INFLUENCE OF DIETARY-PROTEIN LEVELS ON HEPATIC CYSTEINE DIOXYGENASE ACTIVITY IN RAINBOW-TROUT

Fingerling rainbow trout were fed experimental diets having different protein levels for 10 days to determine the sulfur amino acid metaboli sm involving taurine biosynthesis in fish. Either egg white albumin or casein was employed as a sole dietary protein source. Maximum body we ight gain was obtained at about 40% crude protein level in both dietar y groups. However, further increase in dietary protein did not improve the growth rate of the fish. Specific activity of the hepatic cystein e dioxygenase (CDO) [EC 1.12.11.20] in the fish fed the egg white albu min diets increased exponentially as dietary protein level increased, whereas no clear effect on enzyme activity was noted in the casein die tary group. Judging from the difference in the sulfur amino acid conte nts between egg white albumin and casein, the hepatic CDO activity of rainbow trout seems to be closely related to dietary sulfur amino acid levels rather than protein levels. The accumulation of cystathionine in the fish fed a high egg white albumin diet suggests that the capaci ty of the transsulfuration pathway in rainbow trout is limited.