[Kinetic properties of plant malate dehydrogenase]

The kinetic behaviour of highly purified malate dehydrogenase (EC 1.1.1.37) from grape vine leaves has been studied. The curves of the dependence of the initial velocity (v) on oxaloacetate and NADH concentrations at low concentration of the enzyme are hyperbolic, while at relatively high concentrations of the enzyme they are S-shaped (nH = 2 and nH = 1.7 for oxaloacetate and NADH, respectively). The dependence of v upon malate concentrations at low enzyme and substrate concentrations obeys the Michaelis--Menten equation (nH = 1.0); at relatively high concentration of the enzyme at any substrate concentrations of the enzyme it is characterized by "negative" kinetic cooperativity. With respect to NAD malate dehydrogenase at low enzyme concentrations reveals "positive" cooperativity (nH = 2.5), but at relatively high concentrations shows a hyperbolic dependence. The specific malate dehydrogenase activity determined in both directions, i. e. reduction of oxaloacetate and oxidation of malate, strongly depends on the enzyme concentration. At low and relatively high concentrations of the enzyme the specific malate dehydrogenase activity sharply decreases. Therefore malate dehydrogenase can be attributed to reversibly associating oligomeric allosteric enzymes.