Dependence of peroxisomal beta-oxidation on cytosolic sources of NADPH

Growth of Saccharomyces cerevisiae with a fatty acid as carbon source was s hown previously to require function of either glucose-6-phosphate dehydroge nase (ZWF1) or cytosolic NADP(+)-specific isocitrate dehydrogenase (IDP2), suggesting dependence of beta-oxidation on a cytosolic source of NADPH. In this study, we find that Delta IDP2 Delta ZWF1 strains containing disruptio ns in genes encoding both enzymes exhibit a rapid loss of viability when tr ansferred to medium containing oleate as the carbon source. This loss of vi ability is not observed following transfer of a Delta IDP3 strain lacking p eroxisomal isocitrate dehydrogenase to medium with docosahexaenoate, a nonp ermissive carbon source that requires function of IDP3 for beta-oxidation. This suggests that the fatty acid(-) phenotype of Delta IDP2 Delta ZWF1 str ains is not a simple defect in utilization. Instead, we propose that the co mmon function shared by IDP2 and ZWF1 is maintenance of significant levels of NADPH for enzymatic removal of the hydrogen peroxide generated in the fi rst step of peroxisomal beta-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 peroxi de previously reported for Delta ZWF1 mutant strains is less pronounced whe n analyses are conducted with a nonfermentable carbon source, a condition a ssociated with elevated expression of IDP2. Under those conditions, similar slow growth phenotypes are observed for Delta ZWF1 and hIDP2 strains, and co-disruption of both genes dramatically exacerbates the H2O2s phenotype, C ollectively, these results suggest that IDP2, when expressed, and ZWF1 have critical overlapping functions in provision of reducing equivalents for de fense against endogenous or exogenous sources of H2O2.