Lactate dehydrogenase catalyzed reduction of pyruvate. Active substrate and substrate inhibition

The reduction of pyruvate catalyzed by lactate dehydrogenase was studied using a spectrophotometric method in which the rate of oxidation of DPNH was monitored at 340 nm. Detailed kinetic analyses were carried out with respect to the enzymatic activity of beef heart lactate dehydrogenase as a function of pH, pyruvate concentration, and coenzyme concentration. The relative rates of the enzymatic process at various stages of pyruvate hydration, H20 +CH3COCO2- ⇌ CH3C(OH)2CO2-, were investigated and comparative studies involving the deuterated and the protiated pyruvate were carried out. The results obtained indicate that the true substrate involved in this enzymatic process is the unhydrated keto form rather than either a hydrated or an enolized form. Furthermore, the existence of even relatively small quantities of the hydrated pyruvate accounts for significant “substrate” inhibition.