-DNA adducts of DMBA or of B[a]P were produced by doses of EP that produced inhibitions of the binding to DNA. At doses of VP that inhibited covalent binding of both DMBA and B[a]P, no changes in DMBA-DNA adduct distributions were observed but changes in the relative proportions of several B[a]P-DNA adducts were noted. These data are discussed in terms of the potential of aryl acetylenes to act as suicide inhibitors (mechanism-based inactivators) of cytochrome P450-dependent monooxygenase isozymes.

The effects of three aryl acetylenes, 1-ethynylpyrene (EP), 2-ethynylnaphthalene (EN) and 3-ethynylperylene (EPE), upon the metabolism of benzo[a]pyrene (BaP) by microsomes isolated from rat liver were investigated. These aryl acetylenes all inhibited the total metabolism of BaP. Formation of BaP 7,8-dihydrodiol and BaP tetrol products by microsomal preparations from rats that had been pretreated with 3-methylcholanthrene (3MC) were preferentially inhibited. The effects of EP upon the metabolism of BaP 7,8-dihydrodiol by microsomes from rat liver were also studied. This aryl acetylene strongly inhibited the formation of BaP tetrols from BaP 7,8-dihydrodiol by liver microsomes both from untreated rats and from rats pretreated with 3MC, but enhanced the conversion of the BaP dihydrodiol into other metabolites.

Since the N-oxidation of several carcinogenic arylamines has been shown to be catalyzed preferentially by cytochrome P-450IA2 in several species, homologous ethynyl-substituted aromatic hydrocarbons, 2-ethynylnaphthalene, 1-ethynylnaphthalene, and 2-ethynylfluorene, were synthesized and examined as potential mechanism-based inactivators of this monooxygenase. By use of 2-naphthylamine, whose N-oxidation was known to be selectively catalyzed by rat cytochrome P-450ISF-G (P-450IA2), and hepatic microsomes from isosafrole-treated rats, each of these ethynyl derivatives was found to be strongly inhibitory at concentrations of 1 and 10 microM. However, only inhibition by 2-ethynylnaphthalene was significantly enhanced by prior incubation with the microsomal system. The inactivation of 2-naphthylamine N-oxidation was found to be NADPH- and time-dependent and to follow pseudo-first-order kinetics, demonstrating that 2-ethynylnaphthalene is a potent mechanism-based inactivator of the enzymatic activity. The extrapolated kinactivation and KI were 0.23 min-1 and 8 microM, respectively. By use of 2-aminofluorene, whose N-oxidation was known to be catalyzed by both cytochromes P-450ISF-G and P-450 beta NF-B (P-450IA1), and the purified enzymes in a reconstituted system, both 2-ethynylnaphthalene and 1-ethynylnaphthalene were found to be strongly inhibitory. However, 2-ethynylnaphthalene was a more potent inhibitor of the purified P-450ISF-G than of P-450 beta NF-B; and it was also found to be a more potent inhibitor of P-450ISF-G than was 1-ethynylnaphthalene.(ABSTRACT TRUNCATED AT 250 WORDS)