Effect of imidazole on the activity of photosystem II in spinach chloroplasts

The activity of the Hill reaction was accelerated about 2–3times in the presence of 1–5 mM imidazole. However, inchloroplasts uncoupled by carbonyl cyanide m-chlorophenylhydrazone,the acceleration of the Hill activity was not observed in thesame imidazole concentration range, but a moderate inhibitionof the activity was observed. These results suggest that inthe coupled chloroplasts, the enhanced Hill activity by imidazoleis due to uncoupling of phosphorylation. Imidazole-washing of chloroplasts at neutral pH caused a moderateinhibition of the Hill reaction to about 60% of the control,while a complete loss of the Hill activity was observed afterwashing at pH 10.5. In chloroplasts washed with imidazole atpH 10.5, the variable fluorescence yield was also diminished.These activities were restored by adding an artificial electrondonor to photosystem II. (Received October 14, 1978; )