Regulation of Malate Oxidation in Isolated Mung Bean Mitochondria

Effects of ADP and ATP on products of malate oxidation in the presence or absence of respiratory inhibitors and an uncoupler were investigated in mitochondria isolated from mung bean (Phaseolus aureus var. Jumbo) hypocotyls. Changes in levels of products from malate oxidation generally correlated directly with changes in oxygen uptake. Effects of ADP and ATP were indistinguishable from each other when respiratory chain activity was limited. We concluded that adenylates indirectly act on malate oxidation via the oxidation-reduction status of the pyridine nucleotides which are linked to the respiratory chain. The possibility of allosteric action of ADP and ATP on malate dehydrogenase activity was examined in both intact mitochondria and a partially purified enzyme preparation. Although small inhibition, 16% with 500 muM ATP and 8% with 500 muM ADP, was observed at pH 9.5, this effect was abolished by the addition of magnesium ions or by lowering the pH to 7.2. We concluded that these adenylate effects are probably not a significant factor in regulation under physiological conditions. Furthermore, the equilibrium constant of malate dehydrogenase (to 1.5 x 10(-5)) in both mitochondria and the partially purified enzyme calculated from the steady state level of NADH formed suggested that the enzyme functions in an equilibrium manner in intact mitochondria.