MALATE OXIDATION BY ARUM SPADIX MITOCHONDRIA: PARTICIPATION AND CHARACTERISTICS OF NAD MALIC ENZYME *

S ummary When oxygen uptake by mitochondria from Arum spadix was determined at pH 6.5 and 7.7, striking differences were found in the response of malate oxidation to the addition of ADP, KCN, salicylhydroxamic acid (SHAM) and NAD. These responses are consistent with the hypothesis that at pH 6.5 malate was oxidized primarily through the NAD malic enzyme while at pH 7.7 the primary enzyme oxidizing malate was malate dehydrogenase. Oxygen uptake rate plotted as a function of malate concentration up to a concentration of 120 mM produced curves which are best fitted by a model which assumes the presence of two enzymes with different affinities for malate. The change in the relative activities of the two enzymes at pH 6.5 and 7.7 is in agreement with the suggestion that malic enzyme was more active at the lower pH and malate dehydrogenase at the higher. The NAD malic enzyme was purified from Arum spadix mitochondria and was found to utilize either Mg 2+ or Mn 2+ as the required divalent cation. Although the enzyme was capable of using NADP, it did so at a rate only 10% of that with NAD. The enzyme displayed hysteretic behaviour, with bursts of activity under some conditions and lags under others, perhaps indicating a shifting state of aggregation such as has been found with other plant malic enzymes. The Michaelis constants for the components of the reaction were similar to those found with other plant NAD malic enzymes. Sulphate, fumarate, coenzyme A, AMP, ADP and ATP were activators of the enzyme, with activation constants similar to those found with malic enzyme from other plant sources. The relative effectiveness of ADP and ATP was strongly in favour of ADP, so that the enzyme may be under energy charge control in the cell. The exceptionally high concentration of NAD malic enzyme found supports the suggestion that this enzyme may be important in the metabolism of Arum spadix during floral development.