Double bond removal from odd-numbered carbons during peroxisomal β- oxidation of arachidonic acid requires both 2,4-dienoyl-CoA reductase and Δ3,5,Δ2,4-dienoyl-CoA isomerase

Abstract

The pathway for the peroxisomal β-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 Δ3,5,Δ2,4-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 Δ3,5,Δ2,4-dienoyl-CoA isomerase. One complete cycle of 5,8-14:2 and 5,8,11,14-20:4 β-oxidation yields, respectively, 6-dodecenoic and 6,9,12- octadecatrienoic acids.