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Effect of Dietary Amino Acids on Amino Acidcatabolizing Enzymes in Rat Liver

Laboratory of Nutritional Biochemistry, Department of Agricultural Chemistry, Nagoya University, Chikusa, Nagoya, Japan

The effect of dietary amino acids on the level of hepatic amino acidcatabolizing enzymes, e.g., tryptophan pyrrolase, threonine dehydratase and arginase, was studied by their forcible administration to rats depleted of protein. (a) Among 12 amino acids tested, only tryptophan and methionine were found to cause a consistent increase in enzyme activities. Feeding rats an equimolar complete amino acid mixture resulted in a maximal induction of the enzyme, whereas the omission of both tryptophan and methionine caused little or no induction. The administration of these two functional amino acids did not cause any significant increase in the level of plasma urea, compared with that caused by the feeding of the complete amino acid mixture. (b) In adrenalectomized rats hepatic tryptophan pyrrolase, which can be increased in amount in intact rats by feeding either tryptophan or methionine, was activated and increased in amount by feeding tryptophan but not methionine. The feeding of tryptophan resulted in a significant increase in the level of arginase and threonine dehydratase. Methionine produced a similar increase in both threonine dehydratase and arginase activities. (c) Administration of cortisone acetate produced a significant increase in the level of all three enzyme activities. Based on these findings we propose that the level of amino acid-catabolizing enzymes in the liver of rats fed either tryptophan or methionine can be elevated by two separate mechanisms, hormone (adrenal) mediated and hormone independent ones. The nutritional role of tryptophan and methionine in the natural protein diet is discussed.