Effect of carbonylcyanide m-chlorophenylhydrazone on the photochemical reactions of isolated chloroplasts

Carbonylcyanide m -chlorophenylhydrazone is a powerful inhibitor of both photosynthetic phosphorylation and photoreduction of O 2 in isolated spinach chloroplasts. The inhibition of phosphorylation was observed in three cyclic systems (PMS, vitamin K 3 and FMN) and in three non-cyclic systems (ferricyanide-water, TPN + -water and TPN + -ascorbate). Inhibition of O 2 evolution was also obtained in the ferricyanide-water and TPN + -water systems but only at higher concentrations of CCCP. The only photochemical activity of chloroplasts which was found to be resistant to CCCP inhibition was the photoreduction of TPN + with ascorbate-dichlorophenol-indophenol as the electron donor (TPN + -ascorbate system). In the cyclic system (PMS) but not in the non-cyclic system (ferricyanide-water) CCCP inhibition of photophosphorylation varied with light intensity. Thus, in the PMS system, at 1000 lux, 0.4 μM CCCP gave 50% inhibition of photophosphorylation, whereas at 50 000 lux, 8 μM CCCP was required to give 50% inhibition. A model is proposed to account for the differences between cyclic and non-cyclic phosphorylation. The inhibition of photophosphorylation by CCCP was completely reversed by the addition of cysteine or by washing.