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Activities of Branched-Chain Amino Acid-Degrading Enzymes in Liver from Rats Fed Different Dietary Levels of Protein¹

Soetijoso Soemitro², Kevin P. Block³, Pamela L. Crowell^,4 and Alfred E. Harper^,5

Department of Nutritional Sciences Department of Biochemistry, University of Wisconsin, Madison, WI 53706

The relationships among dietary protein intake, plasma branched-chain amino acid (BCAA) and keto acid (BCKA) concentrations, and liver BCAA-degrading enzyme activities were investigated in rats fed, for 5 h/d for 2, 6 or 9 d, diets containing from 0 to 50% casein. Plasma, liver and muscle BCAA concentrations were proportional to protein intake over the entire range tested; plasma BCKA concentration, however, was proportional only in the range from 0 to 20% casein, after which a plateau was reached. By d 2, liver cytosolic BCAA aminotransferase activity had increased in rats fed 50% casein; by d 9, activity had increased in rats fed 0 or 5% casein as well. Liver mitochondrial BCAA aminotransferase activity was unresponsive to dietary treatment. Basal liver BCKA dehydrogenase activity and the percent active complex were proportional to protein intake on d 2 and 6. On d 2, total BCKA dehydrogenase activity was the same in all groups; by d 6, total activity had increased in rats fed 30 or 50% casein. We conclude that although the adaptive changes in BCAA-degrading enzyme activities are small, they are sufficient to compensate for excessively high or low protein intakes, so that tolerable concentrations of BCAA and BCKA are maintained.

KEY WORDS: • branched-chain amino acid aminotransferase • rats • branched-chain -keto acid dehydrogenase • plasma amino acids • branched-chain -keto acids • protein intake

¹ Supported in part by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, and by grants AM 10748 and P30 AM 26659 from the U.S. Public Health Service, National Institutes of Health.

² Present address: Department of Chemistry, FMIPA, Padjadjaran University, 1 Jln. Singaperbangsa, Bandung, Indonesia 40133.

³ Present address: Departments of Medicine and Biochemistry, Medical University of South Carolina, Charleston, SC 29425.

⁴ Present address: Department of Human Oncology, University of Wisconsin-Madison, 600 Highland Avenue, Madison, WI 53792.

⁵ No reprints available.

Manuscript received 15 February 1989. Revision accepted 13 March 1989.

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