Redox turnover of organometallic B12 cofactors recycles vitamin C: Sulfur assisted reduction of dehydroascorbic acid by cob(II)alamin

Abstract

This work reports the reactivity of cob(II)alamin (Cbl(II)) toward reduction of dehydroascorbic (DHA) to ascorbic acid (AA) mediated by sulfur-containing compounds such as glutathione (GSH) and thiocyanate. The reaction supported by GSH proceeded more efficiently than with SCN−. Our findings demonstrate new aspects of interactions between vitamins B12 and C. It has been accepted that simultaneous presence of these vitamins results in their decomposition (viz., irreversible modification of the corrin ring of Cbl and oxidation of AA). We have shown, however, that Cbl(II), the biologically active one-electron reduction product of methyl-Cbl (MeCbl) and adenosyl-Cbl (AdoCbl), is capable of recovering AA in the presence of natural sulfur-containing ligands, within a process that can occur in vivo without glutathione spending, both in a stoichiometric and catalytic manner. Our studies highlight the redox versatility of Cbl(II) and expands the repertoire of reactions whereby redox turnover of the unique B12 organometallic cofactors MeCbl and AdoCbl generates Cbl(II), which in turn recycles oxidized vitamin C.