Scheme 1. .

Linked assay mechanism. ornithine decarboxylase (ODC) catalyzes the decarboxylation of ornithine, releasing CO₂, which is then captured by the basic buffer (pH 8.05) as bicarbonate. Phosphoenolpyruvate carboxylase (PEPC) uses this bicarbonate to generate oxaloacetate from phosphoenolpyruvate. The oxaloacetate is then reduced by malate dehydrogenase (MDH) to malate in a NADH-dependent fashion. There is a 1:1 relationship between the amount of CO₂ produced by ODC and the amount of NADH oxidized by MDH, allowing kinetic parameters for ODC to be calculated from observing NADH levels as a function of time. The assay system has been optimized such that ODC is the rate-limiting step.