Kinetics and mechanism of oxidation of super-reduced cobalamin and cobinamide species by thiosulfate, sulfite and dithionite

Abstract

We studied the kinetics of reactions of cob(I)alamin and cob(I)inamide with thiosulfate, sulfite, and dithionite by UV-Visible (UV-Vis) and stopped-flow spectroscopy. We found that the two Co(I) species were oxidized by these sulfur-containing compounds to Co(II) forms: oxidation by excess thiosulfate leads to penta-coordinate complexes and oxidation by excess sulfite or dithionite leads to hexa-coordinate Co(II)–SO₂⁻ complexes. The net scheme involves transfer of three electrons in the case of oxidation by thiosulfate and one electron for oxidation by sulfite and dithionite. On the basis of kinetic data, the nature of the reactive oxidants was suggested, i.e., HS₂O₃⁻ (for oxidation by thiosulfate), S₂O₅²⁻, HSO₃⁻, and aquated SO₂ (for oxidation by sulfite), and S₂O₄²⁻ and SO₂⁻ (for oxidation by dithionite). No difference was observed in kinetics with cob(I)alamin or cob(I)inamide as reductants.