Oxidative protein damage causes chromium toxicity in yeast

Oxidative damage in microbial cells occurs during exposure to the toxic metal chromium, but it is not certain whether such oxidation accounts for the toxicity of Cr. Here, a Saccharomyces cerevisiae sod1 Delta mutant (defective for the Cu,Zn-superoxide dismutase) was found to be hypersensitive to Cr(VI) toxicity under aerobic conditions, but this phenotype was suppressed under anaerobic conditions. Studies with cells expressing a Sod1p, variant (Sod1(H46c)) showed that the superoxide dismutase activity rather than the metal-binding function of Sod1p was required for Cr resistance. To help identify the macromolecular target(s) of Cr-dependent oxidative damage, cells deficient for the reduction of phospholipid hydroperoxides (gpx3 Delta and gpx1 Delta/gpx2 Delta/gpx3 Delta) and for the repair of DNA oxidation (ogg1 Delta and rad30 Delta/ogg1 Delta) were tested, but were found not to be Cr-sensitive. In contrast, S. cerevisiae msra Delta (mxr1 Delta) and msrb Delta (yc/033c Delta) mutants defective for peptide methionine sulfoxide reductase (msr) activity exhibited a Cr sensitivity phenotype, and cells overexpressing these enzymes were Cr-resistant. Overexpression of MSRs also suppressed the Cr sensitivity of sod1 Delta cells. The inference that protein oxidation is a primary mechanism of Cr toxicity was corroborated by an observed similar to 20-fold increase in the cellular levels of protein carbonyls within 30 min of Cr exposure. Carbonylation was not distributed evenly among the expressed proteins of the cells; certain glycolytic enzymes and heat-shock proteins were specifically targeted by Cr-dependent oxidative damage. This study establishes an oxidative mode of Cr toxicity in S. cerevisiae, which primarily involves oxidative damage to cellular proteins.