Glutathione-dependent one-electron transfer reactions catalyzed by a B₁₂ trafficking protein.

CblC is involved in an early step in cytoplasmic cobalamin processing following entry of the cofactor into the cytoplasm. CblC converts the cobalamin cargo arriving from the lysosome to a common cob(II)alamin intermediate, which can be subsequently converted to the biologically active forms. Human CblC exhibits glutathione (GSH)-dependent alkyltransferase activity and flavin-dependent reductive decyanation activity with cyanocobalamin (CNCbl). In this study, we discovered two new GSH-dependent activities associated with the Caenorhabditis elegans CblC for generating cob(II)alamin: decyanation of CNCbl and reduction of aquocobalamin (OH2Cbl). We subsequently found that human CblC also catalyzes GSH-dependent decyanation of CNCbl and reduction of OH2Cbl, albeit efficiently only under anaerobic conditions. The air sensitivity of the human enzyme suggests interception by oxygen during the single-electron transfer step from GSH to CNCbl. These newly discovered GSH-dependent single-electron transfer reactions expand the repertoire of catalytic activities supported by CblC, a versatile B12-processing enzyme.