Carbon isotope effects on the decarboxylation of carboxylic acids. Comparison of the lactate oxidase reaction and the degradation of pyruvate by H2O2

Publisher Website
Google Scholar
Abstract
The isotope effect at C-1 on the H2O2-catalysed decarboxylation of pyruvate (used as a model reaction for the enzymic reaction) increases between pH 3 and 10 from 1.0007 .+-. 0.0004 to 1.0283 .+-. 0.0014 (25.degree. C). This result indicates a change in the rate-determining step from formation of the tetrahedral intermediate to decarboxylation of this intermediate. Practically no isotope fractionation at C-1 (1.0011 .+-. 0.0002, pH 6.0, 25.degree. C) is found in the lactate oxidase-catalysed decarboxylation of lactate, which is indicative for the existence of an irreversible O2-dependent step prior to the enzyme-catalysed decarboxylation. In addition, the result provides further evidence that dissociation of pyruvate and H2O2 from the enzyme can be excluded. The isotope effect at C-2 of lactate in the enzymic reaction (1.0048 .+-. 0.0004) is attributed to the hydrogen transfer step from lactate to the coenzyme.