The effects of temperature on the control of respiration rate, phosphorylation rate, proton leakage rate, the protonmotive force and the effective ATP/O ratio were determined in isolated rat liver mitochondria over a range of respiratory conditions by applying top-down elasticity and control analyses. Simultaneous measurements of membrane potential, oxidation and phosphorylation rates were performed under various ATP turnover rates, ranging from state 4 to state 3. Although the activities of the three subsystems decreased with temperature (over 30-fold between 37 and 4 °C), the effective ATP/O ratio exhibited a maximum at 25 °C, far below the physiological value. Top-down elasticity analysis revealed that maximal membrane potential was maintained over the range of temperature studied, and that the proton leakage rate was considerably reduced at 4 °C. These results definitely rule out a possible uncoupling of mitochondria at low temperature. At 4 °C, the decrease in ATP/O ratio is explained by the relative decrease in phosphorylation processes revealed by the decrease in depolarization after ADP addition [Diolez and Moreau (1985) Biochim. Biophys. Acta 806, 56–63]. The change in depolarization between 37 and 25 °C was too small to explain the decrease in ATP/O ratio. This result is best explained by the changes in the elasticity of proton leakage to membrane potential between 37 and 25 °C, leading to a higher leak rate at 37 °C for the same value of membrane potential. Top-down control analysis showed that despite the important changes in activities of the three subsystems between 37 and 25 °C, the patterns of the control distribution are very similar. However, a different pattern was obtained at 4 °C under all phosphorylating conditions. Surprisingly, control by the proton leakage subsystem was almost unchanged, although both control patterns by substrate oxidation and phosphorylation subsystems were affected at 4 °C. In comparison with results for 25 and 37 °C, at 4 °C there was evidence for increased control by the phosphorylation subsystem over both fluxes of oxidation and phosphorylation as well as on the ATP/O ratio when the system is close to state 3. However, the pattern of control coefficients as a function of mitochondrial activity also showed enhanced control exerted by the substrate oxidation subsystem under all intermediate conditions. These results suggest that passive membrane permeability to protons is not involved in the effect of temperature on the control of oxidative phosphorylation.

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