Occurrence of a long-chain delta 3,delta 2-enoyl-CoA isomerase in rat liver

Characterization of an acyl-coenzyme A binding protein predominantly expressed in human primitive progenitor cells

Human acyl-coenzyme A binding domain-containing member 6 (ACBD6) is a modular protein that carries an acyl-CoA binding domain at its N terminus and two ankyrin motifs at its C terminus. ACBD6 binds long-chain acyl-CoAs with a strong preference for unsaturated, C18:1-CoA and C20:4-CoA, over saturated, C16:0-CoA, acyl species. Deletion of the C terminus, which is not conserved among the members of this family, did not affect the binding capacity or the substrate specificity of the protein. ACBD6 is not a ubiquitous protein, and its expression is restricted to tissues and progenitor cells with functions in blood and vessel development. ACBD6 was detected in bone marrow, spleen, placenta, cord blood, circulating CD34+ progenitors, and embryonic-like stem cells derived from placenta. In placenta, the protein was only detected in CD34+ progenitor cells present in blood and in CD31+ endothelial cells surrounding the blood vessels. These cells were also positive for the marker CD133, and they probably constitute hemangiogenic stem cells, precursors of both blood and vessels. We propose that human ACBD6 represents a cellular marker for primitive progenitor cells with functions in hematopoiesis and vascular endothelium development.

Intrinsic isomerase activity of medium-chain acyl-CoA dehydrogenase

Mitochondrial medium-chain acyl-CoA dehydrogenase is a key enzyme for the beta oxidation of fatty acids, and the deficiency of this enzyme in patients has been previously reported. We found that the enzyme has intrinsic isomerase activity, which was confirmed using incubation followed with HPLC analysis. The isomerase activity of the enzyme was thoroughly characterized through studies of kinetics, substrate specificity, pH dependence, and enzyme inhibition. E376 mutants were constructed, and mutant enzymes were purified and characterized. It was shown that E376 is the catalytic residue for both dehydrogenase and isomerase activities of the enzyme. The isomerase activity of medium-chain acyl-CoA dehydrogenase is probably a spontaneous process driven by thermodynamic equilibrium with the formation of a conjugated structure after deprotonation of substrate alpha proton. The energy level of the transition state may be lowered by a stable dienolate intermediate, which gains further stabilization via charge transfer with the electron-deficient FAD cofactor of the enzyme. This raises the question as to whether the dehydrogenase might function as an isomerase in vivo in conditions in which the activity of the isomerase is decreased.

The 1.3 A crystal structure of human mitochondrial Delta3-Delta2-enoyl-CoA isomerase shows a novel mode of binding for the fatty acyl group

The crystal structure of Delta3-Delta2-enoyl-CoA isomerase from human mitochondria (hmEci), complexed with the substrate analogue octanoyl-CoA, has been refined at 1.3 A resolution. This enzyme takes part in the beta-oxidation of unsaturated fatty acids by converting both cis-3 and trans-3-enoyl-CoA esters (with variable length of the acyl group) to trans-2-enoyl-CoA. hmEci belongs to the hydratase/isomerase (crotonase) superfamily. Most of the enzymes belonging to this superfamily are hexamers, but hmEci is shown to be a trimer. The mode of binding of the ligand, octanoyl-CoA, shows that the omega-end of the acyl group binds in a hydrophobic tunnel formed by residues of the loop preceding helix H4 as well as by side-chains of the kinked helix H9. From the structure of the complex it can be seen that Glu136 is the only catalytic residue. The importance of Glu136 for catalysis is confirmed by mutagenesis studies. A cavity analysis shows the presence of two large, adjacent empty hydrophobic cavities near the active site, which are shaped by side-chains of helices H1, H2, H3 and H4. The structure comparison of hmEci with structures of other superfamily members, in particular of rat mitochondrial hydratase (crotonase) and yeast peroxisomal enoyl-CoA isomerase, highlights the variable mode of binding of the fatty acid moiety in this superfamily.

The biochemistry of peroxisomal beta-oxidation in the yeast Saccharomyces cerevisiae

Peroxisomal fatty acid degradation in the yeast Saccharomyces cerevisiae requires an array of beta-oxidation enzyme activities as well as a set of auxiliary activities to provide the beta-oxidation machinery with the proper substrates. The corresponding classical and auxiliary enzymes of beta-oxidation have been completely characterized, many at the structural level with the identification of catalytic residues. Import of fatty acids from the growth medium involves passive diffusion in combination with an active, protein-mediated component that includes acyl-CoA ligases, illustrating the intimate linkage between fatty acid import and activation. The main factors involved in protein import into peroxisomes are also known, but only one peroxisomal metabolite transporter has been characterized in detail, Ant1p, which exchanges intraperoxisomal AMP with cytosolic ATP. The other known transporter is Pxa1p-Pxa2p, which bears similarity to the human adrenoleukodystrophy protein ALDP. The major players in the regulation of fatty acid-induced gene expression are Pip2p and Oaf1p, which unite to form a transcription factor that binds to oleate response elements in the promoter regions of genes encoding peroxisomal proteins. Adr1p, a transcription factor, binding upstream activating sequence 1, also regulates key genes involved in beta-oxidation. The development of new, postgenomic-era tools allows for the characterization of the entire transcriptome involved in beta-oxidation and will facilitate the identification of novel proteins as well as the characterization of protein families involved in this process.

Functional characterization of Delta3,Delta2-enoyl-CoA isomerases from rat liver

The degradation of unsaturated fatty acids by beta-oxidation involves Delta(3),Delta(2)-enoyl-CoA isomerases (enoyl-CoA isomerases) that catalyze 3-cis --> 2-trans and 3-trans --> 2-trans isomerizations of enoyl-CoAs and the 2,5 --> 3,5 isomerization of dienoyl-CoAs. An analysis of rat liver enoyl-CoA isomerases revealed the presence of a monofunctional enoyl-CoA isomerase (ECI) in addition to mitochondrial enoyl-CoA isomerase (MECI) in mitochondria, whereas peroxisomes contain ECI and multifunctional enzyme 1 (MFE1). Thus ECI, which previously had been described as peroxisomal enoyl-CoA isomerase, was found to be present in both peroxisomes and mitochondria. This enzyme seems to be identical with mitochondrial long-chain enoyl-CoA isomerase (Kilponen, J.M., Palosaari, P.M., and Hiltunen, J.K. 1990. Biochem. J. 269, 223-226). All three hepatic enoyl-CoA isomerases have broad chain length specificities but are distinguishable by their preferences for one of the three isomerization reactions. MECI is most active in catalyzing the 3-cis --> 2-trans isomerization; ECI has a preference for the 3-trans --> 2-trans isomerization, and MFE1 is the optimal isomerase for the 2,5 --> 3,5 isomerization. A functional characterization based on substrate specificities and total enoyl-CoA isomerase activities in rat liver leads to the conclusion that the 3-cis --> 2-trans and 2,5 --> 3,5 isomerizations in mitochondria are catalyzed overwhelmingly by MECI, whereas ECI contributes significantly to the 3-trans --> 2-trans isomerization. In peroxisomes, ECI is predicted to be the dominant enzyme for the 3-cis --> 2-trans and 3-trans --> 2-trans isomerizations of long-chain intermediates, whereas MFE1 is the key enzyme in the 2,5 --> 3,5 isomerization.

Disorders of mitochondrial fatty acyl-CoA beta-oxidation

In recent years tremendous progress has been made with respect to the enzymology of the mitochondrial fatty acid beta-oxidation machinery and defects therein. Firstly, a number of new mitochondrial beta-oxidation enzymes have been identified, including very-long-chain acyl-CoA dehydrogenase (VLCAD) and mitochondrial trifunctional protein (MTP). Secondly, the introduction of tandem MS for the analysis of plasma acylcarnitines has greatly facilitated the identification of patients with a defect in fatty acid oxidation (FAO). These two developments explain why the number of defined FAO disorders has increased dramatically, making FAO disorders the most rapidly growing group of inborn errors of metabolism. In this review we describe the current state of knowledge of the enzymes involved in the mitochondrial oxidation of straight-chain, branched-chain and (poly)unsaturated fatty acyl-CoAs as well as disorders of fatty acid oxidation. The laboratory diagnosis of these disorders is described, with particular emphasis on the methods used to identify the underlying enzyme defect and the molecular mutations. In addition, a simple flowchart is presented as a guide to the identification of mitochondrial FAO-disorders. Finally, treatment strategies are discussed briefly.

Molecular characterization of Saccharomyces cerevisiae Delta3, Delta2-enoyl-CoA isomerase

We report here the identification of the Saccharomyces cerevisiae peroxisomal Delta3,Delta2-enoyl-CoA isomerase, an enzyme that is essential for the beta-oxidation of unsaturated fatty acids. The yeast gene YLR284C was identified in an in silico screen for genes that contain an oleate response element, a transcription factor-binding site common to most fatty acid-induced genes. Growth on oleic acid resulted in a significant increase in YLR284C mRNA, demonstrating that it is indeed an oleate-induced gene. The deduced product of YLR284C contains a type 1 peroxisomal targeting signal-like sequence at its C terminus and localizes to the peroxisome in a PEX8-dependent manner. Removal of YLR284C from the S. cerevisiae genome eliminated growth on oleic acid, but had no effect on peroxisome biogenesis, indicating a role for YLR284C in fatty acid metabolism. Cells lacking YLR284C had no detectable Delta3,Delta2-enoyl-CoA isomerase activity, and a bacterially expressed form of this protein catalyzed the isomerization of 3-cis-octenoyl-CoA to 2-trans-octenoyl-CoA with a specific activity of 16 units/mg. We conclude that YLR284C encodes the yeast peroxisomal Delta3,Delta2-enoyl-CoA isomerase and propose a new name, ECI1, to reflect its enoyl-CoA isomerase activity.

Peroxisomal Delta3-cis-Delta2-trans-enoyl-CoA isomerase encoded by ECI1 is required for growth of the yeast Saccharomyces cerevisiae on unsaturated fatty acids

We have identified the Saccharomyces cerevisiae gene ECI1 encoding Delta3-cis-Delta2-trans-enoyl-CoA isomerase that acts as an auxiliary enzyme in the beta-oxidation of (poly)unsaturated fatty acids. A mutant devoid of Eci1p was unable to grow on media containing unsaturated fatty acids such as oleic acid but was proficient for growth when a saturated fatty acid such as palmitic acid was the sole carbon source. Levels of ECI1 transcript were elevated in cells grown on oleic acid medium due to the presence in the ECI1 promoter of an oleate response element that bound the transcription factors Pip2p and Oaf1p. Eci1p was heterologously expressed in Escherichia coli and purified to homogeneity. It was found to be a hexameric protein with a subunit of molecular mass 32, 000 Da that converted 3-hexenoyl-CoA to trans-2-hexenoyl-CoA. Eci1p is the only known member of the hydratase/isomerase protein family with isomerase and/or 2-enoyl-CoA hydratase 1 activities that does not contain a conserved glutamate at its active site. Using a green fluorescent protein fusion, Eci1p was shown to be located in peroxisomes of wild-type yeast cells. Rat peroxisomal multifunctional enzyme type I containing Delta3-cis-Delta2-trans-enoyl-CoA isomerase activity was expressed in ECI1-deleted yeast cells, and this restored growth on oleic acid.

Peroxisomal beta-oxidation and polyunsaturated fatty acids

Peroxisomes are capable of oxidizing a variety of substrates including (poly)unsaturated enoyl-CoA esters. The beta-oxidation of unsaturated enoyl-CoA esters in peroxisomes, and also in mitochondria, is not just chain-shortening but also involves the metabolizing of pre-existing carbon-to-carbon double bonds. In addition to the enzymes of the beta-oxidation spiral itself, this metabolism requires the participation of auxiliary enzymes: delta 3, delta 2-enoyl-CoA isomerase; 2,4-dienoyl-CoA reductase; 2-enoyl-CoA hydratase 2 or 3-hydroxyacyl-CoA epimerase; and delta 3,5 delta 2,4-dienoyl-CoA isomerase. Many of these enzymes are present as isoforms, and can be found located in multiple subcellular compartments, for example, peroxisomes, mitochondria or the endoplasmic reticulum, while some of the activities are integral parts of multifunctional enzymes of beta-oxidation systems.

Mammalian mitochondrial beta-oxidation

The enzymic stages of mammalian mitochondrial beta-oxidation were elucidated some 30-40 years ago. However, the discovery of a membrane-associated multifunctional enzyme of beta-oxidation, a membrane-associated acyl-CoA dehydrogenase and characterization of the carnitine palmitoyl transferase system at the protein and at the genetic level has demonstrated that the enzymes of the system itself are incompletely understood. Deficiencies of many of the enzymes have been recognized as important causes of disease. In addition, the study of these disorders has led to a greater understanding of the molecular mechanism of beta-oxidation and the import, processing and assembly of the beta-oxidation enzymes within the mitochondrion. The tissue-specific regulation, intramitochondrial control and supramolecular organization of the pathway is becoming better understood as sensitive analytical and molecular techniques are applied. This review aims to cover enzymological and organizational aspects of mitochondrial beta-oxidation together with the biochemical aspects of inherited disorders of beta-oxidation and the intrinsic control of beta-oxidation.

Purification and mechanism of delta 3,delta 5-t-2,t-4-dienoyl-CoA isomerase from rat liver

A new enzyme, i.e., delta 3,delta 5-t-2,t-4-dienoyl-CoA isomerase, required in the NADPH-dependent metabolic pathway of odd-numbered double bond, unsaturated fatty acids, was isolated and purified to apparent homogeneity from rat liver. In the oxidation of odd-numbered double bond, unsaturated fatty acids, stepwise beta-oxidation leads to cis-5-enoyl-CoA, which is then dehydrogenated and isomerized to delta 3,delta 5-dienoyl-CoA. delta 3,delta 5-t-2,t-4-Dienoyl-CoA isomerase converts delta 3,delta 5-dienoyl-CoA to trans-2,trans-4-dienoyl-CoA, which is a substrate for NADPH-dependent 2,4-dienoyl-CoA reductase. This enzyme was purified through Matrex gel red A, blue Sepharose, DEAE-cellulose, CM-cellulose, hydroxylapatite, and Sepharose CL6B column chromatography of an ammonium sulfate precipitated fraction (30-80%) of rat liver homogenate. A native molecular weight of 200,000 with four subunits of 55,000 each was determined. The isoelectric point was 6.5. This enzyme was located in mitochondria and was inducible by clofibrate treatment. Using delta 3,delta 5-decadienoyl-CoA, delta 3,delta 5-dodecadienoyl-CoA, and delta 3,delta 5-tetradecadienoyl-CoA as substrates, the Vmax ratio was 1:0.5:0.4 and the Km's were 10.9, 5.9, and 1.4 microM, respectively. The specific activity of purified enzyme was 7 units/mg using delta 3,delta 5-decadienoyl-CoA as substrate. The mechanism of isomerization was studied by deuterium labeling. Consistent with the deuterium labeling pattern of the products, the isomerization from trans-2,cis-5-dienoyl-CoA to trans-2,trans-4-dienoyl-CoA was a two-step process through an intermediate delta 3,delta 5-dienoyl-CoA.(ABSTRACT TRUNCATED AT 250 WORDS)

Redox control of beta-oxidation in rat liver mitochondria

Coupled rat liver mitochondria were incubated with [U-14C]hexadecanoate and carnitine which resulted in the formation of acyl-, 2-enoyl- and 3-hydroxyacyl-CoA and carnitine esters. The production of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters was associated with a significant lowering of the NAD+/NADH ratio, in contrast to rat muscle mitochondria [Eaton, S., Bhuiyan, A. K. M. J., Kler, R. S., Turnbull, D. M. & Bartlett, K. (1993) Biochem. J. 289, 161-172], suggesting that control by the respiratory chain is important under normal conditions. When NAD+/NADH ratios were held low by succinate-induced reverse electron flow, 3-enoyl-CoA esters were also detected, probably formed by the action of 3,2-enoyl-CoA isomerase. Measurement of the flux of beta-oxidation at different osmolalities showed that flux was strongly dependent on osmolality changes in the physiological range. Measurement of the CoA and carnitine esters resulting from incubations made at different osmolalities showed that there was an increase in the amounts of the saturated acyl-CoA esters with respect to 2-enoyl-CoA and 3-hydroxyacyl-CoA esters, consistent with control by the electron-transfer flavoprotein-ubiquinone segment [Halestrap, A. P. & Dunlop, J. L. (1986) Biochem. J. 239, 559-565]. This however could not be the only factor operating as indicated by the continued presence of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters at high osmolalities.

The existence of two mitochondrial isoforms of 2,4-dienoyl-CoA reductase in the rat

Isoforms of 2,4-dienoyl-CoA reductase (EC 1.3.1.34), which is the key enzyme in the beta-oxidation of fatty acids with double bonds, have been studied in rat heart and liver. Electrofocusing and adsorption chromatography on hydroxyapatite were used to separate the reductase activity in tissue homogenates into two peaks, one peak in each experiment being identified as the previously purified mitochondrial reductase. The novel activity was partially purified from rat liver by means of ammonium sulphate precipitation, anion-exchange chromatography on DEAE-cellulose (DE-52), hydrophobic chromatography on Phenyl-Sepharose and dye-ligand binding chromatography (Blue Sepharose). Taking into account the contribution of the different reductases to the total activity in rat liver, the overall purification for the novel isoform was 1900-fold. Ultracentrifugation on a sucrose gradient gave an M(r) of 50,000 and size-exclusion chromatography on Superdex 200 an M(r) of 60,000. The antibody against the previously characterised reductase did not cross-react with this novel isoform, but the distribution of the activity peaks in heart and liver tissue, and an electrofocusing experiment with isolated mitochondria, both pointed to a mitochondrial origin. The novel reductase was estimated to account for 80% (50%) of the total reductase activity in rat heart (liver) homogenate measured with 2,4-hexadienoyl-CoA. The present results, together with those previously published, suggest that mammals have at least three reductase isoforms: two in mitochondria and a third one in peroxisomes, but the peroxisomal activity has not been characterised so far.

Immunocytochemical localization of delta 3, delta 2-enoyl-CoA isomerase and NADPH-dependent-2,4-dienoyl-CoA reductase in rat kidney

Localization of delta 3, delta 2-enoyl-CoA isomerase (ECI) and NADPH-dependent-2,4-dienoyl-CoA reductase (DCR) in the rat kidney was investigated by immunocytochemical techniques. The kidneys were perfusion-fixed and embedded in Epon or LR White. For light microscopy, semi-thin sections of Epon-embedded materials were stained by the immunoenzyme technique after the epoxy resin was removed by treatment with sodium ethoxide. For electron microscopy, ultra-thin sections of LR White-embedded materials were stained by the protein A-gold technique. By light microscopy, the S1 segment of the proximal tubule was most heavily stained for ECI and DCR whilst S2 and S3 segments showed intermediate staining. A weak staining reaction was observed in the distal tubule and the medullary collecting tubule. In the cortical collecting tubule, heavily stained cells were present between weakly stained cells. By electron microscopy, gold particles showing the antigenic sites for ECI were confined mainly to the mitochondria, but few particles were observed in the peroxisomes. Gold labeling for DCR was localized both in the mitochondria and the peroxisomes. The labeling intensity of the peroxisomes was much higher than that of the mitochondria. The results suggest that metabolism of unsaturated fatty acids occurs mainly in the mitochondria and the peroxisomes of the proximal tubule in the kidney.

Beta-oxidation of unsaturated fatty acids in humans. Isoforms of delta 3, delta 2-enoyl-CoA isomerase

This investigation was undertaken in order to elucidate the human enzymes which participate in metabolism of the double bonds of unsaturated fatty acids during beta-oxidation. The results indicate that the human monofunctional delta 3, delta 2-enoyl-CoA isomerase (EC 5.3.3.8) with the native M(r) of 70,000 differed significantly from its rat counterpart [Palosaari et al. (1990) J. Biol. Chem. 265, 3347-3353]; the isoelectric point of the human isoform was over three pH-units more acidic, it showed different chromatographic behaviour, the human enzyme did not show any clear-cut substrate chain-length specificity and only a weak immunological cross-reactivity was detected with the antibody to rat liver mitochondrial short-chain enzyme. This explains the failure of attempts to apply the rat data directly to human beings. Another isomerase activity from human liver was found to be a part of the isomerase-hydratase-dehydrogenase polypeptide showing immunological cross-reactivity with the previously characterized peroxisomal multifunctional enzyme (MFE) from rat liver.

Peroxisomal beta-oxidation of polyunsaturated fatty acids

Peroxisomes have been shown to play an important role in the oxidative degradation of (poly)unsaturated fatty acids, and contain the enzyme activities needed for the metabolism of double bonds of unsaturated fatty acids in connection with this physiological function. Our understanding of the metabolic pathways and enzyme activities involved in the degradation of unsaturated acyl-CoAs has undergone a re-evaluation recently, and though many open questions still remain significant progress has been made, especially concerning the reactions metabolizing double bonds. The enzyme activities to be discussed here are 2,4-dienoyl-CoA reductase; 3/2-enoyl-CoA isomerase; 2-enoyl-CoA hydratase 2; 5-enoyl-CoA reductase and 3,5/2,4-dienoyl-CoA isomerase. Some of these activities are integral parts of the multifunctional proteins of beta-oxidation systems, which must also be taken into account in this context.

NADPH-dependent beta-oxidation of unsaturated fatty acids with double bonds extending from odd-numbered carbon atoms

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.

Human liver long-chain 3-hydroxyacyl-coenzyme A dehydrogenase is a multifunctional membrane-bound beta-oxidation enzyme of mitochondria

We have purified to homogeneity the long-chain specific 3-hydroxyacyl-CoA dehydrogenase from mitochondrial membranes of human infant liver. The enzyme is composed of non-identical subunits of 71 kDa and 47 kDa within a native structure of 230 kDa. The pure enzyme is active with 3-ketohexanoyl-CoA and gives maximum activity with 3-ketoacyl-CoA substrates of C10 to C16 acyl-chain length but is inactive with acetoacetyl-CoA. In addition to 3-hydroxyacyl-CoA dehydrogenase activity, the enzyme possesses 2-enoyl-CoA hydratase and 3-ketoacyl-CoA thiolase activities which cannot be separated from the dehydrogenase. None of these enzymes show activity with C4 substrates but all are active with C6 and longer acyl-chain length substrates. They are thus distinct from any described previously. This human liver mitochondrial membrane-bound enzyme catalyses the conversion of medium- and long-chain 2-enoyl-CoA compounds to: 1) 3-ketoacyl-CoA in the presence of NAD alone and 2) to acetyl-CoA (plus the corresponding acyl-CoA derivatives) in the presence of NAD and CoASH. It is therefore a multifunctional enzyme, resembling the beta-oxidation enzyme of E. coli, but unique in its membrane location and substrate specificity. We propose that its existence explains the repeated failure to detect any intermediates of mitochondrial beta-oxidation.

Crystallization and preliminary X-ray diffraction studies of mitochondrial short-chain delta 3,delta 2-enoyl-CoA isomerase from rat liver

Crystals of short-chain delta 3,delta 2-enoyl-CoA isomerase (EC 5.3.3.8) from rat liver mitochondria have been grown using the hanging-drop vapour diffusion technique. The enoyl-CoA isomerase is an auxiliary enzyme in the beta-oxidation pathway of fatty acid metabolism, and catalyzes the isomerization of unsaturated fatty acids to produce the metabolizable delta 2-trans isomer. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) with unit cell dimensions a = 47.9, b = 118.4 and c = 164.8 A, and diffract to 3 A.

Peroxisomes and beta-oxidation of long-chain unsaturated carboxylic acids

Degradation of unsaturated long-chain carboxylic acids by beta-oxidation, which is a compartmentalized process occurring in both mitochondria and peroxisomes in mammalian cells, was studied using rat liver as a model tissue. Inclusion of (poly)unsaturated fatty acids in the perfusion medium resulted in an increased concentration of catalase-H2O2 complex indicating on-going peroxisomal beta-oxidation. For this to occur, an active peroxisomal delta 3, delta 2-enoyl-CoA isomerase was required for metabolism of the double bonds at odd-numbered positions in acyl-CoA. Experiments with isolated subcellular organelles from rat liver confirmed the presence of isomerase in the peroxisomes, and the enzyme was subsequently isolated with apparent homogeneity. Comparison of amino acid sequences from the enzyme with a published sequence for a bifunctional protein from rat liver identified them representing the same molecule. The peroxisomal bifunctional protein can thus act as a multifunctional hydratase-dehydrogenase-isomerase enzyme. Examination of the mitochondrial isomerase revealed that rat liver mitochondria possess two isoenzymes: a long-chain isomerase not induced by clofibrate-treatment and showing a preference for C10-C12 substrates, and a clofibrate-inducible short-chain isomerase which gave the highest catalytic activity with C6 substrates. Experiments with isolated peroxisomes and unsaturated acyl-CoAs demonstrated that the beta-oxidation of fatty acids having double bonds at even-numbered positions was dependent on 2,4-dienoyl-CoA reductase in peroxisomes, as in mitochondria. Immunocytochemical experiments using the protein A-gold labelling technique, and comparison of their physicochemical properties indicated that all the mammalian reductases purified so far are mitochondrial isoenzymes. It turned out during the isolation of 3-hydroxyacyl-CoA epimerase that there was no monofunctional epimerase at all in the rat liver. Instead, the epimerization reaction occurred via dehydration-hydration catalyzed by two distinct stereospecific hydratases, 2-enoyl-CoA hydratase 1 (the classic hydratase) and 2-enoyl-CoA hydratase 2 (a novel hydratase). The present data demonstrate that peroxisomes contain all the enzymes required for the beta-oxidation of unsaturated fatty acids and support the notion that one of the physiological functions of peroxisomal beta-oxidation is to metabolize long-chain hydrophobic carboxylic acids to shorter, more polar metabolites which are then either metabolized further in the body or excreted.