We tested here the hypothesis that some of Caenorhabditis elegans cytochrome P450 (CYP) forms may be involved in the metabolism of polyunsaturated fatty acids (PUFA), especially eicosapentaenoic acid (EPA), the predominant PUFA of this nematode. Microsomes isolated from adult worms contained spectrally active CYP proteins and showed NADPH-CYP reductase (CPR) activities. They metabolized EPA and, with lower activity, also arachidonic acid (AA) to specific sets of regioisomeric epoxy- and w-/(w-1)-hydroxy-derivatives. 17(R),18(S)-epoxyeicosatetraenoic acid was produced as the main EPA metabolite with an enantiomeric purity of 72 %. The epoxygenase and hydroxylase reactions were NADPH-dependent, required the functional expression of the CPR-encoding
emb-8 gene, and were inhibited by 17-ODYA and PPOH, two compounds known to inactivate mammalian AA-metabolizing CYP isoforms. Multiple followed by single RNAi gene silencing experiments identified CYP-29A3 and CYP-33E2 as the major isoforms contributing to EPA metabolism in C. elegans. Liquid chromatography/ mass spectrometry revealed that regioisomeric epoxy- and hydroxy-derivatives of EPA and AA are endogenous constituents of C. elegans. The endogenous EPA metabolite levels were increased by treating the worms with fenofibrate, which also induced the microsomal epoxygenase and hydroxylase activities. Heterologous expression of cyp-29A3 and cyp-33E2 in SF9 insect cells is in progress. These results demonstrate for the first time that C. elegans shares with mammals the capacity to produce CYP-dependent eicosanoids and may thus facilitate future studies on the mechanisms of action of this important class of signaling molecules.