Globins Scavenge Sulfur Trioxide Anion Radical

Background: Sulfite, an intermediate in sulfur metabolism, can be oxidized to the potentially toxic sulfur trioxide anion radical (STAR).Results: Diverse globins efficiently reduced STAR in vitro, and flavohemoglobin protected yeast from STAR toxicity.Conclusion: Globins can function as STAR scavengers.Significance: The data suggest roles for diverse globins in protecting cells against sulfite stress.Ferrous myoglobin was oxidized by sulfur trioxide anion radical (STAR) during the free radical chain oxidation of sulfite. Oxidation was inhibited by the STAR scavenger GSH and by the heme ligand CO. Bimolecular rate constants for the reaction of STAR with several ferrous globins and biomolecules were determined by kinetic competition. Reaction rate constants for myoglobin, hemoglobin, neuroglobin, and flavohemoglobin are large at 38, 120, 2,600, and >= 7,500 x 10(6) m(-1) s(-1), respectively, and correlate with redox potentials. Measured rate constants for O-2, GSH, ascorbate, and NAD(P)H are also large at -100, 10, 130, and 30 x 10(6) M-1 s(-1), respectively, but nevertheless allow for favorable competition by globins and a capacity for STAR scavenging in vivo. Saccharomyces cerevisiae lacking sulfite oxidase and deleted of flavohemoglobin showed an O-2-dependent growth impairment with nonfermentable substrates that was exacerbated by sulfide, a precursor to mitochondrial sulfite formation. Higher O-2 exposures inactivated the superoxide-sensitive mitochondrial aconitase in cells, and hypoxia elicited both aconitase and NADP(+)-isocitrate dehydrogenase activity losses. Roles for STAR-derived peroxysulfate radical, superoxide radical, and sulfo-NAD(P) in the mechanism of STAR toxicity and flavohemoglobin protection in yeast are suggested.