SHORT TITLE:CYP-13A12 of C. elegans is a PUFA-epoxygenase 4 SYNOPSISA specific behavioural response of Caenorhabditis elegans, the rapid increase of locomotion in response to anoxia/reoxygenation called the O2-ON response, has been used to model key aspects of ischemia/reperfusion injury. A genetic suppressor screen demonstrated a direct causal role of CYP-13A12 in this response and suggested that CYP-eicosanoids, which in mammals influence the contractility of cardiomyocytes and vascular smooth muscle cells, might function in C. elegans as specific regulators of the body muscle cell activity. Here we show that co-expression of CYP-13A12 with the NADPH-CYP-reductase EMB-8 in insect cells resulted in the reconstitution of an active microsomal monooxygenase system that metabolised EPA (eicosapentaenoic acid) and also AA (arachidonic acid) to specific sets of regioisomeric epoxy and hydroxy derivatives. Main products included 17,18-EEQ (epoxyeicosatetraenoic acid) from EPA and 14,15-EET (epoxyeicosatrienoic acid) from AA. Locomotion assays showed that the defective O2-ON response of C20-PUFA-deficient, ∆ -12 and ∆ -6 fatty acid desaturase mutants (fat-2 and fat-3, respectively) can be restored by feeding the nematodes AA or EPA, but not ETYA (eicosatetraynoic acid), a non-metabolisable AAanalogue. Already short-term incubation with 17,18-EEQ was sufficient to rescue the impaired locomotion of the fat-3 strain. The endogenous level of free 17,18-EEQ declined during anoxia and was rapidly restored in response to reoxygenation. Based on these results, we suggest that CYP-dependent eicosanoids such as 17,18-EEQ function as signalling molecules in the regulation of the O2-ON response in C. elegans. Remarkably, the exogenously administered 17,18-EEQ increased the locomotion activity already under normoxic conditions and was effective not only with C20-PUFA mutants but to a lesser extent also with wild-type worms. KEYWORDS:Eicosanoids; Polyunsaturated fatty acids; Caenorhabditis elegans; Cytochrome P450; Reoxygenation; 17,18-EEQ ABBREVIATIONS USED: AA, arachidonic acid; CID, collision-induced dissociation; COD, carbon monoxide difference; CPR, NADPH-CYP reductase; CYP, cytochrome P450; DiHETE, dihydroxyeicosatetraenoic acid; DTT, dithiothreitol; EGL, egg laying defect; EGLN, EGL nine; EEQ, epoxyeicosatetraenoic acid; EET, epoxyeicosatrienoic acid; EPA, eicosapentaenoic acid; ETYA, eicosatetraynoic acid; GFP, green fluorescent protein; GPCR, G protein-coupled receptor; HEET, hydroxyepoxyeicosatrienoic acid; HEPE, hydroxyeicosapentaenoic acid; HETE, hydroxyeicosatetraenoic acid; HIF, hypoxia inducible factor; LC, liquid chromatography; MC, pharyngeal marginal cell; MS, mass spectrometry; NGM, nematode growth medium; PUFA, polyunsaturated fatty acid; RP, reversed-phase 5 INTRODUCTIONOxygen deprivation upon restriction of blood supply followed by reperfusion and concomitant reoxygenation causes tissue injury and is involved in the initiation of various human pathologies including ischemic stroke, myocardial infarction, and ac...