Effects of Adenine Nucleotides and of Photophosphorylation on H+ Uptake and the Magnitude of the H+ Gradient in Illuminated Chloroplasts

The steady state extent of H+ uptake in illuminated chloroplasts can be observed routinely to be decreased by ADP in the presence of arsenate when these reagents are present at saturating concentrations. The failure of previous investigators to observe this phenomenon may be explained by the fact that suboptimal concentrations of ADP and especially arsenate were used. ATP (or ADP) in the absence of arsenate enhance the magnitude of the light-induced H+ concentration gradient in chloroplasts (ΔpH) by 0.15 to 0.2 unit. In the presence of arsenate, ΔpH was decreased by ADP up to 0.4 unit and this decrease was prevented by phlorizin, an energy transfer inhibitor. When phosphorylation and ΔpH, assayed under similar conditions, were varied either by decreasing light intensity or by adding an uncoupler, it was apparent that initial phosphorylation rates decreased much more rapidly than ΔpH. Plots of log phosphorylation rate against ΔpH were linear and showed that a decrease in ΔpH of 0.3 to 0.34 unit was accompained by a 10-fold decrease in the rate of phosphorylation. The observed, relatively small, decrease in ΔpH by phosphorylation is consistent with the apparent dependence of phosphorylation rate on ΔpH.