Effect of fasting or feeding diets containing different levels of protein or amino acids on the activities of the liver amino acid-degrading enzymes and amino acid oxidation in rainbow trout ( Oncorhynchus mykiss)

Experiments were done on fingerling rainbow trout at 15 ± 1 °C to determine the effect of: (1) fasting or feeding diets containing different levels of protein on liver glutamate dehydrogenase (GDH), alanine aminotransferase (AAT) and histidase activities; (2) dietary protein level on the oxidation rates of glutamate and phenylalanine in vivo; and ( 3 ) dietary phenylalanine level on the phenylalanine oxidation rate in vivo. Activities of GDH and AAT per g liver or per g protein were higher ( P < 0.05) in fasted than in fed trout. GDH activity per g liver was higher in trout fed diets containing 50% crude protein (a commercial diet) or 35% protein (a casein-based diet) than in fish fed a diet containing 10% protein (a casein-based diet), but AAT activity per g liver was not significantly different between the fed fish. The differences in GDH or AAT activity among the treatments disappeared when the activities were expressed per 100 g body weight. Regardless of method of expression, liver histidase activity was higher ( P < 0.05) in fish fasted or fed a commercial diet containing 50% protein than in fish fed diets containing 10 or 35% protein. The oxidation rates of dietary glutamate and phenylalanine in trout that had previously been fed the 35% protein diet, were about three and five times those found in fish previously fed the 10% protein diet, respectively. This difference was found when fish that had been fed the 10 or 35% protein diet were subsequently fed the 35% protein diet containing 14C-labeled glutamate or phenylalanine, indicating that trout previously fed a high protein diet have a greater capacity to catabolize these amino acids. The oxidation rate of dietary glutamate was approximately five or nine times that of phenylalanine in trout fed diets containing 35 or 10% protein, respectively. The rate of phenylalanine oxidation increased with increasing phenylalanine levels in the diet; no break point or plateau in the dose-response curve was observed. Our studies demonstrated that fasting or feeding a high protein diet increases the amino acid catabolism in trout, and suggested that the amino acid oxidation technique cannot be used to determine the amino acid requirements of trout.