Hepatic glucose phosphorylating activities in perch (Perca fluviatilis) after different dietary treatments

Increased activity of hepatic glucose phosphorylation was observed in perch after feeding previously fasted fish. When a pellet diet containing 14% carbohydrate was given, most of the increased activity had a low affinity towards glucose (S0.5 = 19.5 mM) and resembled the mammalian glucokinase (Hexokinase IV or D) and the glucokinase-like activity previously observed in salmon liver. In addition, increased activity of a hexokinase with high affinity towards glucose (Km = 0.50 mM) was observed with the pellet diet. An increase in the activity of this hexokinase alone was observed when the fish were fed with filet of cod containing less than 0.2% carbohydrate. Perch with a very high hepatic glucokinase-like activity after eating the pellet diet had high activities of pyruvate kinase and glucose-6-phosphate dehydrogenase, indicating a high capacity of glycolysis and carbohydrate utilization. Simultaneously, the activity of glycogen phosphorylase was strongly reduced while the activity of fructose-1,6-bisphosphatase was not significantly changed. These observations were made with perch captured in the spawning season and brought to the laboratory. Assays of glucose phosphorylation in livers of perch eating the natural diet (insects) in the lake showed no glucokinase-like activity.