Sulfhydril Groups and the Concerted Inhibition of NADP+-Linked Isocitrate Dehydrogenase

Mixtures of glyoxalate and oxaloacetate have been shown to inhibit highly purified pig heart NADP+ -linked isocitrate dehydrogenase in a strongly concerted manner. The α-ketoglutarate reductive carboxylase activity of the enzyme was also inhibited, but to a lesser extent than the dehydrogenase activity. Modification of sulfhydryl groups in the enzyme by certain reagents led to a diminution of the inhibition, although the extent of this effect depended on the reagent used. N-Ethylmaleimide was most effective at diminishing the concerted inhibition, whereas oxidation of the sulfhydryl groups by X-irradiation or lipid peroxide had no effect. It was concluded that the effect of sulfhydryl reagents on the inhibition was probably due to the modification of one specific cysteine residue which is possibly adjacent to the binding site on the enzyme for glyoxalate.