Formation and release of a peroxisome-dependent arachidonic acid metabolite by human skin fibroblasts

Mapping the myristoylome through a complete understanding of protein myristoylation biochemistry

Protein myristoylation is a C14 fatty acid modification found in all living organisms. Myristoylation tags either the N-terminal alpha groups of cysteine or glycine residues through amide bonds or lysine and cysteine side chains directly or indirectly via glycerol thioester and ester linkages. Before transfer to proteins, myristate must be activated into myristoyl coenzyme A in eukaryotes or, in bacteria, to derivatives like phosphatidylethanolamine. Myristate originates through de novo biosynthesis (e.g., plants), from external uptake (e.g., human tissues), or from mixed origins (e.g., unicellular organisms). Myristate usually serves as a molecular anchor, allowing tagged proteins to be targeted to membranes and travel across endomembrane networks in eukaryotes. In this review, we describe and discuss the metabolic origins of protein-bound myristate. We review strategies for in vivo protein labeling that take advantage of click-chemistry with reactive analogs, and we discuss new approaches to the proteome-wide discovery of myristate-containing proteins. The machineries of myristoylation are described, along with how protein targets can be generated directly from translating precursors or from processed proteins. Few myristoylation catalysts are currently described, with only N-myristoyltransferase described to date in eukaryotes. Finally, we describe how viruses and bacteria hijack and exploit myristoylation for their pathogenicity.

Conjugated linoleic acids (CLA) as precursors of a distinct family of PUFA

One of the possibilities for distinct actions of c9,t11- and the t10,c12-conjugated linoleic acid (CLA) isomers may be at the level of metabolism since the conjugated diene structure gives to CLA isomers and their metabolites a distinct pattern of incorporation into the lipid fraction and metabolism. In fact, CLA appears to undergo similar transformations as linoleic acid but with subtle isomer differences, which may account for their activity in lowering linoleic acid metabolites in those tissues rich in neutral lipids where CLA is preferentially incorporated. Furthermore, c9,t11 and t10,c12 isomers are metabolized at a different rate in the peroxisomes, where the shortened metabolite from t10,c12 is formed at a much higher proportion than the metabolite from c9,t11. This may account for the lower accumulation of t10,c12 isomer into cell lipids. CLA isomers may therefore be viewed as a "new" family of polyunsaturated fatty acids (PUFA) producing a distinct range of metabolites using the same enzymatic system as the other (i.e., n-3, n-6 and n-9) PUFA families. It is likely that perturbation of PUFA metabolism by CLA will have an impact on eicosanoid formation and metabolism, closely linked to the biological activities attributed to CLA.

A nucleotide insertion in the transcriptional regulatory region of FADS2 gives rise to human fatty acid delta-6-desaturase deficiency

Fatty acid delta-6-desaturase (FADS2) is the rate-limiting enzyme in mammalian synthesis of long-chain polyunsaturated fatty acids. We investigated the molecular mechanism of FADS2 deficiency in skin fibroblasts from a patient deficient in this enzyme. Expression analyses demonstrated an 80% to 90% decrease in the steady-state level of FADS2 mRNA in patient-derived cells compared with normal controls that was consistent with previous metabolic biochemical studies. In vitro transcription assays indicated an 80% decrease in the rate of transcriptional initiation in patient-derived cells, thus implicating transcriptional regulation as the mechanism for the decreased transcript levels. Sequence analysis of the 5' end of the gene revealed the insertion of a thymidine between positions -941 and -942 upstream of the translation start site in patient-derived cells compared with normal cells and published sequences. Promoter-reporter assays demonstrated a 6-fold decrease in promoter activity in the polymorphic variant FADS2 regulatory region compared with the normal gene, confirming the functional relevance of the insertion mutation to the decreased expression of the gene in the patient-derived cells. These findings indicate that fatty acid delta-6-desaturase deficiency and decreased FADS2 transcription are caused by a nucleotide insertion in the transcriptional regulatory region of the human FADS2 gene.

Effect of iron overload and dietary fat on indices of oxidative stress and hepatic fibrogenesis in rats

BACKGROUND/AIMS

Oxidative stress is presumed to play an important role in hepatic fibrogenesis. Diets high in polyunsaturated fatty acids (PUFA) enhance fibrosis and have been associated with increased oxidative damage in some models of liver injury. The aim of this study was to determine the effects of dietary fat of varying PUFA content on iron-induced oxidative stress and fibrosis.

METHODS

Rats were given parenteral iron and diets supplemented with coconut oil, safflower oil or menhaden oil.

RESULTS

Hepatic iron overload was associated with induction of heme oxygenase-1, a sensitive indicator of oxidative stress, and with modest increases in hydroxyproline and procollagen I mRNA levels without histologically evident fibrosis, all of which were unaffected by dietary fat. In addition, iron loading was associated with increases in cysteine, gamma-glutamylcysteine and glutathione. Dietary fat brought about the expected alterations in peroxidizability, but did not alter indices of oxidative damage.

CONCLUSION

These data highlight the distinction between oxidative stress and oxidative damage and suggest that the former is not sufficient to elicit overt fibrosis. Furthermore, while hepatic iron overload leads to oxidative stress, there is an associated upregulation of antioxidant defenses involving thiol metabolism that may be a critical factor limiting the accumulation of oxidative damage.

Effect of the delta6-desaturase inhibitor SC-26196 on PUFA metabolism in human cells

The objective of this study was to determine the effect of 2,2-diphenyl-5-(4-[[(1 E)-pyridin-3-yl-methylidene]amino]piperazin-1-yl)pentanenitrile (SC-26196), a delta6-desaturase inhibitor, on PUFA metabolism in human cells. SC-26196 inhibited the desaturation of 2 microM [1-14C] 18:2n-6 by 87-95% in cultured human skin fibroblasts, coronary artery smooth muscle cells, and astrocytes. By contrast, SC-26196 did not affect the conversion of [1-14C]20:3n-6 to 20:4 in the fibroblasts, demonstrating that it is selective for delta6-desaturase. The IC50 values for inhibition of the desaturation of 2 microM [1-14C] 18:3n-3 and [3-14C]24:5n-3 in the fibroblasts, 0.2-0.4 microM, were similar to those for the inhibition of [1-14C 18:2n-6 desaturation, and the rates of recovery of [1-14C]18:2n-6 and [3-14C]24:5n-3 desaturation after removal of SC-26196 from the culture medium also were similar. SC-26196 reduced the conversion of [3-14C]22:5n-3 and [3-14C]24:5n-3 to DHA by 75 and 84%, respectively, but it had no effect on the retroconversion of [3-14C]24:6n-3 to DHA. These results demonstrate that SC-26196 effectively inhibits the desaturation of 18- and 24-carbon PUFA and, therefore, decreases the synthesis of arachidonic acid, EPA, and DHA in human cells. Furthermore, they provide additional evidence that the conversion of 22:5n-3 to DHA involves delta6-desaturation.

Human coronary endothelial cells convert 14,15-EET to a biologically active chain-shortened epoxide

Cytochrome P-450 epoxygenase-derived epoxyeicosatrienoic acids (EETs) play an important role in the regulation of vascular reactivity and function. Conversion to the corresponding dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolases is thought to be the major pathway of EET metabolism in mammalian vascular cells. However, when human coronary artery endothelial cells (HCEC) were incubated with (3)H-labeled 14,15-EET, chain-shortened epoxy fatty acids, rather than DHET, were the most abundant metabolites. After 4 h of incubation, 23% of the total radioactivity remaining in the medium was converted to 10,11-epoxy-hexadecadienoic acid (16:2), a product formed from 14,15-EET by two cycles of beta-oxidation, whereas only 15% was present as 14,15-DHET. Although abundantly present in the medium, 10,11-epoxy-16:2 was not detected in the cell lipids. Exogenously applied (3)H-labeled 10,11-epoxy-16:2 was neither metabolized nor retained in the cells, suggesting that 10,11-epoxy-16:2 is a major product of 14,15-EET metabolism in HCEC. 10,11-Epoxy-16:2 produced potent dilation in coronary microvessels. 10,11-Epoxy-16:2 also potently inhibited tumor necrosis factor-alpha-induced production of IL-8, a proinflammatory cytokine, by HCEC. These findings implicate beta-oxidation as a major pathway of 14,15-EET metabolism in HCEC and provide the first evidence that EET-derived chain-shortened epoxy fatty acids are biologically active.

Comparison of 20-, 22-, and 24-carbon n-3 and n-6 polyunsaturated fatty acid utilization in differentiated rat brain astrocytes

Astrocytes convert n-6 fatty acids primarily to arachidonic acid (20:4n-6), whereas n-3 fatty acids are converted to docosapentaenoic (22:5n-3) and docosahexaenoic (22:6n-3) acids. The utilization of 20-, 22- and 24-carbon n-3 and n-6 fatty acids was compared in differentiated rat astrocytes to determine the metabolic basis for this difference. The astrocytes retained 81% of the arachidonic acid ([(3)H]20:4n-6) uptake and retroconverted 57% of the docosatetraenoic acid ([3-(14)C]22:4n-6) uptake to 20:4n-6. By contrast, 68% of the eicosapentaenoic acid ([(3)H]20:5n-3) uptake was elongated, and only 9% of the [3-(14)C]22:5n-3 uptake was retroconverted to 20:5n-3. Both tetracosapentaenoic acid ([3-(14)C]24:5n-3) and tetracosatetraenoic acid ([3-(14)C]24:4n-6) were converted to docosahexaenoic acid (22:6n-3) and 22:5n-6, respectively. Therefore, the difference in the n-3 and n-6 fatty acid products formed is due primarily to differences in the utilization of their 20- and 22-carbon intermediates. This metabolic difference probably contributes to the preferential accumulation of docosahexaenoic acid in the brain.

Arachidonate dilates basilar artery by lipoxygenase-dependent mechanism and activation of K(+) channels

Dilatation of cerebral arterioles in response to arachidonic acid is dependent on activity of cyclooxygenase. In this study, we examined mechanisms that mediate dilatation of the basilar artery in response to arachidonate. Diameter of the basilar artery (baseline diameter = 216 +/- 7 micrometer) (means +/- SE) was measured using a cranial window in anesthetized rats. Arachidonic acid (10 and 100 microM) produced concentration-dependent vasodilatation that was not inhibited by indomethacin (10 mg/kg iv) or N(G)-nitro-L-arginine (100 microM) but was inhibited markedly by baicalein (10 micrometerM) or nordihydroguaiaretic acid (NDGA; 10 microM), inhibitors of the lipoxygenase pathway. Dilatation of the basilar artery was also inhibited markedly by tetraethylammonium ion (TEA; 1 mM) or iberiotoxin (50 nM), inhibitors of calcium-dependent potassium channels. For example, 10 microM arachidonate dilated the basilar artery by 19 +/- 7 and 1 +/- 1% in the absence and presence of iberiotoxin, respectively. Measurements of membrane potential indicated that arachidonate produced hyperpolarization of the basilar artery that was blocked completely by TEA. Incubation with [(3)H]arachidonic acid followed by reverse-phase and chiral HPLC indicated that the basilar artery produces relatively small quantities of prostanoids but large quantities of 12(S)-hydroxyeicosatetraenoic acid (12-S-HETE), a lipoxygenase product. Moreover, the production of 12-HETE was inhibited by baicalein or NDGA. These findings suggest that dilatation of the basilar artery in response to arachidonate is mediated by a product(s) of the lipoxygenase pathway, with activation of calcium-dependent potassium channels and hyperpolarization of vascular muscle.

Pathways of epoxyeicosatrienoic acid metabolism in endothelial cells. Implications for the vascular effects of soluble epoxide hydrolase inhibition

Epoxyeicosatrienoic acids (EETs) are products of cytochrome P-450 epoxygenase that possess important vasodilating and anti-inflammatory properties. EETs are converted to the corresponding dihydroxyeicosatrienoic acid (DHET) by soluble epoxide hydrolase (sEH) in mammalian tissues, and inhibition of sEH has been proposed as a novel approach for the treatment of hypertension. We observed that sEH is present in porcine coronary endothelial cells (PCEC), and we found that low concentrations of N,N'-dicyclohexylurea (DCU), a selective sEH inhibitor, have profound effects on EET metabolism in PCEC cultures. Treatment with 3 microM DCU reduced cellular conversion of 14,15-EET to 14,15-DHET by 3-fold after 4 h of incubation, with a concomitant increase in the formation of the novel beta-oxidation products 10,11-epoxy-16:2 and 8,9-epoxy-14:1. DCU also markedly enhanced the incorporation of 14,15-EET and its metabolites into PCEC lipids. The most abundant product in DCU-treated cells was 16,17-epoxy-22:3, the elongation product of 14,15-EET. Another novel metabolite, 14,15-epoxy-20:2, was present in DCU-treated cells. DCU also caused a 4-fold increase in release of 14,15-EET when the cells were stimulated with a calcium ionophore. Furthermore, DCU decreased the conversion of [3H]11,12-EET to 11,12-DHET, increased 11,12-EET retention in PCEC lipids, and produced an accumulation of the partial beta-oxidation product 7,8-epoxy-16:2 in the medium. These findings suggest that in addition to being metabolized by sEH, EETs are substrates for beta-oxidation and chain elongation in endothelial cells and that there is considerable interaction among the three pathways. The modulation of EET metabolism by DCU provides novel insight into the mechanisms by which pharmacological or molecular inhibition of sEH effectively treats hypertension.

12-lipoxygenase in porcine coronary microcirculation: implications for coronary vasoregulation

Noncyclooxygenase metabolites of arachidonic acid (AA) have been proposed to mediate endothelium-dependent vasodilation in the coronary microcirculation. Therefore, we examined the formation and bioactivity of AA metabolites in porcine coronary (PC) microvascular endothelial cells and microvessels, respectively. The major noncyclooxygenase metabolite produced by microvascular endothelial cells was 12(S)-hydroxyeicosatetraenoic acid (HETE), a lipoxygenase product. 12(S)-HETE release was markedly increased by pretreatment with 13(S)-hydroperoxyoctadecadienoic acid but not by the reduced congener 13(S)-hydroxyoctadecadienoic acid, suggesting oxidative upregulation of 12(S)-HETE output. 12(S)-HETE produced potent relaxation and hyperpolarization of PC microvessels (EC(50), expressed as -log[M] = 13.5 +/- 0.5). Moreover, 12(S)-HETE potently activated large-conductance Ca(2+)-activated K(+) currents in PC microvascular smooth muscle cells. In contrast, 12(S)-HETE was not a major product of conduit PC endothelial AA metabolism and did not exhibit potent bioactivity in conduit PC arteries. We suggest that, in the coronary microcirculation, 12(S)-HETE can function as a potent hyperpolarizing vasodilator that may contribute to endothelium-dependent relaxation, particularly in the setting of oxidative stress.

Overexpression of human catalase gene decreases oxidized lipid-induced cytotoxicity in vascular smooth muscle cells

Reactive oxygen metabolites such as hydrogen peroxide (H(2)O(2)) and oxidized fatty acids are proinflammatory and are involved in the pathophysiology of various diseases including atherosclerosis. The effects of these oxidants could be inhibited by the external addition of an antioxidant, suggesting the promotion or propagation of further oxidation. In this study, we describe the stable overexpression of human catalase in smooth muscle cells and the resistance of these cells to cytotoxicity induced not only by the addition of H(2)O(2) but also by the addition of 13-hydroperoxyoctadecadienoic acid (13-HPODE). The results pose an intriguing possibility of the generation of H(2)O(2) from a peroxidized fatty acid. Accordingly, incubation of cells with both 13-HPODE and 13-hydroxyoctadecadienoic acid resulted in the generation of intracellular H(2)O(2). To explain the observed results by which catalase could overcome the effects of 13-HPODE, we propose that oxidized fatty acids are degraded in the cellular peroxisomes, resulting in the generation of H(2)O(2). In other words, the cellular effects of peroxidized fatty acids could be attributed to the generation of H(2)O(2).

Role of peroxisomal oxidation in the conversion of arachidonic acid to eicosatrienoic acid in human skin fibroblasts

Human skin fibroblasts converted [5,6,8,9,11,12,14,15-3H]arachidonic acid ([3H]20:4) to eicosatrienoic acid (20:3), but appreciable amounts of radiolabeled 20:3 were not detected in corresponding incubations with [1-(14)C]20:4. This indicates that the main pathway for synthesizing 20:3 from arachidonic acid in the fibroblast involves oxidative removal of the carboxyl group of arachidonic acid. Fibroblasts deficient in long-chain acyl coenzyme A dehydrogenase (LCAD) converted [3H]20:4 to [3H]20:3. However, Zellweger fibroblasts that are deficient in peroxisomal fatty acid oxidation did not, indicating that the oxidative removal of the carboxyl group occurs in the peroxisomes. [3H]Hexadecatrienoic acid (16:3) was the main product that accumulated when [3H]20:4 was incubated with normal, LCAD deficient, and very long-chain acyl coenzyme A dehydrogenase (VLCAD) deficient fibroblasts, but Zellweger fibroblasts did not form this product. Normal fibroblasts converted [3H]16:3 to radiolabeled 20:3 and arachidonic acid. These findings suggest that some of the 16:3 produced from arachidonic acid by peroxisomal beta-oxidation can be recycled and that this recycling process constitutes a novel pathway for the conversion of arachidonic acid to 20:3 in human fibroblasts.

A comparison of the metabolism of [3-14C]-labeled 22- and 24-carbon (n-3) and (n-6) unsaturated fatty acids by rat testes and liver

The unsaturated fatty acid composition of phospholipids from different tissues frequently varies. Rat liver phospholipids contain esterified 22:6(n-3) while 22:5(n-6) is the major esterified 22-carbon acid in testes phospholipids. Both testes and liver synthesize polyunsaturated fatty acids. Microsomes, particularly from liver, have been used extensively to measure reaction rates as they relate to polyunsaturated fatty acid and phospholipid biosynthesis. None of these rate studies explain why specific acids are synthesized and subsequently esterified. In this study we compared the metabolism of [3-14C]-labeled (n-3) and (n-6) acids when injected via the tail vein, as a measure of hepatic metabolism, versus when they were injected directly into the testes. Liver preferentially metabolizes [3-14C]-labeled 24:5(n-3) and 24:6(n-3) to yield esterified 22:6(n-3), when compared with the conversion of [3-14C]-labeled 24:4(n-6) and 24:5(n-6) to yield 22:5(n-6). Both 24-carbon (n-3) acids were also converted to 22:5(n-3) but no labeled 22:4(n-6) was detected after injecting the two 24-carbon (n-6) acids. Differences in the hepatic metabolism of 24-carbon (n-3) and (n-6) acids to 22:6(n-3) and 22:5(n-6), versus their partial beta-oxidation to 22:5(n-3) and 22:4(n-6), are important in vivo controls. Surprisingly, in testes a higher percentage of radioactivity was found in esterified 22:6(n-3) versus 22:5(n-6) following injections, respectively, of [3-14C]-labeled 22:5(n-3) versus 22:4(n-6), which is the corresponding metabolic analog. Corresponding pairs of 24-carbon (n-3) and (n-6) acids, as they relate to metabolism, were processed in similar ways by testes. The relative absence of esterified 22-carbon (n-3) fatty acids, versus the abundance of 22- and 24-carbon (n-6) acids in testes phospholipids, does not appear per se to be due to differences in the ability of testes to metabolize (n-3) and (n-6) fatty acids. It remains to be determined if there is selective uptake of specific fatty acids by testes for use as precursors to synthesize polyunsaturated fatty acids.

The beta-oxidation of arachidonic acid and the synthesis of docosahexaenoic acid are selectively and consistently altered in skin fibroblasts from three Zellweger patients versus X-adrenoleukodystrophy, Alzheimer and control subjects

The beta-oxidation of [3H] arachidonic acid (AA; 20:4 n-6) and the conversion of [1-14C]eicosapentaenoic acid (EPA, 20:5 n-3) to docosahexaenoic acid (DHA, 22:6 n-3) have been studied in skin fibroblasts from patients with inherited peroxisomal diseases, such as Zellweger (ZW) and X-linked adrenoleukodystrophy (X-ALD), from patients with Alzheimer's disease (AD), a non-inherited neuropathology, and from controls. EPA is not converted to DHA, while there is enhanced formation of the intermediate product 22:5 n-3 in ZW, when compared to X-ALD, AD and controls. We also confirmed that AA is not beta-oxidized to 4,7,10-hexadecatrienoic acid (16:3), a metabolite produced by peroxisomes, while being more effectively converted to the elongation product 22:4, in ZW, in comparison to X-ALD, AD and controls. The data demonstrate a defect in DHA synthesis and in AA beta-oxidation, and the occurrence of associated adaptative modifications in the metabolism of these long chain PUFA, in three Italian ZW patients.

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.

Recent advances in the biology of n-6 fatty acids

The intensive research carried out in the last 10 years on the unique biological functions of n-3 fatty acids (FA), has promoted comparative investigations on various aspects (metabolic, functional) of the biology of n-6 FA. The involvement of peroxisomes in fatty acid metabolism, initially described for the n-3 acids, has now been shown also for the n-6 FA (formation of 22 carbon delta 4 unsaturated FA, formation of newly identified products of beta-oxidation of arachidonic acid, AA). Additional pathways of AA conversion, beyond the classical eicosanoids, give rise to a series of biologically active products, such as the epoxides, involved in the modulation of vascular functions, through the cytochrome p450 system, and to the AA-ethanolamide, anandamide, an endogenous ligand of the cannabinoid receptors, through a phospholipase-mediated process. Finally, nonenzymatic oxidation products of AA, the isoprostanes, isomers of prostaglandins, also endowed of potent biological activities, are generated both in in vitro-induced lipid oxidation and in vivo, being considered as reliable markers of in vivo oxidative stress. As to the nutritional aspects of the n-6 FA, attention is now paid to the intake of preformed long-chain polyunsaturated FA (PUFA) in the n-6 series, mainly AA, through the diet, in analogy with the intake of the long-chain n-3 FA, in fish-eating populations. The importance of the dietary intake of preformed AA is now recognized in newborns, through maternal milk. The ranges of the intakes of AA in population groups, not currently adequately estimated, appear to be wider than generally assumed, and the elevated intakes in some population groups, in the order of several hundred milligrams per day, may be partly responsible of yet unexplored population-based differences in physiologic variables. Recent research on the functional effects of n-6 FA has confirmed their lipid-lowering effects, which can be observed also in neonates, and has shown that, in cooperation with the n-3, they directly and indirectly contribute to modulate functional parameters at the cellular level, such as receptor function, ion channels, and gene expression. From a nutritional point of view, it is clear that PUFA represent the biologically most active component of dietary fat, and the n-6 are quantitatively the most relevant fraction in our diet. In the light of the diversified activities of n-6 and n-3 PUFA, a correct balance between the various fatty acids is recommended.

Conversion of arachidonic acid to tetradecadienoic acid by peroxisomal oxidation

Human skin fibroblasts convert [5,6,8,9,11,12,14,15-3H]arachidonic acid to two radiolabeled polar metabolites that accumulate in the culture medium. Previous studies identified the most abundant of these products as 4,7,10-hexadecatrienoic acid (16:3). We have now identified the second metabolite as 5,8-tetradecadienoic acid (14:2). Fibroblasts deficient in mitochondrial long-chain acyl coenzyme A dehydrogenase produce increased amounts of 14:2 from arachidonic acid. By contrast, Zellweger fibroblasts which are deficient in peroxisomal beta-oxidation do not convert arachidonic acid to either 14:2 or 16:3. These results demonstrate that 14:2 can be synthesized from arachidonic acid, that this oxidative process occurs in the peroxisomes, and that the pathway does not function in Zellweger's syndrome and similar diseases where there is a genetic deficiency in peroxisomal beta-oxidation.

Metabolites produced during the peroxisomal beta-oxidation of linoleate and arachidonate move to microsomes for conversion back to linoleate

When [1-(14)C]4,7,10-16:3, a product produced after two cycles of arachidonate beta-oxidation, was incubated with rat liver peroxisomes and microsomes it was metabolized to 2-trans-4,7,10-16:4, a catabolic product; 6,9,12-18:3 and 8,11,14-20:3, anabolic products made via microsomal chain elongation of the substrate; and 7,10-16:2 and 9,12-18:2. Analysis of the acyl-CoAs produced when 6,9,12-18:3 and its catabolic product, 4,7,10-16:3, where incubated under the above conditions showed that the acyl-CoAs of all of the above compounds, as well as 5,8-14:2-CoA and 6:0-CoA accumulated. Our results show that when 5,8-14:2 and 4,7,10-16:3 are produced by peroxisomal beta-oxidation they can be further degraded to hexanoyl-CoA or move to microsomes for conversion back to linoleate, which is a precursor of arachidonate.

Functional implications of a newly characterized pathway of 11,12-epoxyeicosatrienoic acid metabolism in arterial smooth muscle

Epoxyeicosatrienoic acids (EETs) are potent vasodilators derived from cytochrome P-450 metabolism of arachidonic acid. The rapid conversion of EETs to their corresponding dihydroxyeicosatrienoic acids (DHETs) has been proposed as a process whereby EETs are rendered biologically inactive. However, the vascular metabolism of EETs and the vasoactivities of EET metabolites have not been extensively studied. Accordingly, 11,12-EET metabolism was characterized in porcine aortic smooth muscle cells. The cells converted [3H]11,12-EET to 11,12-DHET and to a newly identified metabolite, 7,8-dihydroxy-hexadecadienoic acid (DHHD). 11,12-DHET accumulation in the medium reached a maximum in 2 to 4 hours and then declined, whereas 7,8-DHHD accumulation increased continuously and exceeded the amount of 11,12-DHET by 8 hours. [3H]11,12-EET conversion to radiolabeled 7,8-DHHD was reduced in the presence of unlabeled 11,12-DHET, indicating that 11,12-DHET is an intermediate in the conversion of 11,12-EET to 7,8-DHHD. This is consistent with a pathway whereby 11,12-EET is converted by an epoxide hydrolase to 11,12-DHET, which then undergoes two beta-oxidations to form 7,8-DHHD. In porcine coronary artery rings contracted with a thromboxane mimetic, 11,12-DHET produced relaxation similar in magnitude to that produced by 11,12-EET (77% versus 64% relaxation at 5 mumol/L, respectively). 7,8-DHHD also produced vasorelaxation. Thus, the vasoactivity of 11,12-EET is not eliminated by conversion to 11,12-DHET and 7,8-DHHD. These results suggest that 11,12-DHET and its metabolite, 7,8-DHHD, may contribute to the regulation of vascular tone in the porcine coronary artery and possibly other vascular tissues.

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.

Formation of a novel arachidonic acid metabolite in peroxisomes

A new radiolabeled metabolite was released into the extracellular fluid by normal human skin fibroblasts that were labeled with [5,6,8,9,11,12,14,15-3H] arachidonic acid. This product continued to accumulate during a 24 h incubation, and its formation was not saturated at arachidonic acid concentrations up to 15 mumol/L. The compound, identified as hexadecatrienoic acid, was not produced by Zellweger fibroblasts which are deficient in peroxisomal fatty acid beta-oxidation. By contrast, radiolabeled hexadecatrienoic acid was produced by mutant fibroblasts having other peroxisomal defects, including X-linked adrenoleukodystrophy, adult Refsum's disease, and rhizomelic chondrodysplasia punctata. This radiolabeled metabolite also was produced by mutant fibroblasts that cannot oxidize long-chain fatty acids in the mitochondria. These results indicate that hexadecatrienoic acid is synthesized from arachidonic acid by peroxisomal beta-oxidation. The absence of this pathway may account for some of the biochemical and functional abnormalities that occur in Zellweger's syndrome.