Primary deuterium isotope effect in the oxidation of an iminium ion by aldehyde oxidase

Abstract

Studies of the rates of aldehyde oxidase-mediated oxidation of iminium ion ( I) and its specifically labeled counterpart containing deuterium at the methine and methylene carbons of its seven-membered ring ( I- d 3), are reported. In separate incubations, the isotope effect ( k H k D ) on V max was 3.3 and that on K m was 1.5. Co-incubation of equimolar amounts of I and I- d 3 with aldehyde oxidase resulted in an isotope effect of 2.2 on product ratio. These results support hydrogen transfer from the methine carbon of the seven-membered ring to the molybdenum center of the enzyme as the rate-determining step in the conversion of I to its lactam metabolite ( II).