In humans, cobalamin (vitamin B12) is delivered to two target enzymes via a complex intracellular trafficking pathway comprising transporters and chaperones. CblC is a processing chaperone that removes the upper axial ligand of cobalamin derivatives, forming an intermediate in the pathway that is subsequently converted to the active cofactor derivatives. Mutations in the cblC gene lead to methylmalonic aciduria and homocystinuria. Researchers in the Banerjee lab report that nitrosylcobalamin (NOCbl), which was developed as an antiproliferative agent and is purported to cause cell death by releasing nitric oxide, is highly unstable in air and is rapidly oxidized to nitrocobalamin (NO2Cbl). They demonstrate that CblC catalyzes the glutathione-dependent denitration of NO2Cbl to form 5-coordinate cob(II)alamin, which had one of two fates: it could be oxidized to aquo-cob(III)alamin or enter a futile thiol oxidase cycle forming glutathione disulfide. Arg-161 in the active site of CblC suppressed the NO2Cbl-dependent thiol oxidase activity whereas the disease-associated R161G variant stabilized cob(II)alamin and promoted futile cycling. They also report that CblC exhibits nitrite reductase activity, converting cob(I)alamin and nitrite to NOCbl. Finally, the denitration activity of CblC supported cell proliferation in the presence of NO2Cbl, which can serve as a cobalamin source. The newly described nitrite reductase and denitration activities of CblC extend its catalytic versatility, adding to its known decyanation and dealkylation activities. In summary, upon exposure to air, NOCbl is rapidly converted to NO2Cbl, which is a substrate for the B12-trafficking enzyme CblC.
Read the JBC paper HERE.