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Effect of Dietary Carbohydrate and Fat on Amino Acid-degrading Enzymes in Relation to Their Protein Sparing Action

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

The effect of dietary carbohydrate and fat on protein metabolism was investigated using rats grown under various nutritional conditions. The activity of hepatic amino acid-degrading enzymes, e.g., threonine dehydratase, tryptophan pyrrolase, and arginase, and the amount of urinary nitrogen output were determined. It was found that supplementation of a high protein diet with either carbohydrate or fat caused an appreciable reduction of the enzyme activity accompanied by a marked conservation of the body nitrogen. These findings support in principle the concept of the protein sparing action of either carbohydrate or fat diet. It was also demonstrated that protein metabolism is markedly influenced by the previous diet. That is, the activity of all the hepatic amino acid-degrading enzymes in rats previously fed the high fat diet was much less than that in rats previously fed the high carbohydrate diet. In parallel, the amount of urinary nitrogen output was found to be less in rats fed the high fat diet compared with the high carbohydrate diet. Overall results thus indicate that the hepatic amino acid-degrading enzymes may be one of the possible regulatory sites involved in the protein metabolism of rats. The unique inhibitory effect elicited by the previous high fat diet may be ascribed to the deteriorated carbohydrate metabolism, presumably resulting in hormone imbalance in the rat. This hypothetical view was supported by analyzing the level of various blood constituents, e.g., sugar, free fatty acid and amino acid, in rats grown under comparable nutritional conditions.