Effects of quantity and quality of dietary protein on certain enzyme activities in rainbow trout

The effects of variation in quality and quantity of dietary protein on certain tissue enzymes in rainbow trout (Salmo gairdneri) were examined. Trout were given for 9 weeks diets containing proteins of different quality (fish-meal, casein and corn gluten) and with protein energy levels ranging from 26 to 74% of total metabolizable energy. In the first experiment, activities of a number of enzymes were monitored by only hepatic serine pyruvate transaminase (SPT) activity changed in response to the dietary treatments--increasing as protein energy level was raised. In the second experiment, opposing glycolytic an gluconeogenic enzyme activities [pyruvate kinase (PK) and phosphoenolpyruvate carboxykinase (PEPCK); phosphofructokinase (PFK) and fructose diphosphatase (FDP)] were measured. Gluconeogenic enzyme activities correlated positively and significantly with dietary protein energy level; glycolytic enzymes correlated negatively and significantly with this parameter for all three proteins. There was no consistent relationship between presumed equilibrium point of opposing enzyme activities and maximum weight gain for the three proteins. It is suggested that hepatic activities of SPT, PFK, PK, FDP and PEPCK will provide useful indices of protein status in trout.