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By use of carboxyl-labeled α-ketoisocaproic, α-ketoisovaleric, and α-keto-β-methylvaleric acids, branched chain keto acid dehydrogenase activities were measured in rat tissues. α-Ketoisocaproic acid dehydrogenase was shown to be a mitochondrial enzyme and was found almost exclusively in liver, in contrast to leucine-α-ketoglutarate aminotransferase which was found predominantly in kidney. α-Ketoisocaproic, α-ketoisovaleric, and α-keto-β-methylvaleric acid dehydrogenase specific activities in liver increased linearly and in constant proportion to each other as the protein content of the diets was increased from 0 to 30%. Modulation of dehydrogenase activity appears to be the resultant of at least two superimposable mechanisms. One, cycloheximide-sensitive, is apparently an adaptation to dietary protein intake. The other, cycloheximide-insensitive, involves a daily activation-deactivation cycle.
Wohlhueter et al. (Fri,) studied this question.