Dependence of peroxisomal β-oxidation on cytosolic sources of NADPH

Growth of Saccharomyces cerevisiae with a fatty acid as carbon source was shown previously to require function of either glucose-6-phosphate dehydrogenase (ZWF1) or cytosolic NADP⁺-specific isocitrate dehydrogenase (IDP2), suggesting dependence of β-oxidation on a cytosolic source of NADPH. In this study, we find that ΔIDP2ΔZWF1 strains containing disruptions in genes encoding both enzymes exhibit a rapid loss of viability when transferred to medium containing oleate as the carbon source. This loss of viability is not observed following transfer of a ΔIDP3 strain lacking peroxisomal isocitrate dehydrogenase to medium with docosahexaenoate, a nonpermissive carbon source that requires function of IDP3 for β-oxidation. This suggests that the fatty acid^- phenotype of ΔIDP2ΔZWF1 strains is not a simple defect in utilization. Instead, we propose that the common function shared by IDP2 and ZWF1 is maintenance of significant levels of NADPH for enzymatic removal of the hydrogen peroxide generated in the first step of peroxisomal β-oxidation in yeast and that inadequate levels of the reduced form of the cofactor can produce lethality. This proposal is supported by the finding that the sensitivity to exogenous hydrogen peroxide previously reported for ΔZWF1 mutant strains is less pronounced when analyses are conducted with a nonfermentable carbon source, a condition associated with elevated expression of IDP2. Under those conditions, similar slow growth phenotypes are observed for ΔZWF1 and ΔIDP2 strains, and co-disruption of both genes dramatically exacerbates the H₂O₂(s) phenotype. Collectively, these results suggest that IDP2, when expressed, and ZWF1 have critical overlapping functions in provision of reducing equivalents for defense against endogenous or exogenous sources of H₂O₂.