Abstract
The magnitude of the kinetic isotope effect observed in the oxidation of tetradeuteriosuccinate depends on the concentration of oxidant and not on the concentration of succinate. Fumarate and dideuteriofumarate are identically effective as competitive inhibitors of succinic dehydrogenase against both succinate and tetradeuterio- succinate We conclude (1) that the isotope effect occurs at a step prior to the attack of oxidant, presumably upon the reduction of enzyme by succinate, and (2) that this step is rate determining when the step involving attack by oxi- dant is not rate determining.