PURIFICATION OF NADP-MALIC ENZYME FROM APPLE FRUIT AND ITS CHANGE WITH DEVELOPMENT AND RIPENING

The molecular weight of NADP-malic enzyme (ME) in apple fruit is about 270,000 and the enzyme molecule seems to be a tetramer composed of the same 4 subunits. The enzyme shows absolute specificity for NADP+ and no detectable activity when NAD+ replaced NADP+ in the assay mixture. The enzyme also requires a divalent cation such as MN++ and Mg++ for its activity. In the presence of MN++, optimum pH for the reaction was 7.5. The Km values for malate and NADP+ were 1.0 mM and 20.8-mu-M, respectively. In the presence of Mg++, the Km values for malate and NADP+ at optimum pH of 7.2 were 4.0 mM and 60-mu-M, respectively. The ME activity increased with ripening, concomitant with an increase in the amount of enzyme protein determined by the ELISA method. The rate of increase in enzyme activity with ripening seems to be highest in late maturing apple cultivars.