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Relation between Liver Amino Acid-Catabolizing Enzymes and Free Amino Acids of Liver and Plasma in Adult Cockerels Fed Diets Containing Graded Levels of Protein¹

Laboratory of Animal Nutrition, Faculty of Agriculture, Nagoya University, Nagoya 464, Japan

Activities of liver amino acid-catabolizing enzymes and liver and plasma free amino acid concentrations in adult cockerels fed diets containing 3% to 21% casein were determined 2 hours after a meal. Liver tryptophan pyrrolase activity and liver tryptophan increased with an increase in dietary casein level, but plasma tryptophan decreased with the increase in dietary casein level. As the dietary casein level increased, liver lysine-ketoglutarate reductase activity decreased, but liver and plasma lysine increased proportionally, and the rate of increase was larger in the plasma than in the liver. Liver phenylalanine hydroxylase activity increased with the increase in dietary casein level from 3% to 12%, and thereafter remained unchanged. Reflecting the change of this enzyme, phenylalanine increased in the liver and decreased in the plasma with the increase in dietary casein level from 3% to 12%, and thereafter the rate of increase in liver phenylalanine became small and plasma phenylalanine increased. Liver tyrosine was not influenced by the dietary casein level, whereas plasma tyrosine increased sharply from 3% to 12%, and thereafter the rate of increase decreased. Thus, the difference between liver and plasma free amino acid responses might be due to the changes in the activities of liver amino acid-catabolizing enzymes.

KEY WORDS: • liver amino acid-catabolizing enzymes • cockerels • free amino acids • dietary protein level

¹ Send the reprint requests to the second author.

² Present address: Department of Physiology, Ehime University School of Medicine, Shigenobu, Ehime 791-02, Japan.

Manuscript received 21 June 1976.