(Bi)sulfite oxidation by copper, zinc-superoxide dismutase: Sulfite-derived, radical-initiated protein radical formation.

Sulfur dioxide, formed during the combustion of fossil fuels, is a major air pollutant near large cities. Its two ionized forms in aqueous solution, sulfite and (bi)sulfite, are widely used as preservatives and antioxidants to prevent food and beverage spoilage. (Bi)sulfite can be oxidized by peroxidases to form the very reactive sulfur trioxide anion radical (*SO(3)-). This free radical further reacts with oxygen to form the peroxymonosulfate anion radical (-O(3)SOO*) and sulfate anion radical (SO(4)*-). To explore the critical role of these radical intermediates in further oxidizing biomolecules, we examined the ability of copper,zinc-superoxide dismutase (Cu,Zn-SOD) to initiate this radical chain reaction, using human serum albumin (HSA) as a model target. We used electron paramagnetic resonance, optical spectroscopy, oxygen uptake, and immuno-spin trapping to study the protein oxidations driven by sulfite-derived radicals. We found that when Cu,Zn-SOD reacted with (bi)sulfite, *SO(3)- was produced, with the concomitant reduction of SOD-Cu(II) to SOD-Cu(I). Further, we demonstrated that sulfite oxidation mediated by Cu,Zn-SOD induced the formation of radical-derived 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin-trapped HSA radicals. The present study suggests that protein oxidative damage resulting from (bi)sulfite oxidation promoted by Cu,Zn-SOD could be involved in oxidative damage and tissue injury in (bi)sulfite-exacerbated allergic reactions.