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Effect of Dietary Branched-chain -Keto Acids on Hepatic Branched-chain -Keto Acid Dehydrogenase in the Rat¹

Department of Surgery and Medicine, Emory University School of Medicine, Atlanta, Georgia 30322

Male albino rats (80–100 g) were tube-fed for 3 days with (a) a complete purified amino acid diet minus valine, or this diet containing 70 to 210 µmole/g of valine (1 to 3 times the minimal daily requirement, MDR) or equimolar amounts of its -keto analogue (KIV); (b) complete diet minus leucine, or this diet containing 85 to 225 µmole/g of either leucine (1 to 3 times the MDR) or its -keto analogue (KIC); (c) complete diet minus valine, leucine and isoleucine, or this diet containing 63 to 170 µmole/g of these amino acids (1 to 2 times the MDR) or their -keto analogues (KIV, KIC, KMV). Liver and kidney were then assayed for dehydrogenase activity towards these substrates: KIV, KIC, KMV, pyruvate, and -ketoglutarate. Both the branched-chain amino acids (BCAA) and their -keto analogues (BCKA) stimulated the activity of branchedchain hepatic dehydrogenases. BCKA were 2 to 9 times more potent than BCAA in this respect. The effect was specific for enzyme and organ, since dehydrogenase activity for -ketoglutarate and pyruvate in liver, and dehydrogenase activity for BCKA, pyruvate and -ketoglutarate in kidney, were not increased. Dietary BCKA (1 to 2 times MDR) accelerated the decarboxylation of KIC by slices of liver 2 to 6 times without altering the rate of transamination to leucine. Decarboxylation of KIC by kidney and muscle slices was unaffected. The stimulation of hepatic branched-chain dehydrogenase by BCKA may play a role in the limited nutritional efficiency of these nitrogen-free substitutes for the BCAA compared to -keto analogues of other essential amino acids.

KEY WORDS: • Branched-chain keto acid dehydrogenases • branched-chain keto acids • branched-chain amino acids

¹ Supported by USPHS grants AM15736 and RR00039.

² Present address: Department of Physiology, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232.

³ Address all correspondence to this author.

Manuscript received 3 November 1976.