Processing of glutathionylcobalamin by a bovine B12 trafficking chaperone bCblC involved in intracellular B12 metabolism

Glutathionylcobalamin (GSCbl) is a biologically relevant vitamin B12 derivative and contains glutathione as the upper axial ligand thought formation of a cobalt-sulfur bond. GSCbl has been shown to be an effective precursor of enzyme cofactors, however processing of the cobalamin in intracellular B12 metabolism has not been fully elucidated. In this study, we discovered that bCblC, a bovine B12 trafficking chaperone, catalyzes elimination of the glutathione ligand from GSCbl by using the reduced form of glutathione (GSH). Deglutathionylation products are base-off cob(II)alamin and glutathione disulfide, which are generated stoichiometrically to GSH. Although cob(I)alamin was not detected due to its instability, deglutathionylation is likely analogous to dealkylation of alkylcobalamins, which uses the thiolate of GSH for nucleophilic displacement. The catalytic turnover number for the deglutathionylation of GSCbl is ⩾1.62 ± 0.13 min−1, which is, at least, an order of magnitude higher than that for elimination of upper axial ligands from other cobalamins. Considering the prevalence of GSH at millimolar concentrations in cells, our results explain the previous finding that GSCbl is more effective than other cobalamins for synthesis of enzyme cofactors.