Temperature dependence of horse heart cytochromec oxidation by membrane preparations ofAnacystis nidulans: Characterization of two distinct arrhenius discontinuities

The oxidation of reduced horse heart cytochromec by membranes isolated from the cyanobacteriumAnacystis nidulans after growth at different temperatures was studied between 4°C and 41°C in the light and the dark using both spectrophotometric and polarographic techniques. Arrhenius plots of the temperature dependence of cytochromec photooxidation showed a single discontinuity at 25°C, 15°C, and 12°C in membranes derived from cells grown at 40°C, 30°C, and 25°C, respectively. By contrast. Arrhenius plots of the temperature dependence of dark respiratory cytochromec oxidation always displayed two distinct breaks at 25 and 18°C, 15 and 8.5°C, and 12 and 5.5°C in membranes isolated from cells grown at 40°C, 30°C, and 25°C, respectively. The results are discussed in terms of the thermotropic lipid-phase transitions known to take place in the membranes ofA. nidulans. Special reference will be made to possibly distinct localizations of the membrane-bound cytochromec oxidase complexes in respiration and photosynthesis.