We report here the identification of the Saccharomyces cerevisiae peroxisomal ⌬ 3 ,⌬ 2 -enoyl-CoA isomerase, an enzyme that is essential for the ␤-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 ⌬ 3 ,⌬ 2 -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 ⌬ 3 ,⌬ 2 -enoyl-CoA isomerase and propose a new name, ECI1, to reflect its enoyl-CoA isomerase activity.

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 ␤-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 ␤-oxidation cycle or be converted by ⌬ 3 ,⌬ 2 -enoyl-CoA isomerase to 3,5,7-decatrienoyl-CoA. The latter compound was isomerized by a novel enzyme, named ⌬ 3,5,7 ,⌬ 2,4,6 -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 ␤-oxidation. ⌬ 3,5,7 ,⌬ 2,4,6 -TrienoylCoA isomerase was purified from pig heart to apparent homogeneity and found to be a component enzyme of ⌬ 3,5 ,⌬ 2,4 -dienoyl-CoA isomerase. Although the direct ␤-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 ␤-oxidation of this intermediate.The ␤-oxidation of typical polyunsaturated fatty acids requires the involvement of three auxiliary enzymes in addition to the enzymes that catalyze the four basic reactions of the ␤-oxidation spiral (1). The auxiliary enzymes are ⌬ 3 ,⌬ 2 -enoylCoA isomerase (⌬ 3 -cis-⌬ 2 -trans-enoyl-CoA isomerase, EC 5.3.3.8; referred to hereafter as enoyl-CoA isomerase), 2,4-dienoyl-CoA reductase (4-enoyl-CoA reductase (NADPH), EC 1.3.1.34), and ⌬ 3,5 ,⌬ 2,4 -dienoyl-CoA isomerase (referred to hereafter as dienoyl-CoA isomerase). These enzymes catalyze either the reduction or isomerization of double bonds once the double bonds are close to the thioester function as the result of chain shortening. Consequently, double bonds either are reductively removed or are shifted to yield 2-trans-enoyl-CoAs, which are intermediates of the ␤-oxidation spiral. Polyunsaturated fatty acid with conjugated double bonds may yield intermediates with more extended chromophores. For example, a fatty acid with two conjugated double bonds at even-numbered positions is assumed to be chain-shortened by ␤-oxidation to 4,6-dienoyl-CoA and is then converted to 2,4,6-trienoyl-CoA by acyl-CoA dehydrogenase. The further metabolism of this intermediate is facilitated by 2,4-dienoyl-CoA reductase, which catalyzes the reduction of one double bond of the 2,4,6-trienoylCoA chromophore to yield 3,6-dienoyl-CoA (2).The ␤-oxidation of a fatty acid with two conjugated double bonds at odd-numbered positions would produce 5,7-dienoylCoA, which may be dehydrogenated by acyl-CoA dehydrogenase to 2,5,7-trienoyl-CoA. The latter compound may be chainshortened to 3,5-dienoyl-CoA or isomerized to 3,5,7-trienoylCoA by enoyl-CoA isomerase. The further metabolism of 3,5,7-trieno...