Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. III. Conditions for the Operation of an Apparent Q-Cycle in Thylakoids

Abstract

The proton-to-electron ratio in pea thylakoids, considering proton uptake with ferricyanide as electron acceptor, was reconfirmed as 1 in periods of single-turnover (<0.5 µs, 2-3 mJ) flashes delivered at 5-50 Hz and at pH values 6.4-8.3. Addition of valinomycin in the presence of K⁺ increased proton uptake in a way depending on [val], flash frequency and flash number.

A maximal stimulation by valinomycin of up to about 1.8× controls was observed in fresh preparations. Half-maximal stimulation was caused by c. 2 nM valinomycin at 10-20 Hz, at c. 3 Hz with 10 nM valinomycin, and after c. five flashes at 50 Hz with 10 nM valinomycin.

The results are discussed in terms of recent models for the Q-cycle. It is suggested that such a cycle operates in chloroplasts only when the intramembrane electric field induced in a series of flashes is kept small by the presence of valinomycin. Preliminary observations of 'P518', the thylakoid component probably indicating the electric field, are consistent with this idea. This field may control the transverse electron flow between the two cytochrome b₆ molecules in the b/f complexes.