The effect of changing dietary fat and carbohydrate on the enzymes of amino acid metabolism

Changes in the activities of alanine aminotransferase (Ala-AT), aspartate aminotransferase (Asp-AT), glutamate dehydrogenase (GDH), serine dehydratase (SDH), and branched chain amino acid dehydrogenase were monitored in weanling and adult male rats fed diets differing in fat content: 11% (low), 22% (medium), and 42% (high) of total energy from fat. The carbohydrate and fiber contents were adjusted to maintain the caloric density of the diets. The results showed that changing the total dietary fat and carbohydrate altered the activities of enzymes predominantly in liver and white muscle. This effect was dependent on the age of the animal. In adult animals, high fat diets decreased liver Ala-AT and SDH activity and increased GDH activity. Muscle Ala-AT activity was lowered in adults fed a high fat diet. In weanlings, high fat diets lowered liver Ala-AT and Asp-AT activities as well as muscle Ala-AT activity. Animals were fasted overnight to determine the effect of a high fat diet on fasting enzyme levels. In adults, fasting eliminated the decrease in muscle Ala-AT activity seen in high fatfed animals and decreased liver Ala-AT and SDH activity. In weanlings, fasting increased liver GDH activity and white muscle Ala-AT and Asp-AT activities. These results suggest a complex dependence of amino acid utilization that may depend on dietary regime and the age of the animal. (J. Nutr. Biochem. 6:414-421, 1995.)