Kinetic Studies on the Decomposition of Thiolatocobalamins in Acidic Solution

Thiolatocobalamins (RSCbls), or vitamin B-12 derivatives with a thiolato ligand coordinated at the beta-axial site of the cobalamin, have been shown to have superior antioxidant properties relative to the currently available therapeutic forms of cobalamin. It has also recently been shown that small amounts of glutathionylcobalamin are isolable from mammalian cells in the presence of a ligand trap. Kinetic studies are now presented for four representative thiolatocobalamins of glutathione, N-acetylcysteine, homocysteine, and captopril, which show that RSCbls spontaneously decompose to aquacobalamin (H(2)OCbl(+)) at pH < 3. A mechanism is proposed in which rapid protonation at the thiol sulfur of RSCbl precedes rate-determining decomposition. These results indicate that protection of RSCbls from acidic environments such as that found in the stomach is essential if RSCbls are to remain intact.