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 Sod1 p variant (Sod[1.sup.H46C]) showed that the superoxide dismutase activity rather than the metal-binding function of Sod1 p 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] (yc1033c[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 Or toxicity was corroborated by an observed ~ 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.