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DPN-dependent isocitrate dehydrogenase is composed of three distinct types of subunits: alpha, beta, and gamma which have molecular weights of about 40,000 but differ in isoelectric points. The relationship of subunit diversity to function was probed by use of chemical modification. 3-Bromo-2-ketoglutarate, a substrate and affinity label for the active site of isocitrate dehydrogenase, was shown to cause significant modification of all types of subunits. The substrate affinity label, 3-ene-2-keto-glutarate, labels each of the subunits equally. Approximately equal labeling of subunits was also found upon modification by cyanate of an essential lysyl residue in the isocitrate binding site. When enzyme was inactivated by a carbodiimide in the presence of glycine ethyl ester, both glutamate and aspartate residues reacted, and labeling of each type of subunit occurred. These studies suggest that the structurally distinct subunits of DPN-dependent isocitrate dehydrogenase are functionally similar and each type of subunit contains a substrate binding site.
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