Kinetic studies on pig heart cytoplasmic malate dehydrogenase

Kinetic studies on the pig heart cytoplasmic malate dehydrogenase have been performed over a wide range of conditions using the full time course of the reaction and computer simulation to obtain the kinetic parameters. The maximum velocity and Michaelis constants for the oxidation of reduced coenzyme have been determined as a fundtion of pH in 0.05 M phosphate buffer at 15 degrees. At pH 7.5 and at low substrate concentrations, the kinetic data are consistent with a sequential addition of substrates, coenzyme binding first, and involving the formation of at least one ternary complex. No oxalacetate binding to the enzyme was observed. The rate constants for the dissociation of coenzyme from the enzyme-coenzyme complex are small enough to define the maximum velocity in either direction of the reaction. These data, plus data using deuterated reduced coenzyme, indicate that the chemical transformation step is not rate determining. It is also shown that DPNH binding can be tight enough to practically exclude the possibility of obtaining initial velocities when measuring the reduction of DPN. Kinetic abnormalities do appear at higher substrate or product concentrations, but these do not appear to be related to the formation of inactive abortice, complexes.