Activity of Energy and Carbohydrate Metabolism Enzymes in the Juvenile Pink Salmon Oncorhynchus gorbuscha (Walb.) during the Transition from Freshwater to a Marine Environment

Biochemical adaptations of energy metabolism and some pathways of glucose oxidation during a change in salinity of the environment in larvae and smolts of the pink salmon Oncorhynchus gorbuscha (Walb.) inhabiting the White Sea were studied. We assayed the activity of energy and carbohydrate metabolism enzymes (cytochrome c oxidase (COХ), lactate dehydrogenase (LDH), glucose-6-phosphate dehydrogenase (G6PDH), 1-glycerophosphate dehydrogenase (1-GPDH), and aldolase) in pink salmon larvae in a short-term aquarium experiment and in pink salmon smolts in a long-term cage experiment simulating the transition of juveniles from freshwater to a marine environment. A decrease in the activity of COX, LDH, 1‑GPDH, and aldolase already in the first hour after the transfer of larvae to seawater was shown. Smolts kept in the estuary and in the sea had low levels of activity of 1-GPDH and aldolase in comparison with individuals from the river. Most likely, in the salmon juveniles studied, there was a redistribution of carbohydrates between the reactions of aerobic and anaerobic metabolism in favor of anaerobic ATP synthesis. No changes in the enzyme activity of the pentose phosphate pathway, G-6-PDH, were found in either larvae or smolts compared with the individuals kept in freshwater. Maintenance of the required levels of anaerobic metabolism and of the pentose phosphate pathway is probably one of the mechanisms of biochemical adaptation of pink salmon to changes in salinity.