EFFECT OF PROTEIN INTAKE AND COLD EXPOSURE ON SELECTED LIVER ENZYMES ASSOCIATED WITH AMINO ACID METABOLISM

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Abstract
Two experiments were conducted to examine the possibility that increased activities of some enzymes in cold-exposed animals could be substrate-induced rather than a direct effect of cold, since the cold-exposed animals increase appreciably their food intake. Two groups of cold-exposed rats were allowed to eat only as much protein as their warm mates, and their extra energy requirement was met by sugar or fat. Another cold-exposed group of animals was allowed to eat the complete diet freely. After a 4-week cold exposure, the activities of 5 liver enzymes involved in amino acid metabolism were markedly increased in all groups of cold-exposed rats. However, the activities of 3 of these enzymes, arginase, glutamic-oxalacetic and glutamicpyruvic transaminase were increased only as a result of a cold-induced higher protein intake. In contrast, the activities of tryptophan pyrrolase and tyrosine-a-ketoglutaric transaminase were increased by cold per se. The data demonstrate that both substrateinduced and cold-induced enzymatic changes occur in cold-exposed animals.