Temperature compensation of hepatic microsomal cytochrome P-450 activity in rainbow trout. I. Thermodynamic regulation during water cooling in autumn

Abstract 1. 1. The molecular mechanism of temperature compensation in the hepatic polysubstrate monooxygenase system during the water cooling period in autumn was studied in rainbow trout ( Salmo gairdneri Richardson). 2. 2. When 7-ethoxycoumarin O -deethylase and benzo(a)pyrene hydroxylase activities were measured at ambient temperatures, practically no changes could be observed between August (20°C) and November (5°C). When measured at a constant temperature of 18°C over the same period, a three-fold increase occurred. 3. 3. The total amount of cytochrome P-450 remained constant. 4. 4. Cytochrome c reductase activity and the rate of the first reduction step of cytochrome P-450 increased two-fold without a change in the activation energy. 5. 5. Only minor changes in the microsomal fatty acid composition were seen during the adaptation. No phase shifts (breaks) in the temperature dependence of cytochrome c and cytochrome P-450 reduction, were observed either. 6. 6. The temperature compensation can mostly be explained by the reduction of cytochrome P-450 facilitated by an increased reductase/cytochrome P-450 ratio.