Delta 3,5, delta 2,4-dienoyl-CoA isomerase from rat liver mitochondria. Purification and characterization of a new enzyme involved in the beta-oxidation of unsaturated fatty acids

Normobaric hypoxia shows enhanced FOXO1 signaling in obese mouse gastrocnemius muscle linked to metabolism and muscle structure and neuromuscular innervation

Skeletal muscle relies on mitochondria for sustainable ATP production, which may be impacted by reduced oxygen availability (hypoxia). Compared with long-term hypoxia, the mechanistic in vivo response to acute hypoxia remains elusive. Therefore, we aimed to provide an integrated description of the Musculus gastrocnemius response to acute hypoxia. Fasted male C57BL/6JOlaHsd mice, fed a 40en% fat diet for six weeks, were exposed to 12% O2 normobaric hypoxia or normoxia (20.9% O2) for six hours (n = 12 per group). Whole-body energy metabolism and the transcriptome response of the M. gastrocnemius were analyzed and confirmed by acylcarnitine determination and Q-PCR. At the whole-body level, six hours of hypoxia reduced energy expenditure, increased blood glucose and tended to decreased the respiratory exchange ratio (RER). Whole-genome transcriptome analysis revealed upregulation of forkhead box-O (FOXO) signalling, including an increased expression of tribbles pseudokinase 3 (Trib3). Trib3 positively correlated with blood glucose levels. Upregulated carnitine palmitoyltransferase 1A negatively correlated with the RER, but the significantly increased in tissue C14-1, C16-0 and C18-1 acylcarnitines supported that β-oxidation was not regulated. The hypoxia-induced FOXO activation could also be connected to altered gene expression related to fiber-type switching, extracellular matrix remodeling, muscle differentiation and neuromuscular junction denervation. Our results suggest that a six-hour exposure of obese mice to 12% O2 normobaric hypoxia impacts M. gastrocnemius via FOXO1, initiating alterations that may contribute to muscle remodeling of which denervation is novel and warrants further investigation. The findings support an early role of hypoxia in tissue alterations in hypoxia-associated conditions such as aging and obesity.

Overexpression of human peroxisomal enoyl-CoA delta isomerase2 HsPECI2, an ortholog of bamboo expressed during gregarious flowering alters salinity stress responses and polar lipid content in tobacco

Peroxisomal enoyl-CoA delta isomerase2 (PECI2) is one of the key enzymes that has critical role in lipid metabolism and plant development during salt stress. Seven out of ten tobacco plants overexpressing human PECI2 (HsPECI2) with PTS1-sequence showed hypersensitivity to salt. Under salt-stress, T2 transformed plants (HsPECI2) displayed reduced primary root, delayed shoot-growth, and visibly smaller rosette leaves turning pale yellow as compared to the pKYLX71 vector control plant. Also, we found altered reactive oxygen species (ROS) levels and reduced catalase activity in 100mM sodium chloride (NaCl) treated HsPECI2 transformed plant compared with the pKYLX71 counterpart. ESI-MS/MS data showed that the polar lipids were differentially modulated upon salt treatment in HsPECI2 transformed and pKYLX71 plants as compared with the respective untreated counterpart. Notably, the levels of monogalactosyldiacylglycerol and phosphatidic acid varied significantly, whereas phosphatidylcholine, phosphatidylserine and digalactosyldiacylglycerol contents were moderately upregulated. In parallel, abscisic acid (ABA) responsiveness assay confirmed insensitivity of HsPECI2 transformed plant towards ABA. Overall our data proclaim that HsPECI2 play multifunctional role in normal development and response to salinity stress apart from its primary role in β-oxidation.

Oleate beta-oxidation in yeast involves thioesterase but not Yor180c protein that is not a dienoyl-CoA isomerase

The beta-oxidation of oleic acid in Saccharomyces cerevisiae (S. cerevisiae) was studied by comparing the growth of wild-type cells on oleic acid or palmitic acid with the growth of mutants that either had a deletion in the YOR180c (DCI1) gene reported to encode delta3,5,delta2,4-dienoyl-CoA isomerase (dienoyl-CoA isomerase) or in the PTE1 gene encoding peroxisomal thioesterase 1. Growth of wild-type cells was indistinguishable from that of YOR180c mutant cells on either palmitic acid or oleic acid, whereas the PTE1 mutant grew slower and to a lower density on oleic acid but not on palmitic acid. The identification of 3,5-tetradecadienoic acid in the medium of wild-type cells but not in the medium of the PTE1 mutant proves the operation of the thioesterase-dependent pathway of oleate beta-oxidation in S. cerevisiae. Dienoyl-CoA isomerase activity was very low in wild-type cells, fourfold higher in the YOR180c mutant, and not associated with purified Yor180c protein. These observations support the conclusion that the YOR180c gene does not encode dienoyl-CoA isomerase.

Mitochondrial 2,4-dienoyl-CoA reductase deficiency in mice results in severe hypoglycemia with stress intolerance and unimpaired ketogenesis

The mitochondrial β-oxidation system is one of the central metabolic pathways of energy metabolism in mammals. Enzyme defects in this pathway cause fatty acid oxidation disorders. To elucidate the role of 2,4-dienoyl-CoA reductase (DECR) as an auxiliary enzyme in the mitochondrial β-oxidation of unsaturated fatty acids, we created a DECR–deficient mouse line. In Decr^−/− mice, the mitochondrial β-oxidation of unsaturated fatty acids with double bonds is expected to halt at the level of trans-2, cis/trans-4-dienoyl-CoA intermediates. In line with this expectation, fasted Decr^−/− mice displayed increased serum acylcarnitines, especially decadienoylcarnitine, a product of the incomplete oxidation of linoleic acid (C_18:2), urinary excretion of unsaturated dicarboxylic acids, and hepatic steatosis, wherein unsaturated fatty acids accumulate in liver triacylglycerols. Metabolically challenged Decr^−/− mice turned on ketogenesis, but unexpectedly developed hypoglycemia. Induced expression of peroxisomal β-oxidation and microsomal ω-oxidation enzymes reflect the increased lipid load, whereas reduced mRNA levels of PGC-1α and CREB, as well as enzymes in the gluconeogenetic pathway, can contribute to stress-induced hypoglycemia. Furthermore, the thermogenic response was perturbed, as demonstrated by intolerance to acute cold exposure. This study highlights the necessity of DECR and the breakdown of unsaturated fatty acids in the transition of intermediary metabolism from the fed to the fasted state.

Fatty acids released from triacylglycerols or obtained from the diet serve as a main energy provider to the heart and skeletal muscle, and when carbohydrates are scarce, fatty acids provide energy for the whole organism. Inherited disorders of mitochondrial β-oxidation are among the most common inborn errors of metabolism affecting infants and children. Under normal conditions, patients are usually asymptomatic; but when challenged with metabolic stress, severe phenotypes arise. Here we describe the generation of a mouse model in which the total degradation of unsaturated fatty acids is prevented by disruption of an auxiliary enzyme of β-oxidation. Although degradation of saturated fatty acids proceeds normally, the phenotype presented here is in many ways similar to mouse models of the disrupted classical β-oxidation pathway, but with additional unique features. The null mutant mice are asymptomatic until exposed to fasting, during which they switch on ketogenesis, but simultaneously develop hypoglycemia. A number of human patients suffer from idiopathic hypoglycemia (hypoglycemia of unknown cause). Our mouse model links this disease state to a specific defect in the breakdown of polyunsaturated fatty acids. Furthermore, it shows that degradation of unsaturated fatty acids is essential for balanced fatty acid and energy metabolism, as well as adaptation to metabolic stress.

An alternative pathway of oleate beta-oxidation in Escherichia coli involving the hydrolysis of a dead end intermediate by a thioesterase

The degradation of 2-trans,5-cis-tetradecadienoyl-CoA, a metabolite of oleic acid, by the purified complex of fatty acid oxidation from Escherichia coli was studied to determine how much of the metabolite is converted to 3,5-cis-tetradecadienoyl-CoA and thereby diverted from the classical, isomerase-dependent pathway of oleate beta-oxidation. Approximately 10% of the 2,5-intermediate was converted to the 3,5-isomer. When the latter compound was allowed to accumulate, it strongly inhibited the flux through the main pathway. Since Delta(3,5),Delta(2,4)-dienoyl-CoA isomerase was not detected in E. coli cells grown on oleate, the 3,5-intermediate cannot be metabolized via the reductase-dependent pathway. However, it was hydrolyzed by a thioesterase, which was most active with 3,5-cis-tetradecadienoyl-CoA as substrate and which was induced by growth of E. coli on oleate. An analysis of fatty acids present in the medium after growth of E. coli on oleate revealed the presence of 3,5-tetradecadienoate, which was not detected after cells were grown on palmitate or glucose. Altogether, these data prompt the conclusion that oleate is mostly degraded via the classical, isomerase-dependent pathway in E. coli but that a small amount of 2-trans,5-cis-tetradecadienoyl-CoA is diverted from the pathway via conversion to 3,5-cis-tetradecadienoyl-CoA by Delta(3),Delta(2)-enoyl-CoA isomerase. The 3,5-intermediate, which would strongly inhibit beta-oxidation if allowed to accumulate, is hydrolyzed, and the resultant 3,5-tetradecadienoate is excreted into the growth medium. This study provides evidence for the novel function of a thioesterase in beta-oxidation.

Metabolic functions of the two pathways of oleate beta-oxidation double bond metabolism during the beta-oxidation of oleic acid in rat heart mitochondria

Unsaturated fatty acids with odd-numbered double bonds, e.g. oleic acid, can be degraded by beta-oxidation via the isomerase-dependent pathway or the reductase-dependent pathway that differ with respect to the metabolism of the double bond. In an attempt to elucidate the metabolic functions of the two pathways and to determine their contributions to the beta-oxidation of unsaturated fatty acids, the degradation of 2-trans,5-cis-tetradecadienoyl-CoA, a metabolite of oleic acid, was studied with rat heart mitochondria. Kinetic measurements of metabolite and cofactor formation demonstrated that more than 80% of oleate beta-oxidation occurs via the classical isomerase-dependent pathway whereas the more recently discovered reductase-dependent pathway is the minor pathway. However, the reductase-dependent pathway is indispensable for the degradation of 3,5-cis-tetradecadienoyl-CoA, which is formed from 2-trans,5-cis-tetradecadienoyl-CoA by delta(3),delta(2)-enoyl-CoA isomerase, the auxiliary enzyme that is essential for the operation of the major pathway of oleate beta-oxidation. The degradation of 3,5-cis-tetradecadienoyl-CoA is limited by the capacity of 2,4-dienoyl-CoA reductase to reduce 2-trans,4-trans-tetradecadienoyl-CoA, which is rapidly formed from its 3,5 isomer by delta(3,5),delta(2,4)-dienoyl-CoA isomerase. It is concluded that both pathways are essential for the degradation of unsaturated fatty acids with odd-numbered double bonds inasmuch as the isomerase-dependent pathway facilitates the major flux through beta-oxidation and the reductase-dependent pathway prevents the accumulation of an otherwise undegradable metabolite.

Disruption of mitochondrial beta -oxidation of unsaturated fatty acids in the 3,2-trans-enoyl-CoA isomerase-deficient mouse

Cellular energy metabolism is largely sustained by mitochondrial beta-oxidation of saturated and unsaturated fatty acids. To study the role of unsaturated fatty acids in cellular lipid and energy metabolism we generated a null allelic mouse, deficient in 3,2-trans-enoyl-CoA isomerase (ECI) (eci(-/-) mouse). ECI is the link in mitochondrial beta-oxidation of unsaturated and saturated fatty acids and essential for the complete degradation and for maximal energy yield. Mitochondrial beta-oxidation of unsaturated fatty acids is interrupted in eci(-/-)mice at the level of their respective 3-cis- or 3-trans-enoyl-CoA intermediates. Fasting eci(-/-) mice accumulate unsaturated fatty acyl groups in ester lipids and deposit large amounts of triglycerides in hepatocytes (steatosis). Gene expression studies revealed the induction of peroxisome proliferator-activated receptor activation in eci(-/-) mice together with peroxisomal beta- and microsomal omega-oxidation enzymes. Combined peroxisomal beta- and microsomal omega-oxidation of the 3-enoyl-CoA intermediates leads to a specific pattern of medium chain unsaturated dicarboxylic acids excreted in the urine in high concentration (dicarboxylic aciduria). The urinary dicarboxylate pattern is a reliable diagnostic marker of the ECI genetic defect. The eci(-/-) mouse might be a model of a yet undefined inborn mitochondrial beta-oxidation disorder lacking the enzyme link that channels the intermediates of unsaturated fatty acids into the beta-oxidation spiral of saturated fatty acids.

Delta 3,5,delta 2,4-dienoyl-CoA isomerase is a multifunctional isomerase. A structural and mechanistic study

Delta(3,5),Delta(2,4)-Dienoyl-CoA isomerase (DI), an auxiliary enzyme of unsaturated fatty acid beta-oxidation, was purified from rat mitochondria and peroxisomes and subjected to N-terminal sequencing to facilitate a mechanistic study of this enzyme. The mature mitochondrial DI from rat heart was lacking its 34 N-terminal amino acid residues that have the properties of a mitochondrial targeting sequence. The peroxisomal isomerase was identified as a product of the same gene with a truncated and ragged N terminus. Expression of the cDNA coding for the mature mitochondrial DI in Escherichia coli yielded an enzyme preparation that was as active as the native DI. Because the recombinant DI also exhibited Delta(3,5,7),Delta(2,4,6)-trienoyl-CoA isomerase (TI) activity, both isomerases reside on the same protein. Mutations of any of the 3 acidic amino acid residues located at the active site (Modis, Y., Filppula, S. A., Novikov, D. K., Norledge, B., Hiltunen, J. K., and Wierenga, R. K. (1998) Structure 6, 957-970) caused activity losses. In contrast to only a 10-fold decrease in activity upon replacement of Asp(176) by Ala, substitutions of Asp(204) by Asn and of Glu(196) by Gln resulted in 10(5)-fold lower activities. Such activity losses are consistent with the direct involvement of these latter two residues in the proposed proton transfers at carbons 2 and 6 or 8 of the substrates. Probing of the wild-type and mutants forms of the enzyme with 2,5-octadienoyl-CoA as substrate revealed low Delta(2),Delta(3)-enoyl-CoA isomerase and Delta(5),Delta(4)-enoyl-CoA isomerase activities catalyzed by Glu(196) and Asp(204), respectively. Altogether, these data reveal that positional isomerizations of the diene and triene are facilitated by simultaneous proton transfers involving Glu(196) and Asp(204), whereas each residue alone can catalyze, albeit less efficiently, a monoene isomerization.

beta-oxidation - strategies for the metabolism of a wide variety of acyl-CoA esters

Living organisms are exposed to a number of different fatty acids and their various derivatives arising either via endogenous synthesis or from exogenous sources. These hydrophobic compounds can play specific metabolic, structural or endocrinic functions in the organisms before their elimination, which can be metabolism to CO(2) or to more polar lipid metabolites allowing their excretion. Quantitatively, one of the major pathways metabolizing fatty acids is beta-oxidation, which consists of a set of four reactions operating at the carbons 2 or 3 of acyl-CoA esters and shortening of the acyl-chain. To allow the beta-oxidation of acyl groups with various steric variants to proceed, different strategies have been developed. These strategies include evolution of beta-oxidation enzymes as paralogues showing specificity with respect to either chain-length or modified acyl-chain, metabolic compartmentalization in eukaryotic cells, controlling of substrate transport across membranes, development of auxiliary enzyme systems, acquisition of enzymes with adaptive active sites and recruiting and optimizing enzymes from non-homologous sources allowing them to catalyze a parallel set of reactions with different substrate specificities.

Characterization of PECI, a novel monofunctional Delta(3), Delta(2)-enoyl-CoA isomerase of mammalian peroxisomes

We report here the identification and characterization of human and mouse PECI, a novel gene that encodes a monofunctional peroxisomal Delta(3),Delta(2)-enoyl-CoA isomerase. Human and mouse PECI were identified on the basis of their sequence similarity to Eci1p, a recently characterized peroxisomal Delta(3),Delta(2)-enoyl-CoA isomerase from the yeast Saccharomyces cerevisiae. Cloning and sequencing of the human PECI cDNA revealed the presence of a 1077-base pair open reading frame predicted to encode a 359-amino acid protein with a mass of 39.6 kDa. The corresponding mouse cDNA contains a 1074-base pair open reading frame that encodes a 358-amino acid-long protein with a deduced mass of 39.4 kDa. Northern blot analysis demonstrated human PECI mRNA is expressed in all tissues. A bacterially expressed form of human PECI catalyzed the isomerization of 3-cis-octenoyl-CoA to 2-trans-octenoyl-CoA with a specific activity of 27 units/mg of protein. The human and mouse PECI proteins contain type-1 peroxisomal targeting signals, and human PECI was localized to peroxisomes by both subcellular fractionation and immunofluorescence microscopy techniques. The potential roles for this monofunctional Delta(3),Delta(2)-enoyl-CoA isomerase in peroxisomal metabolism are discussed.

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.

Delta3,5,7,Delta2,4,6-trienoyl-CoA isomerase, a novel enzyme that functions in the beta-oxidation of polyunsaturated fatty acids with conjugated double bonds

The mitochondrial metabolism of unsaturated fatty acids with conjugated double bonds at odd-numbered positions, e.g. 9-cis, 11-trans-octadecadienoic acid, was investigated. These fatty acids are substrates of beta-oxidation in isolated rat liver mitochondria and hence are expected to yield 5,7-dienoyl-CoA intermediates. 5, 7-Decadienoyl-CoA was used to study the degradation of these intermediates. After introduction of a 2-trans-double bond by acyl-CoA dehydrogenase or acyl-CoA oxidase, the resultant 2,5, 7-decatrienoyl-CoA can either continue its pass through the beta-oxidation cycle or be converted by Delta3,Delta2-enoyl-CoA isomerase to 3,5,7-decatrienoyl-CoA. The latter compound was isomerized by a novel enzyme, named Delta3,5,7,Delta2,4, 6-trienoyl-CoA isomerase, to 2,4,6-decatrienoyl-CoA, which is a substrate of 2,4-dienoyl-CoA reductase (Wang, H.-Y. and Schulz, H. (1989) Biochem. J. 264, 47-52) and hence can be completely degraded via beta-oxidation. Delta3,5,7,Delta2,4,6-Trienoyl-CoA isomerase was purified from pig heart to apparent homogeneity and found to be a component enzyme of Delta3,5,Delta2,4-dienoyl-CoA isomerase. Although the direct beta-oxidation of 2,5,7-decatrienoyl-CoA seems to be the major pathway, the degradation via 2,4,6-trienoyl-CoA makes a significant contribution to the total beta-oxidation of this intermediate.

Polyunsaturated fatty acid biosynthesis: a microsomal-peroxisomal process

The synthesis of 22-carbon fatty acids, with their first double bond at position 4, requires the participation of enzymes in both peroxisomes and the endoplasmic reticulum as well as the controlled movement of fatty acids between these two cellular compartments. It has been observed that there is generally an inverse relationship between rates of peroxisomal beta-oxidation vs those for the microsomal esterification of fatty acids into 1-acyl-sn-glycero-3-phosphocholine. With a variety of different substrates it was found that when a fatty acid is produced in peroxisomes, with its first double bond at position 4, its preferred metabolic fate is to move to microsomes for esterification rather than to serve as a substrate for continued degradation. The required movement, and the associated reactions, in peroxisomes and microsomes is not restricted to the synthesis of 4,7,10,13,16-docosapentaenoic acid and 4,7,10,13,16,19-docosahexaenoic acid. When microsomes and peroxisomes were incubated with NAD, NADPH and malonyl-CoA it was found that 6,9,12-octadecatrienoic acid was metabolized to linoleate. Collectively our findings suggest that there may be considerably more recycling of fatty acids between peroxisomes and the endoplasmic reticulum than was previously recognized.

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.

The crystal structure of dienoyl-CoA isomerase at 1.5 A resolution reveals the importance of aspartate and glutamate sidechains for catalysis

BACKGROUND

The degradation of unsaturated fatty acids is vital to all living organisms. Certain unsaturated fatty acids must be catabolized via a pathway auxiliary to the main beta-oxidation pathway. Dienoyl-coenzyme A (dienoyl-CoA) isomerase catalyzes one step of this auxiliary pathway, the isomerization of 3-trans,5-cis-dienoyl-CoA to 2-trans,4-trans-dienoyl-CoA, and is imported into both mitochondria and peroxisomes. Dienoyl-CoA isomerase belongs to a family of CoA-binding proteins that share the enoyl-CoA hydratase/isomerase sequence motif.

RESULTS

The crystal structure of rat dienoyl-CoA isomerase has been determined at 1.5 A resolution. The fold closely resembles that of enoyl-CoA hydratase and 4-chlorobenzoyl-CoA dehalogenase. Dienoyl-CoA isomerase forms hexamers made up of two trimers. The structure contains a well ordered peroxisomal targeting signal type-1 which is mostly buried in the inter-trimer space. The active-site pocket is deeply buried and entirely hydrophobic, with the exception of the acidic residues Asp176, Glu196 and Asp204. Site-directed mutagenesis of Asp204 revealed that this residue is essential for catalysis. In a molecular modeling simulation, a molecule of 3-trans,5-cis-octadienoyl-CoA was docked into the active site.

CONCLUSIONS

The structural data, supported by the mutagenesis data, suggest a reaction mechanism where Glu196 acts as a proton acceptor and Asp204 acts as a proton donor. Asp176 is paired with Glu196 and is important for optimizing the catalytic proton transfer properties of Glu196. In the predicted mode of substrate binding, an oxyanion hole stabilizes the transition state by binding the thioester oxygen. The presence of a buried peroxisomal targeting signal suggests that dienoyl-CoA isomerase is prevented from reaching its hexameric structure in the cytosol.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Significance of the reductase-dependent pathway for the beta-oxidation of unsaturated fatty acids with odd-numbered double bonds. Mitochondrial metabolism of 2-trans-5-cis-octadienoyl-CoA

The beta-oxidation of unsaturated fatty acids with odd-numbered double bonds proceeds by reduction of the double bond (reductase-dependent pathway) in addition to the well established isomerization of the double bond (isomerase-dependent pathway). The metabolic significance of the reductase-dependent pathway was assessed with 2-trans-5-cis-octadienoyl-CoA (2,5-octadienoyl-CoA) and its products, all of which are metabolites of alpha-linolenic acid. A kinetic evaluation of beta-oxidation enzymes revealed that the presence of a 5-cis double bond in the substrate most adversely affected the activity of 3-ketoacyl-CoA thiolase although not enough to become rate-limiting. Concentration-dependent and time-dependent measurements indicated that most (80%) of 2,5-octadienoyl-CoA is metabolized via the isomerase-dependent pathway. The reason for the greater flux through the isomerase-dependent pathway is the higher activity of L-3-hydroxyacyl-CoA dehydrogenase as compared with Delta3,Delta2-enoyl-CoA isomerase. These two enzymes catalyze the rate-limiting steps in the isomerase-dependent and reductase-dependent pathways, respectively. Once 2,5-octadienoyl-CoA is converted to 3,5-octadienoyl-CoA (perhaps fortuitously because of the presence of Delta3,Delta2-enoyl-CoA isomerase), the only effective route for its degradation is via the reductase-dependent pathway. It is concluded that the reductase-dependent pathway assures the degradation of 3,5-dienoyl-CoA intermediates, thereby preventing the depletion of free coenzyme A and a likely impairment of mitochondrial oxidative function.

Analysis of the acyl-CoAs that accumulate during the peroxisomal beta-oxidation of arachidonic acid and 6,9,12-octadecatrienoic acid

The biosynthesis of 4,7,10,13,16-22:5 and 4,7,10,13,16,19-22:6 requires that when 6,9,12,15,18-24:5 and 6,9,12,15,18,21-24:6 are produced in microsomes they must move to peroxisomes for partial beta-oxidation. When the 24-carbon acids were incubated with peroxisomes, 22-carbon acids with their first double bond at position 4 accumulated as did those with their first two double bonds at the 2-trans-4-cis-positions (D. L. Luthria, S. B. Mohammed, and H. Sprecher, J. Biol. Chem. 271, 16020-16025, 1996; and B. S. Mohammed, D. L. Luthria, S. P. Baykousheva, and H. Sprecher, Biochem. J., 326, 425-430, 1997). In the study reported here we analyzed the acyl-CoAs that accumulated when peroxisomes were incubated with 5,8,11,14-20:4 and 6,9,12-18:3, a metabolite that would be produced via one cycle of arachidonate degradation via the pathway requiring both NADPH-dependent 2,4-dienoyl-CoA reductase and delta 3,5, delta 2,4-dienoyl-CoA isomerase. With both substrates the acyl-CoAs of 2-trans-4-10:2, 4-10:1, 2-trans-4,7,10-16:4, and 4,7,10-16:3 accumulated. These results further establish that the reductase catalyzes a control step in the peroxisomal degradation of unsaturated fatty acids. It was not possible to detect any 18- or 12-carbon acyl-CoA when arachidonate was the substrate, nor did any 12-carbon catabolite accumulate from 6,9,12-18:3. The fractional amount of 5,8-14:2 and arachidonate catabolized via the pathway using only the enzymes of saturated fatty acid degradation versus the pathway that also uses the reductase and the isomerase could thus not be estimated.

Delta3,5-delta2,4-dienoyl-CoA isomerase from rat liver. Molecular characterization

rECH1, a recently identified rat cDNA (FitzPatrick, D. R., Germain-Lee, E., and Valle, D. (1995) Genomics 27, 457-466) encodes a polypeptide belonging to the hydratase/isomerase superfamily. We modeled the structure of rECH1 based on rat mitochondrial 2-enoyl-CoA hydratase 1. The model predicts that rECH1p has the hydratase fold in the core domain and two domains for interaction with other subunits. When we incubated 3,5,8,11, 14-eicosapentaenoyl-CoA with purified rECH1p, the spectral data suggested a switching of the double bonds from the Delta3-Delta5 to the Delta2-Delta4 positions. This was confirmed by demonstrating that the product was a valid substrate for 2,4-dienoyl-CoA reductase. These results indicate that rECH1p is Delta3,5-Delta2,4-dienoyl-CoA isomerase. Subcellular fractionation and immunoelectron microscopy using antibodies to a synthetic polypeptide derived from the C terminus of rECH1p showed that rECH1p is located in the matrix of both mitochondria and peroxisomes in rat liver. Consistent with these observations, the 36,000-Da rECH1p has a potential N-terminal mitochondrial targeting signal as well as a C-terminal peroxisomal targeting signal type 1. Transport of the protein into the mitochondria with cleavage of the targeting signal results in a mature mitochondrial form with a molecular mass of 32,000 Da; transport to peroxisomes yields a protein of 36,000 Da.

Identification of novel metabolites of prostaglandin E2 formed by isolated rat hepatocytes

The metabolism of prostaglandin E2 (PGE2) in isolated rat hepatocytes led to the formation of four major as well as several minor products which were structurally characterized using electrospray tandem mass spectrometry. The major metabolites identified included dinor-PGE1, dinor-PGE2, and tetranor-PGE1 and the taurine conjugates of dinor-PGE1 and dinor-PGE2. Several minor metabolites including the taurine conjugates of PGE2 and tetranor PGE1 along with a glucuronide conjugate of PGE2 were also identified. These taurine conjugates had not been previously identified in studies of PGE2 metabolism, yet comprised nearly 50% of the mixture of metabolites after 40-min incubations. Experiments carried out with deuterium-labeled PGE2 ([3,3,4,4-D4]PGE2) resulted in the complete loss of all deuterium atoms in dinor-PGE1, dinor-PGE2, and tetranor metabolites during incubation with hepatocytes. Metabolism via classic beta-oxidation pathways would predict one deuterium atom retained by dinor-PGE1 and two deuterium atoms retained by dinor-PGE2. When PGE2 was incubated with isolated rat hepatocytes in buffer containing 30% D2O, substantial incorporation (30%) of one deuterium atom could be observed in the dinor metabolites along with 10% incorporation into the tetranor and residual PGE2. Deuterium-labeled PGE1 ([3,3,4,4-D4]PGE1) was metabolized to D2-dinor-PGE1, tetranor-PGE1, and the taurine conjugate of D2-dinor-PGE1 by isolated rat hepatocytes. The loss of deuterium during metabolism of the deuterated substrates of PGE2, but not PGE1, as well as the incorporation of deuterium atoms from the aqueous solvent into PGE2 metabolites suggested that the delta 5 double bond and sequential isomerization reactions lead to eventual exchange of the protons from carbon atom 4 of PGE2 with water.

Tracer studies with crude U-13C-lipid mixtures. Biosynthesis of the lipase inhibitor lipstatin

The biosynthesis of the pancreatic lipase inhibitor lipstatin was investigated by fermentation experiments using cultures of Streptomyces toxytricini, which were supplied with soybean oil and a crude mixture of U-13C-lipids obtained from algal biomass cultured with 13CO2. Lipstatin was analyzed by one- and two-dimensional NMR spectroscopy. 13C total correlation spectroscopy and INADEQUATE experiments show that two fatty acid fragments containing 14 and 8 carbon atoms, respectively, are incorporated en bloc into lipstatin. The 14-carbon fragment is preferentially derived from the unsaturated fatty acid fraction, as shown by an experiment with hydrogenated U-13C-lipid mixture, which is conducive to labeling of the 8-carbon moiety but not of the 14-carbon moiety. The data indicate that the lipstatin molecule can be assembled by Claisen condensation of octanoyl-CoA with 3-hydroxy-delta5,8-tetradecanoyl-CoA obtained by beta oxidation of linoleic acid. The formation of lipstatin from acetate units by a polyketide-type pathway is ruled out conclusively by these data. The data show that surprisingly clear labeling patterns can be obtained in studies with crude, universally 13C-labeled precursor mixtures that are proffered together with a large excess of unlabeled material. One- and two-dimensional 13C total correlation spectroscopy analyses are suggested as elegant methods for the delineation of contiguously 13C-labeled biosynthetic blocks.

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.

Metabolism of 6-trans-isomers of leukotriene B4 in cultured hepatoma cells and in human polymorphonuclear leukocytes. Identification of a delta 6-reductase metabolic pathway

The intermediate metabolic events which degrade hydroxy polyunsaturated fatty acids is largely unknown. Such molecules are common products of lipid peroxidation and lipoxygenase catalyzed oxidation of arachidonic acid. Metabolism of two 5,12-dihydroxyeicosatetraenoic acids, 6-trans-LTB4 (leukotriene B4), and 6-trans-12-epi-LTB4 was studied in HepG2 cells (a human-derived hepatoma cell line). Extensive metabolism was observed with a major metabolite identified as 4-hydroxy-6-dodecenoic acid for both epimers. Incubation of 6-trans-LTB4 epimers at shorter times revealed the formation of intermediate metabolites, including 6-hydroxy-4,8-tetradecadienoic acid and 8-hydroxy-4,6,10-hexadecatrienoic acid suggesting beta-oxidation as the major pathway leading to the formation of the common terminal metabolite. Two additional metabolites were structurally elucidated as 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 which have not been previously described. Formation of 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 were also observed during metabolism of 6-trans-12-epi-LTB4 in human polymorphonuclear leukocytes. Of particular interest is the metabolism of these compounds by beta-oxidation from the carboxyl terminus, a process which is not observed with leukotriene B4 or leukotriene C4. Identification of these metabolites suggested the operation of the 5-hydroxyeicosanoid dehydrogenase pathway followed by a delta 6-reductase metabolic pathway which has not been previously described. This pathway of beta-oxidation may limit the activity of various 5,12-diHETEs including nonenzymatic hydrolysis products of LTA4 and also the recently described B4-isoleukotrienes.

Double bond removal from odd-numbered carbons during peroxisomal beta-oxidation of arachidonic acid requires both 2,4-dienoyl-CoA reductase and delta 3,5,delta 2,4-dienoyl-CoA isomerase

The pathway for the peroxisomal beta-oxidation of arachidonic acid (5,8,11,14-20:4) was elucidated by comparing its metabolism with 4,7,10-hexadecatrienoic acid (4,7,10-16:3) and 5,8-tetradecadienoic acid (5,8-14:2) which are formed, respectively, after two and three cycles of arachidonic acid degradation. When [1-14C]4,7,10-16:3 was incubated with peroxisomes in the presence of NAD+ and NADPH, it resulted in a time-dependent increase in the production of acid-soluble radioactivity which was accompanied by the synthesis of 2-trans-4,7,10-hexadecatetraenoic acid and two 3,5,7,10-hexadecatetraenoic acid isomers. The formation of conjugated trienoic acids suggests that peroxisomes contain delta 3,5,delta 2,4-dienoyl-CoA isomerase with the ability to convert 2-trans-4,7,10-hexadecatetraenoic acid to 3,5,7,10-hexadecatetraenoic acid. When 1-14C-labeled 6,9,12-octadecatrienoic acid or 7,10,13,16-docosatetraenoic acid was incubated without nucleotides, the 3-hydroxy metabolites accumulated, since further degradation requires NAD(+)-dependent 3-hydroxyacyl-CoA dehydrogenase. When [1-14C]5,8,11,14-20:4 was incubated under identical conditions, no polar metabolite was detected, but 2-trans-4,8,11,14-eicosapentaenoic acid accumulated. When NADPH was added to incubations, 3-hydroxy-8,11,14-eicosatrienoic, 2-trans-4,8,11,14-eicosapentaenoic, 2-trans-8,11,14-eicosatetraenoic, and 8,11,14-eicosatrienoic acids were produced. Analogous compounds were formed from [1-14C]5,8-14:2. Our results show that the removal of double bonds from odd-numbered carbons in arachidonic acid thus requires both NADPH-dependent 2,4-dienoyl-CoA reductase and delta 3,5,delta 2,4-dienoyl-CoA isomerase. One complete cycle of 5,8-14:2 and 5,8,11,14-20:4 beta-oxidation yields, respectively, 6-dodecenoic and 6,9,12-octadecatrienoic acids.

Isomerization of trans-2,delta 5-dienoyl-CoA's to delta 3,delta 5-dienoyl-CoA's in the beta-oxidation of delta 5-unsaturated fatty acids

The NADPH-dependent reduction pathway for the metabolism of delta 5-unsaturated fatty acids involves the isomerization of trans-2,delta 5-dienoyl-CoA, initially formed from the dehydrogenation of delta 5-enoyl-CoA, to isomeric delta 3,delta 5-dienoyl-CoA. The latter intermediates were then isomerized to trans-2,trans-4-dienoyl-CoA, which then follows the NADPH-dependent pathway mediated by 2,4-dienoyl-CoA reductase. The isomerization from trans-2,delta 5-dienoyl-CoA to delta 3,delta 5-dienoyl-CoA is catalyzed by delta 3,delta 2-enoyl-CoA isomerase. In this investigation, we identified the stereoisomers of delta 3,delta 5-dienoates that were formed in the reaction. Starting from trans-2,cis-5-decadienoyl-CoA, the isomerization produced cis-3,cis-5- and trans-3,cis-5-decadienoates. On the other hand, trans-2,trans-5-decadienoyl-CoA yielded cis-3,trans-5- and trans-3,trans-5-decadienoates. In addition to purified rat liver delta 3,delta 2-enoyl-CoA isomerase, acyl-CoA oxidase from Arthrobacter also catalyzed the isomerization from trans-2,cis-5-dienoyl-CoA. However, this acyl-CoA oxidase could not catalyze the similar isomerization of trans-2,trans-5-dienoyl-CoA. delta 3,delta 5-t-2,t-4-Dienoyl-CoA isomerase used cis-3,cis-5-, trans-3,cis-5-, and cis-3,trans-5-dienoyl-CoA's as substrates and converted them to trans-2,trans-4-dienoyl-CoA. In contrast, trans-3,trans-5-dienoyl-CoA was not a substrate for this isomerization. Extensive purification of acyl-CoA oxidase through column chromatography could not remove or diminish the isomerization activity associated with acyl-CoA oxidase. Acyl-CoA oxidases derived from Candida and rat liver also possess isomerization activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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)