Seasonal variation of the xenobiotic metabolizing enzyme activities in the liver of male and female vendace ( Coregonus albula L.)

Abstract

The activities of xenobiotic metabolizing enzymes measured in the liver of freshwater vendace ( Coregonus albula L.) varied with the seasons. The determinations were made at a constant temperature (18°C) and at the environmental (water) temperature, in which the fish were caught. The monooxygenase activities (benzo[ a]pyrene, 7-ethoxycoumarin and 7-ethoxyresorufin as substrates) decreased gradually in the autumn before spawning and increased thereafter. The activities in the male vendace were higher than in the female fish. Slight thermal compensation was seen during water cooling in the autumn from temperatures of 10 to 1°C when benzo[ a]pyrene or 7-ethoxyresorufin were used as substrates. UDP-glucuronosyltransferase activity, p-nitrophenol as substrate, increased slowly before spawning and just after the event the activity rose to much higher levels. It did not show any thermal compensation during water cooling in the autumn. The amounts of reduced glutathione, the cosubstrate of the enzyme glutathione S-transferase, varied, being highest in the spring and lowest in the autumn. When studying the xenobiotic metabolizing enzyme activities of fish the sexual maturation and reproduction cycle have to be taken into consideration.