Tritium isotope effects in adenosylcobalamin-dependent methylmalonyl-CoA mutase

The effects of tritium loss from 5'-[3H]adenosylcobalamin during the isomerization of methylmalonyl-CoA and succinyl-CoA and of its subsequent appearance in the substrate and reaction product were investigated by measuring the kinetics of the reaction. The primary kinetic tritium isotope of 4.9 on enzyme-catalyzed hydrogen transfer was found to be lower than the normal range, suggesting that hydrogen transfer is not a rate-limiting step in the reaction.