Inhibition of Spinach Leaf NADPH(NADH)-Glyoxylate Reductase by Acetohydroxamate, Aminooxyacetate, and Glycidate

Acetohydroxamate (AHA) and aminooxyacetate (AOA) were found to be potent inhibitors of purified NADPH(NADH)-dependent glyoxylate reductase from spinach (Spinacia oleracea) leaves. AHA was a noncompetitive (ro mixed) inhibitor of the NADPH-dependent activity of the reductase with a K_i of 0.33 millimolar. With NADH serving as a cofactor, AHA preferentially bound to the same form of the enzyme as glyoxylate, exhibiting a K_i of 0.31 millimolar. Glycine hydroxamate and l-glutamic acid-γ-hydroxamate were also inhibitory, but to a lesser extent than AHA. Inhibition by AOA (K_i of 1.8 millimolar) was enhanced by increased concentrations of glyoxylate, indicating that the inhibitor preferentially reacted with the glyoxylate-bound form of the enzyme. Glycidate, an effector of glycolate metabolism in leaves, was found to be a much weaker inhibitor of the enzyme with a K_i of 21 millimolar. While the inhibition by both AHA and AOA was fully reversible, glycidate acted as a tight-binding inhibitor. These findings are discussed with respect to the use of AHA, AOA, and glycidate as inhibitors of photorespiratory carbon metabolism in leaves. Caution is recommended in the use of these inhibitors with intact tissue experiments due to their lack of specificity.