The reaction of 4-iodoacetamidosalicylic acid with TPN-dependent isocitrate dehydrogenase from pig heart

Salicylic acid acts as a reversible inhibitor of pig heart TPN-dependent isocitrate dehydrogenase competitive with manganous isocitrate (K sub(i) = 18.2 mM) but not with TPN. The salicylate derivative, 4-iodoacetamidosalicylic acid (0.5 to 4 mM), causes an irreversible time-dependent loss of both the isocitrate dehydrogenase and oxalosuccinate decarboxylase activities of the enzyme when incubated at pH 7.0 and 30 degree C. Enzyme reacted first with unlabeled ISA in the presence of protecting substrate plus metal ion, and then with radioactive ISA in the absence of ligands yields only radioactive epsilon -dicarboxymethyllysine from the hydrolysate of the inactive enzyme. It is concluded that in this case a lysine residue attacked by 4-iodoacetamidosalicylic acid is critical for the function of isocitrate dehydrogenase and is located within the metal-isocitrate binding site.