Physarum polycephalum malate dehydrogenase: inhibitor analyses of the mitochondrial and supernatant isozymes.

The effects of naturally occurring metabolites were tested on the malate dehydrogenase (L-malate: NAD+oxidoreductase, EC 1.1.1.37) isozymes from the eucaryotic protist Physarum polycephalum. Several of the Krebs cycle intermediates were inhibitors for each isozyme indicating that a similar catalytic process was involved for both forms. The metabolites ATP, ADP, and AMP were inhibitors competitive with NAD for the mitochondrial isozyme but not the supernatant form. Several other nucleoside phosphates had no effects. Tests of protein sulfhydryl, arginine- and tyrosine-modifying reagents revealed a similar functional sensitivity by both isozymes to these reagents. Those results are compared with data on isozymes from more complex tissue with comments on the physiological significance of those combined data.