Malate Dehydrogenase and NAD Malic Enzyme in the Oxidation of Malate by Sweet Potato Mitochondria

Over a range of concentrations from less than 0.1 mm to more than 70 mm, sweet potato root mitochondria display a bimodal substrate saturation isotherm for malate. The high affinity portion of the isotherm has an apparent Km for malate of 0.85 mm and fits a rectangular hyperbolic function. The low affinity portion of the isotherm is sigmoid in character and gives an apparent S(0.5) of 40.6 mm and a Hill number of 3.7.Extracts of sweet potato mitochondria contain both malate dehydrogenase and NAD malic enzyme. The malate dehydrogenase, assayed in the forward direction at pH 7.2, shows typical Michaelis-Menten kinetics with a Km for malate of 0.38 mm. The NAD malic enzyme shows pronounced sigmoidicity in response to malate with a Hill number of 3.5 and an S(0.5) of 41.6 mm.On the basis of the normal kinetics, the Km, and the fact that oxaloacetate production from malate by mitochondria appears most active at low malate concentrations, the high affinity portion of the malate isotherm with mitochondria is attributed to malate dehydrogenase. The low affinity portion of the malate isotherm with mitochondria is thought, on the basis of the similarity of S(0.5) values, the Hill numbers, and the greater production of pyruvate from malate at high malate concentrations, to represent the activity of the NAD malic enzyme.