CHANGES IN THE STRUCTURE AND FUNCTION OF THE EPAXIAL MUSCLE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN RELATION TO RATION AND AGE .2. ACTIVITY OF KEY ENZYMES IN ENERGY-METABOLISM

The activity levels of four enzymes, representing vital energy-creating pathways (glycolysis by phosphofructokinase, beta-oxidation by 3-hydroxyacyl-CoA dehydrogenase, citric acid cycle by citrate synthetase and the respiratory chain by cytochrome oxidase), were measured from hatching to sexual maturation. The activities of all enzymes investigated were affected more by age than by ration level. Only in fish kept on very restricted rations were reductions in activity levels noted. However, phosphofructokinase activity in white muscle decreased significantly also at moderately restricted ration levels. From 0.3 to 1 year of age, enzyme activities of glycolysis increased in white and decreased in red muscle while the opposite was true for enzymes in the beta-oxidation and citric acid cycle, indicating an ongoing differentiation process as regards substrate utilization in the two muscle types. An increase in the respiratory chain in both muscle types during this period suggests an increased importance of aerobic catabolism of the different substrates. Enzyme activities decreased markedly at the time of saltwater transfer, with the exception of white muscle glycolytic activity. An increase in enzyme activities in red muscle occurred between 2 and 2.4 years, but no relation was observed between sex or degree of maturation and enzyme activity.