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The final steps in bile acid biosynthesis take place in peroxisomes and involve oxidative cleavage of the side chain of C27-5β-cholestanoic acids leading to the formation of the primary bile acids cholic acid and chenodeoxycholic acid. The enoyl-CoA hydratase and β-hydroxy acyl-CoA dehydrogenase reactions involved in the chain shortening of C27-5β-cholestanoic acids are catalyzed by the recently identified peroxisomal d-bifunctional protein. Deficiencies of d-bifunctional protein lead, among others, to an accumulation of 3α,7α,12α,24-tetrahydroxy-5β-cholest-26-oic acid (varanic acid). The ability to resolve the four C24, C25 diastereomers of varanic acid has, so far, only been carried out on biliary bile acids using p-bromophenacyl derivatives. Here, we describe a sensitive gas chromatography–mass spectrometry (GC/MS) method that enables good separation of the four varanic acid diastereomers by use of 2R-butylester-trimethylsilylether derivatives. This method showed the specific accumulation of (24R,25R)-varanic acid in the serum of a patient with isolated deficiency of the d-3-hydroxy acyl-CoA dehydrogenase part of peroxisomal d-bifunctional protein, whereas this diastereomer was absent in a serum sample from a patient suffering from complete d-bifunctional protein deficiency. In samples from both patients an accumulation of (24S,25S)-varanic acid was observed, most likely due to the action of l-bifunctional protein on Δ24E-THCA-CoA. This GC/MS method is applicable to serum samples, obviating the use of bile fluid, and is a helpful tool in the subclassification of patients with peroxisomal d-bifunctional protein deficiency.—Vreken, P., A. van Rooij, S. Denis, E. G. van Grunsven, D. A. Cuebas, and R. J. A. Wanders. Sensitive analysis of serum 3α,7α,12α,24-tetrahydroxy-5β-cholestan-26-oic acid diastereomers using gas chromatography–mass spectrometry and its application in peroxisomal d-bifunctional protein deficiency. J. Lipid Res. 1998. 39: 2452–2458.

Rat liver peroxisomes contain two multifunctional enzymes: (1) perMFE-1 [2-enoyl-CoA hydratase 1/Delta3,Delta2-enoyl-CoA isomerase/(S)-3-hydroxyacyl-CoA dehydrogenase] and (2) perMFE-2 [2-enoyl-CoA hydratase 2/(R)-3-hydroxyacyl-CoA dehydrogenase]. To investigate the role of the hydratase activity of perMFE-2 in beta-oxidation, a truncated version of perMFE-2 was expressed in Escherichia coli as a recombinant protein. The protein catalyses the hydration of straight-chain (2E)-enoyl-CoAs to (3R)-hydroxyacyl-CoAs, but it is devoid of hydratase 1 [(2E)-enoyl-CoA to (3S)-hydroxyacyl-CoA] and (3R)-hydroxyacyl-CoA dehydrogenase activities. The purified enzyme (46 kDa hydratase 2) can be stored as an active enzyme for at least half a year. The recombinant enzyme hydrates (24E)-3alpha,7alpha,12alpha-trihydroxy- 5beta-cholest-24-enoyl-CoA to (24R,25R)-3alpha,7alpha,12alpha, 24-tetrahydroxy-5beta-cholestanoyl-CoA, which has previously been characterized as a physiological intermediate in bile acid synthesis. The stereochemistry of the products indicates that the hydration reaction catalysed by the enzyme proceeds via a syn mechanism. A monofunctional 2-enoyl-CoA hydratase 2 has not been observed as a wild-type protein. The recombinant 46 kDa hydratase 2 described here survives in a purified form under storage, thus being the first protein of this type amenable to application as a tool in metabolic studies.

The ability of the bifunctional 2-enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase from rat liver peroxisomes to metabolize (24E)-3alpha, 7alpha, 12alpha-trihydroxy-5beta-cholest-24-enoyl-CoA, a presumed intermediate during the beta-oxidative degradation of the steroid side chain in the formation of cholic acid, was investigated. The bifunctional enzyme efficiently hydrated the above compound specifically to (24S,25S)-3alpha, 7alpha, 12alpha, 24-tetrahydroxy-5beta-cholestanoyl-CoA, but the dehydrogenase component of the enzyme was virtually inactive toward this product. In contrast, the bifunctional enzyme efficiently catalyzed the dehydrogenation of the (24S,25R) diastereomer of the above hydroxy intermediate to two products whose uv absorbance and chemical properties were consistent with those of alpha-methyl-beta-ketoacyl-CoAs. These results suggest that the bifunctional enzyme is not sufficient for the formation of a 24-keto intermediate in bile acid biosynthesis.

Sodium phenylbutyrate is used for treating urea cycle disorders, providing an alternative for ammonia excretion. Following conversion to its CoA ester, phenylbutyryl-CoA is postulated to undergo one round of β-oxidation to phenylacetyl-CoA, the active metabolite. Molecular modeling suggests that medium chain acyl-CoA dehydrogenase (MCAD; EC 1.3.99.3), a key enzyme in straight chain fatty acid β-oxidation, could utilize phenylbutyryl-CoA as substrate. Moreover, phenylpropionyl-CoA has been shown to be a substrate for MCAD and its intermediates accumulate in patients with MCAD deficiency. We have examined the involvement of MCAD and other acyl-CoA dehydrogenases (ACADs) in the metabolism of phenylbutyryl-CoA. Anaerobic titration of purified recombinant human MCAD with phenylbutyryl-CoA caused changes in the MCAD spectrum that are similar to those induced by octanoyl-CoA, its bona fide substrate, and unique to the development of the charge transfer ternary complex. The calculated apparent dissociation constant (K(D app)) for these substrates was 2.16 μM and 0.12 μM, respectively. The MCAD reductive and oxidative half reactions were monitored using the electron transfer flavoprotein (ETF) fluorescence reduction assay. The catalytic efficiency and the K(m) for phenylbutyryl-CoA were 0.2 mM 34(-1)·sec(-1) and 5.3 μM compared to 4.0 mM(-1)·sec(-1) and 2.8 μM for octanoyl-CoA. Extracts of wild type and MCAD-deficient lymphoblast cells were tested for the ability to reduce ETF using phenylbutyryl-CoA as substrate. While ETF reduction activity was detected in extracts of wild type cells, it was undetectable in extracts of cells deficient in MCAD. The results are consistent with MCAD playing a key role in phenylbutyrate metabolism.

A major deficiency in our understanding of primary biliary cirrhosis (PBC) is the identification of mechanisms that lead to the selective destruction of small intrahepatic bile ducts. PBC is characterized by a multi-lineage T and B cell response against the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2) (1, 2), which is contained in the mitochondria of all nucleated cells. However, only the epithelial cells of small bile ducts and, to a lesser extent, salivary glands are targeted in this autoimmune disease. PBC can also reoccur following liver transplantation, even in the absence of MHC matching, suggesting that there is no restricted phenotype of the target cells and that bile ducts from any host can be destroyed (3). We have recently shown that following apoptosis, human intrahepatic biliary epithelial cells (HiBEC), but not other epithelial cells, translocate immunologically intact PDC-E2 into the apoptotic body (AB), forming an apotope. This could explain the biliary selectivity of autoimmune damage in PBC (4, 5). While our previous study focused only on PDC-E2, it has been reported that 23% and 57% patients with PBC also produce autoantibodies against two other 2-oxo acid dehydrogenase enzymes, the E2 subunit of the oxo-glutarate dehydrogenase complex (OGDC-E2) and the E2 subunit of the branched chain 2-oxo acid dehydrogenase complex (BCOADC-E2), respectively (1, 6-9). Furthermore, AMA- PBC patients do not have autoantibodies against PDC-E2, OGDC-E2 or BCOADC-E2 but often have autoantibodies to nuclear autoantigens (8, 10-13). This raises the possibility that an immune response against other proteins in ABs of HiBEC could also cause selective biliary damage in PBC. We therefore determined whether OGDC-E2 or BCOADC-E2, as well as other potential mitochondrial autoantigens or candidate nuclear autoantigens are also immunologically intact in the ABs of HiBEC. We report that PDC-E2, OGDC-E2 and BCOADC-E2 are all translocated immunologically intact to the ABs of HiBEC. Furthermore, another mitochondrial enzyme, 2, 4-dienoyl CoA reductase 1 (DECR1), is also found intact within the ABs of HiBEC and a small number of patients with PBC have serum autoantibodies against this protein.

The mitochondrial metabolism of 5-enoyl-CoAs, which are formed during the beta-oxidation of unsaturated fatty acids with double bonds extending from odd-numbered carbon atoms, was studied with mitochondrial extracts and purified enzymes of beta-oxidation. Metabolites were identified spectrophotometrically and by high performance liquid chromatography. 5-cis-Octenoyl-CoA, a putative metabolite of linolenic acid, was efficiently dehydrogenated by medium-chain acyl-CoA dehydrogenase (EC 1.3.99.3) to 2-trans-5-cis-octadienoyl-CoA, which was isomerized to 3,5-octadienoyl-CoA either by mitochondrial delta 3,delta 2-enoyl-CoA isomerase (EC 5.3.3.8) or by peroxisomal trifunctional enzyme. Further isomerization of 3,5-octadienoyl-CoA to 2-trans-4-trans-octadienoyl-CoA in the presence of soluble extracts of either rat liver or rat heart mitochondria was observed and attributed to a delta 3,5,delta 2,4-dienoyl-CoA isomerase. Qualitatively similar results were obtained with 2-trans-5-trans-octadienoyl-CoA formed by dehydrogenation of 5-trans-octenoyl-CoA. 2-trans-4-trans-Octadienoyl-CoA was a substrate for NADPH-dependent 2,4-dienoyl-CoA reductase (EC 1.3.1.34). A soluble extract of rat liver mitochondria catalyzed the isomerization of 2-trans-5-cis-octadienoyl-CoA to 2-trans-4-trans-octadienoyl-CoA, which upon addition of NADPH, NAD+, and CoA was chain-shortened to hexanoyl-CoA, butyryl-CoA, and acetyl-CoA. Thus we conclude that odd-numbered double bonds, like even-numbered double bonds, can be reductively removed during the beta-oxidation of polyunsaturated fatty acids.

The mechanism of 3-hydroxyacyl-CoA epimerase (EC 5.1.2.3), which is associated with the multienzyme complex of fatty acid oxidation from Escherichia coli, was studied with D-3-hydroxy-4-trans-decenoyl-CoA as a substrate. The E. coli complex catalyzes the rapid and direct dehydration of D-3-hydroxy-4-trans-decenoyl-CoA to 2-trans,4-trans-decadienoyl-CoA, which is slowly hydrated to L-3-hydroxy-4-trans-decenoyl-CoA. A kinetic analysis of the epimerase and its partial reactions established that epimerization of 3-hydroxyacyl-CoAs occurs solely by a dehydration/hydration mechanism. The results of a substrate competition study with L-3-hydroxy-4-trans-decenoyl-CoA and its D-isomer, together with the conclusion from a sequence analysis of the large subunit of the E. coli complex (Yang, X.-Y., Schulz, H., Elzinga, M., and Yang, S.-Y. (1991) Biochemistry 30, 6788-6795), prompt the suggestion that a single active site is responsible for the dehydration of the D- and L-isomers of 3-hydroxyacyl-CoAs.

We developed a method to determine the urinary concentrations of metabolites in the synthetic pathway for carnitine from N(6)-trimethyllysine and applied this method to determine their excretion in control individuals. In addition, we investigated whether newborns are capable of carnitine synthesis from deuterium-labeled N(6)-trimethyllysine.Urine samples were first derivatized with methyl chloroformate. Subsequently, the analytes were separated by ion-pair, reversed-phase HPLC and detected online by electrospray tandem mass spectrometry. Stable-isotope-labeled reference compounds were used as internal standards.The method quantified all carnitine biosynthesis metabolites except 4-N-trimethylaminobutyraldehyde. Detection limits were 0.05-0.1 micromol/L. The interassay imprecision (CV) for urine samples with added compounds was 6-12%. The intraassay imprecision (CV) was 1-5% (3-10 micromol/L). Recoveries were 94-106% at 10-20 micromol/L and 98-103% at 100-200 micromol/L. The mean (SD) excretions of N(6)-trimethyllysine and 3-hydroxy-N(6)-trimethyllysine were 2.8 (0.8) and 0.45 (0.15) mmol/mol creatinine, respectively. gamma-Butyrobetaine and carnitine excretions were more variable with values of 0.27 (0.21) and 15 (12) mmol/mol creatinine, respectively. After oral administration of deuterium-labeled N(6)-trimethyllysine, all urines of newborns contained deuterium-labeled N(6)-trimethyllysine, 3-hydroxy-N(6)-trimethyllysine, gamma-butyrobetaine, and carnitine.HPLC in combination with electrospray ionization tandem mass spectrometry allows rapid determination of urinary carnitine biosynthesis metabolites. Newborns can synthesize carnitine from exogenous N(6)-trimethyllysine, albeit at a low rate.

Bile acids are synthesized de novo in the liver from cholesterol and conjugated to glycine or taurine via a complex series of reactions involving multiple organelles. Bile acids secreted into the small intestine are efficiently reabsorbed and reutilized. Activation by thioesterification to CoA is required at two points in bile acid metabolism. First, 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid, the 27-carbon precursor of cholic acid, must be activated to its CoA derivative before side chain cleavage via peroxisomal beta-oxidation. Second, reutilization of cholate and other C24 bile acids requires reactivation prior to re-conjugation. We reported previously that homolog 2 of very long-chain acyl-CoA synthetase (VLCS) can activate cholate (Steinberg, S. J., Mihalik, S. J., Kim, D. G., Cuebas, D. A., and Watkins, P. A. (2000) J. Biol. Chem. 275, 15605-15608). We now show that this enzyme also activates chenodeoxycholate, the secondary bile acids deoxycholate and lithocholate, and 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid. In contrast, VLCS activated 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate, but did not utilize any of the C24 bile acids as substrates. We hypothesize that the primary function of homolog 2 is in the reactivation and recycling of C24 bile acids, whereas VLCS participates in the de novo synthesis pathway. Results of in situ hybridization, topographic orientation, and inhibition studies are consistent with the proposed roles of these enzymes in bile acid metabolism.

Background: We developed a method to determine the urinary concentrations of metabolites in the synthetic pathway for carnitine from N6-trimethyllysine and applied this method to determine their excretion in control individuals. In addition, we investigated whether newborns are capable of carnitine synthesis from deuterium-labeled N6-trimethyllysine.Methods: Urine samples were first derivatized with methyl chloroformate. Subsequently, the analytes were separated by ion-pair, reversed-phase HPLC and detected online by electrospray tandem mass spectrometry. Stable-isotope-labeled reference compounds were used as internal standards.Results: The method quantified all carnitine biosynthesis metabolites except 4-N-trimethylaminobutyraldehyde. Detection limits were 0.05–0.1 μmol/L. The interassay imprecision (CV) for urine samples with added compounds was 6–12%. The intraassay imprecision (CV) was 1–5% (3–10 μmol/L). Recoveries were 94–106% at 10–20 μmol/L and 98–103% at 100–200 μmol/L. The mean (SD) excretions of N6-trimethyllysine and 3-hydroxy-N6-trimethyllysine were 2.8 (0.8) and 0.45 (0.15) mmol/mol creatinine, respectively. γ-Butyrobetaine and carnitine excretions were more variable with values of 0.27 (0.21) and 15 (12) mmol/mol creatinine, respectively. After oral administration of deuterium-labeled N6-trimethyllysine, all urines of newborns contained deuterium-labeled N6-trimethyllysine, 3-hydroxy-N6-trimethyllysine, γ-butyrobetaine, and carnitine.Conclusions: HPLC in combination with electrospray ionization tandem mass spectrometry allows rapid determination of urinary carnitine biosynthesis metabolites. Newborns can synthesize carnitine from exogenous N6-trimethyllysine, albeit at a low rate.

Unconjugated bile acids must be activated to their CoA thioesters before conjugation to taurine or glycine can occur. A human homolog of very long-chain acyl-CoA synthetase, hVLCS-H2, has two requisite properties of a bile acid:CoA ligase, liver specificity and an endoplasmic reticulum subcellular localization. We investigated the ability of this enzyme to activate the primary bile acid, cholic acid, to its CoA derivative. When expressed in COS-1 cells, hVLCS-H2 exhibited cholate:CoA ligase (choloyl-CoA synthetase) activity with both non-isotopic and radioactive assays. Other long- and very long-chain acyl-CoA synthetases were incapable of activating cholate. Endogenous choloyl-CoA synthetase activity was also detected in liver-derived HepG2 cells but not in kidney-derived COS-1 cells. Our results are consistent with a role for hVLCS-H2 in the re-activation and re-conjugation of bile acids entering liver from the enterohepatic circulation rather than in de novo bile acid synthesis.

Peroxisomes play an essential role in a number of different metabolic pathways, including the β-oxidation of a distinct set of fatty acids and fatty acid derivatives. The importance of the peroxisomal β-oxidation system in humans is made apparent by the existence of a group of inherited diseases in which peroxisomal β-oxidation is impaired. This includes X-linked adrenoleukodystrophy and other disorders with a defined defect. On the other hand, many patients have been described with a defect in peroxisomal β-oxidation of unknown etiology. Resolution of the defects in these patients requires the elucidation of the enzymatic organization of the peroxisomal β-oxidation system. Importantly, a new peroxisomal β-oxidation enzyme was recently described called d -bifunctional protein with enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activity primarily reacting with α-methyl fatty acids like pristanic acid and di- and trihydroxycholestanoic acid. In this patient we describe the first case of d -bifunctional protein deficiency as resolved by enzyme activity measurements and mutation analysis. The mutation found (Gly 16 Ser) is in the dehydrogenase coding part of the gene in an important loop of the Rossman fold forming the NAD + -binding site. The results show that the newly identified d -bifunctional protein plays an essential role in the peroxisomal β-oxidation pathway that cannot be compensated for by the l -specific bifunctional protein.

The effects of several short-chain mercapto acids on the rate of respiration supported by either palmitoylcarnitine, octanoate, or pyruvate was studied with coupled rat heart mitochondria. 3-Mercaptopropionic acid was found to be a potent inhibitor of respiration sustained by palmitoylcarnitine or octanoate, whereas under identical conditions respiration with pyruvate as a substrate was unaffected. 2-Mercaptoacetic acid also inhibits palmitoylcarnitine-supported respiration, but only at much higher concentrations of the inhibitor. 2-Mercaptopropionic acid has virtually no effect. Incubation of mitochondria with 3-mercaptopropionic acid did not cause the irreversible inactivation of any beta-oxidation enzyme. Since 3-mercaptopropionic acid did not inhibit beta-oxidation in uncoupled mitochondria, it appears that this compound must first be metabolized in an energy-dependent reaction before it becomes inhibitory. 3-Mercaptopropionyl-CoA and three of its S-acyl derivatives, all of which are likely mitochondrial metabolites of 3-mercaptopropionic acid, were tested for their capacity to inhibit the individual enzymes of beta-oxidation. 3-Mercaptopropionyl-CoA inhibits only acyl-CoA dehydrogenase, whereas S-myristoyl-3-mercaptopropionyl-CoA inhibits reversibly several beta-oxidation enzymes. All observations together lead us to suggest that the inhibition of beta-oxidation by 3-mercaptopropionic acid in coupled rat heart mitochondria is most likely a consequence of the reversible inhibition of acyl-CoA dehydrogenase by long-chain S-acyl-3-mercaptopropionyl-CoA thioesters and possibly by 3-mercaptopropionyl-CoA.

The activities of cis -Δ 3 - trans -Δ 2 -enoyl-CoA isomerase (EC 5.3.3.8), 3-hydroxyacyl-CoA epimerase (EC 5.1.2.3), and 2,4-dienoyl-CoA reductase, all of which have been proposed to function as auxiliary enzymes in the β-oxidation of polyunsaturated fatty acids, have been determined in mitochondria from rat heart and rat liver. In heart mitochondria the activity of 3-hydroxyacyl-CoA epimerase was lower, whereas that of 2,4-dineoyl-CoA reductase was higher than the observed rate of linoleate degration. This observation suggests that 2,4-dienoyl-CoA reductase and not 3-hydroxyacyl-CoA epimerase functions as an auxiliary enzyme in the metabolism of polyunsaturated fatty acids in heart. A modified pathway of linoleate degradation is presented.

Chromatography of a rat liver extract on DEAE-cellulose resulted in the near total loss of 3-hydroxyacyl-CoA epimerase activity. The activity was regained either when fractions were recombined or when purified crotonase was added to the early column fractions. A new enoyl-CoA hydratase present in these early fractions catalyzes the conversion of D-3-hydroxyacyl-CoA to 2- trans -enoyl-CoA which can be hydrated by crotonase or the peroxisomal bifunctional enzyme to L-3-hydroxyacyl-CoA. Thus, the 3-hydroxyacyl-CoA epimerase activity is due to the combined actions of two enoyl-CoA hydratases with opposite stereospecificities.

The metabolism of 3-mercaptopropionic acid in mitochondria was studied by use of purified mitochondrial enzymes and rat heart mitochondria. Metabolites of 3-mercaptopropionic acid were separated by high performance liquid chromatography and identified by comparing them with chemically synthesized derivatives of 3-mercaptopropionic acid. The initial step in the metabolism of 3-mercaptopropionic acid is its conversion to a CoA thioester, most likely catalyzed by medium-chain acyl-CoA synthetase. The resulting 3-mercaptopropionyl-CoA is a poor substrate of acyl-CoA dehydrogenase but substitutes effectively for CoASH in reactions catalyzed by 3-ketoacyl-CoA thiolase and acetoacetyl-CoA thiolase. S-Acyl-3-mercaptopropionyl-CoA thioesters formed in the thiolase-catalyzed reactions are not at all or only poorly acted upon by acyl-CoA dehydrogenases. However, they are hydrolyzed by thioesterase(s) to CoASH and S-acyl-3-mercaptopropionic acid. The hydrolysis of S-acyl-3-mercaptopropionyl-CoA thioesters proceeds more rapidly than the hydrolysis of fatty acyl-CoA thioesters of comparable chain lengths. Free CoASH is also regenerated from S-acetyl-3-mercaptopropionyl-CoA and more rapidly from 3-mercaptopropionyl-CoA as a result of their reactions with carnitine catalyzed by carnitine acetyltransferase. These findings lead to the suggestion that the major mitochondrial CoA-containing metabolites of 3-mercaptopropionic acid are S-acyl-3-mercaptopropionyl-CoA thioesters.

The beta-oxidation of 2-trans,4-cis-decadienoyl-CoA, an assumed metabolite of linoleic acid, by purified enzymes from mitochondria, peroxisomes, and Escherichia coli was studied. 2-trans,4-cis-Decadienoyl-CoA is an extremely poor substrate of the beta-oxidation system reconstituted from mitochondrial enzymes. The results of a kinetic evaluation lead to the conclusion that in mitochondria 2-trans,4-cis-decadienoyl-CoA is not directly beta-oxidized, but instead is reduced by NADPH-dependent 2,4-dienoyl-CoA reductase prior to its beta-oxidation. Hence, the mitochondrial beta-oxidation of 2-trans,4-cis-decadienoyl-CoA does not require 3-hydroxyacyl-CoA epimerase, a conclusion which agrees with the finding that 3-hydroxyacyl-CoA epimerase is absent from mitochondria (Chu, C.-H., and Schulz, H. (1985) FEBS Lett. 185, 129-134). However, 2-trans,4-cis-decadienoyl-CoA can be slowly oxidized by the bifunctional beta-oxidation enzyme from rat liver peroxisomes, as well as by the fatty acid oxidation complex from E. coli. The observed rates of 2-trans,4-cis-decadienoyl-CoA degradation by these two multi-functional proteins were significantly higher than the values calculated according to steady-state velocity equations derived for coupled enzyme reactions. This is attributed to the direct transfer of L-3-hydroxy-4-cis-decenoyl-CoA from the active site of enoyl-CoA hydratase to that of 3-hydroxyacyl-CoA dehydrogenase on the same protein molecule. All observations together lead to the suggestion that the chain shortening of 2-trans,4-cis-decadienoyl-CoA in peroxisomes and in E. coli occurs simultaneously by two different pathways. The major pathway involves the NADPH-dependent 2,4-dienoyl-CoA reductase, whereas 3-hydroxyacyl-CoA epimerase functions in the metabolism of D-3-hydroxyoctanoyl-CoA which is formed via the minor pathway.

Rates of the NAD+-dependent oxidation of 2-trans,4-trans-decadienoyl-CoA, a metabolite of trans-omega-6-unsaturated fatty acids, catalyzed by the mitochondrial enoyl-CoA hydratase plus 3-hydroxyacyl-CoA dehydrogenase and by the corresponding enzymes from peroxisomes, as well as Escherichia coli, were compared. The study of the mitochondrial system revealed that the conventional kinetic theory of coupled enzyme reactions cannot be applied to systems in which the primary reaction has a small equilibrium constant, and/or the concentration of coupling enzyme is higher than 0.01 Km for the intermediate and higher than the steady-state concentration of the intermediate. In contrast to the results obtained with the mitochondrial beta-oxidation system of unlinked enzymes, the steady-state velocities of 2-trans,4-trans-decadienoyl-CoA degradation catalyzed by either the peroxisomal bifunctional enzyme or by the E. coli fatty acid oxidation complex were found to be equal to the activities of enoyl-CoA hydratase even though the concentration of coupling enzyme was equal to that of the primary enzyme, and the quotient of Vmax/Km for the dehydration of 3-hydroxy-4-trans-decenoyl-CoA is much larger than the Vmax/Km for its dehydrogenation. The extraordinarily high efficiencies of these two multifunctional proteins in catalyzing the degradation of 2-trans,4-trans-decadienoyl-CoA is best explained by the direct transfer of the 3-hydroxy-4-trans-decenoyl-CoA intermediate from the active site of enoyl-CoA hydratase to that of 3-hydroxyacyl-CoA dehydrogenase. The discovery of an intermediate channeling mechanism on the peroxisomal bifunctional enzyme explains on the molecular level why the peroxisomal beta-oxidation system is well suited for the degradation of trans-fatty acids.

The enzymatic degradation of 2,4-decadienoyl-CoA, an assumed metabolite of linoleic acid, was studied in vitro. 2-trans,4-trans-Decadienoyl-CoA, a metabolite of unsaturated fatty acids with trans double bonds, was readily degraded by a beta-oxidation system reconstituted from purified enzymes, as well as by rat heart mitochondria. In contrast, a mixture of 2-cis,4-cis-decadienoyl-CoA and 2-trans,4-cis-decadienoyl-CoA, the latter of which is assumed to be a metabolite of linoleic acid, was acted upon neither by the reconstituted beta-oxidation system nor by the beta-oxidation system present in rat heart mitochondria. However, all three 2,4-decadienoyl-CoA isomers were rapidly reduced by the NADPH-dependent 2,4-dienoyl-CoA reductase of rat heart mitochondria. Based on these observations, we conclude that 2-trans,4-cis-decadienoyl-CoA, a metabolite of linoleic acid, is not directly degraded via the beta-oxidation cycle, but instead is reduced by the NADPH-dependent 2,4-dienoyl-CoA reductase to 3-decenoyl-CoA which is further degraded via the beta-oxidation pathway.