Effect of Protein Intake and Cold Exposure on Selected Liver Enzymes Associated with Amino Acid Metabolism

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-α-ketoglutaric transaminase were increased by cold per se. The data demonstrate that both substrate-induced and cold-induced enzymatic changes occur in cold-exposed animals.