EFFECTS OF OXALATE ON REDUCTION OF HYDROXYPYRUVATE AND GLYOXYLATE IN LEAVES

Oxalate was found to be a potent uncompetitive, or mixed, inhibitor of the partially purified NADPH-preferring hydroxypyruvate reductase (HPR-2) from a mutant of barley which lacks the NADH-preferring HPR (HPR-1). Both hydroxypyruvate and glyoxylate-dependent rates of the HPR-2 enzyme were inhibited by oxalate, with the experimentally determined K(i) values of 3-5-mu-M, depending on variable substrate. Oxalate did not affect the activity of barley NADPH-preferring glyoxylate-specific reductase (GR-1), and had only a negligible effect on HPR-1 from wild-type barley. The selective effect of oxalate, as seen for barley HPR-2, was confirmed in studies on partially purified reductases from leaves of maize, pea, soybean and spinach. In all cases, oxalate acted independently from acetohydroxamate, a selective inhibitor of GR-1. Inhibition by oxalate provides an accurate measure of the contribution of HPR-2 to the overall activities of reductases in leaf extracts and partially purified preparations.