Modulation of proton efflux from chloroplasts in the light by external pH

Abstract

The effects of external pH on the efflux of protons from illuminated spinach chloroplasts have been studied by monitoring the rates of proton-pumping electron transport under a variety of steady-state conditions. Phosphorylation-coupled proton efflux through the ATP synthase (CF 0-CF 1), determined from the rates of ATP formation and that portion of the total electron transport attributable to phosphorylation, is strongly dependent upon pH over the range 6–9, with little activity below pH 7 and half-maximal activity at pH ≈ 7.6. Noncoupled proton efflux through the ATP synthase, determined in the absence of ADP and phosphate, was also strongly pH sensitive, with little activity below pH 7.5 and half-maximal activity at pH ∼- 7.9. When proton efflux via CF 0 was prevented by triphenyltin, the rate of passive proton leakage across the membrane was very low and practically insensitive to external pH indicating that the major pH-sensitive pathway(s) for proton efflux in the light involves CF 0 · CF 1. Modification of CF 1 sulfhydryls by Ag + resulted in an apparent increase in proton efflux via the normally coupled CF 0 · CF 1 pathway (half-maximal activity = pH 7.6), whereas modification by Hg 2+ resulted in an apparent increase in proton efflux via the noncoupled CF 0 · CF 1 pathway (half-maximal activity = pH 7.9).