Inhibitory site of carbonyl cyanide m-chlorophenylhydrazone in the electron transfer system of the chloroplasts

Abstract

The inhibitory effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the electron transport system of chloroplasts was investigated. At concentrations higher than 1 · 10 −7 M, CCCP acted as an inhibitor of the Hill reaction. Two different processes of CCCP inhibition were discernible; the one which was accomplished very rapidly and independently of light, and the other which developed slowly and only in light. Both processes of inhibition were not reversed by washing the inhibited chloroplasts. The rapid type of inhibition in the dark was observed not only in the Hill reaction in normal (untreated) chloroplasts, but also in ascorbate plus hydroquinonesupported NADP + photoreduction and manganese-supported 2,6-dichlorophenol-indophenol (DCIP) photoreduction in heated chloroplasts, whereas NADP + photoreduction supported by reduced DCIP was not inhibited. CCCP inactivated the Hill reaction by affecting the quantum efficiency of the reaction without influencing the rate constant of the dark reaction. There was no loss in the manganese content in chloroplasts caused by incubation with CCCP in the dark. The variable fluorescence in chloroplasts was decreased by CCCP at concentrations higher than 1 · 10 −7 M, with concomitant elimination of the induction phenomenon. However, on addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to the CCCP-inhibited chloroplasts there occurred a recovery of the induction phase of fluorescence. The pool size of the electron acceptor of photosystem II decreased with increasing concentrations of CCCP, but the pool size of the primary electron acceptor determined in the presence of DCMU was not affected by the poison. It is postulated that the site of CCCP inhibition is located between the endogenous redox substance (Y 1) which receives electrons from artificial electron donors such as ascorbate, and the primary electron donor of photosystem II (Y 2).