Effect of nutritional factors and cortisol on tyrosine aminotransferase activity in liver of brook trout, Salvelinus fontinalis Mitchill.

1. Starvation for over 50 days increased the activity of Tyrosine aminotransferase (TAT) from brook trout liver and decreased liver glycogen. 2. Cortisol (6-40 mg/100 g body wt) increased TAT activity, optimally 72 hr post-injection; liver glycogen response to cortisol was variable while tissue water either increased or remained constant. 3. Fish fed high-protein/low-carbohydrate had greater TAT activity than fish fed low-protein/high carbohydrate. There was food-induced peak of activity indicating a "diurnal rhythm" analogous to that of rat TAT. 4. TAT activity in brook trout appears to be correlated with protein catabolism.