Conversion of arachidonic acid to tetradecadienoic acid by peroxisomal oxidation

Is autism a disorder of fatty acid metabolism? Possible dysfunction of mitochondrial beta-oxidation by long chain acyl-CoA dehydrogenase

Long chain acyl-CoA dehydrogenase (LCAD) has recently been shown to be the mitochondrial enzyme responsible for the beta-oxidation of branched chain and unsaturated fatty acids [Biochim. Biophys. Acta 1393 (1998) 35; Biochim. Biophys. Acta 1485 (2000) 121]. Whilst disorders of short, medium and very long chain acyl dehydrogenases are known, there is no known disorder of LCAD deficiency in humans. Experimental LCAD deficiency in mice shows an acyl-carnitine profile with prominent elevations of unsaturated fatty acid metabolites C14:1 and C14:2 [Hum. Mol. Genet. 10 (2001) 2069]. A child with autism whose acyl-carnitine profile also shows these abnormalities is presented, and it is hypothesized that the child may have LCAD deficiency. Additional metabolic abnormalities seen in this patient include alterations of TCA energy production, ammonia detoxification, reduced synthesis of omega-3 DHA, and abnormal cholesterol metabolism. These metabolic changes are also seen as secondary abnormalities in dysfunction of fatty acid beta-oxidation, and have also been reported in autism. It is hypothesized that LCAD deficiency may be a cause of autism. Similarities between metabolic disturbances in autism, and those of disorders of fatty acid beta-oxidation are discussed.

Intermediates of unsaturated fatty acid oxidation are incorporated in triglycerides but not in phospholipids in tissues from patients with mitochondrial beta-oxidation defects

The fatty acid composition was determined of liver, skeletal muscle and heart obtained post mortem from patients with medium-chain acyl-CoA dehydrogenase deficiency (MCADD), multiple acyl-CoA dehydrogenase deficiency (MADD) and very long-chain acyl-CoA dehydrogenase deficiency (VLCADD). Increased amounts of 4-decenoic acid 10:1(n-6), 5-dodecenoic acid 12:1(n-7), 5-tetradecenoic acid 14:1(n-9), 5,8-tetradecadienoic acid 14:2(n-6) and 7,10-hexadecadienoic acid 16:2(n-6)--intermediates of unsaturated fatty acid oxidation--were found. Fractionation into different lipid classes showed that these fatty acids were exclusively present in the triglyceride fraction. They could not be detected in the free fatty acid fraction or in the phospholipid fraction. Our results suggest that intermediates of unsaturated fatty acid oxidation that accumulate as a consequence of MCADD, MADD and VLCADD are transported to the endoplasmic reticulum for esterification into neutral glycerolipids. The pattern of accumulation is characteristic for each disease, which makes fatty acid analysis of total lipid of post-mortem tissues a useful tool in the detection of mitochondrial fatty acid oxidation defects in patients who died unexpected, for example with sudden infant death syndrome.

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.

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.