Alternatives to the Isomerase-dependent Pathway for the β-Oxidation of Oleic Acid Are Dispensable in Saccharomyces cerevisiae

Fatty acids with double bonds at odd-numbered positions such as oleic acid can enter β-oxidation via a pathway relying solely on the auxiliary enzyme Δ3-Δ2-enoyl-CoA isomerase, termed the isomerase-dependent pathway. Two novel alternative pathways have recently been postulated to exist in mammals, and these additionally depend on Δ3,5-Δ2,4-dienoyl-CoA isomerase (di-isomerase-dependent) or on Δ3,5-Δ2,4-dienoyl-CoA isomerase and 2,4-dienoyl-CoA reductase (reductase-dependent). We report the identification of the Saccharomyces cerevisiae oleic acid-inducible DCI1 (YOR180c) gene encoding peroxisomal di-isomerase. Enzyme assays conducted on soluble extracts derived from yeast cells overproducing Dci1p using 3,5,8,11,14-eicosapentenoyl-CoA as substrate demonstrated a specific di-isomerase activity of 6 nmol × min−1 per mg of protein. Similarly enriched extracts from eci1Δ cells lacking peroxisomal 3,2-isomerase additionally contained an intrinsic 3,2-isomerase activity that could generate 3,5,8,11,14-eicosapentenoyl-CoA from 2,5,8,11,14-eicosapentenoyl-CoA but not metabolize trans-3-hexenoyl-CoA. Amplification of this intrinsic activity replaced Eci1p since it restored growth of the eci1Δ strain on petroselinic acid for which di-isomerase is not required whereas Eci1p is. Heterologous expression in yeast of rat di-isomerase resulted in a peroxisomal protein that was enzymatically active but did not re-establish growth of the eci1Δ mutant on oleic acid. A strain devoid of Dci1p grew on oleic acid to wild-type levels, whereas one lacking both Eci1p and Dci1p grew as poorly as the eci1Δ mutant. Hence, we reasoned that yeast di-isomerase does not additionally represent a physiological 3,2-isomerase and that Dci1p and the postulated alternative pathways in which it is entrained are dispensable for degrading oleic acid.