FURTHER STUDIES ON PHOTOPHOSPHORYLATION ACCOMPANYING THE HILL REACTION WITH FERRICYANIDE.

Abstract

Abstract The striking pH dependence of the Hill reaction with ferricyanide was confirmed for reactions carried out in a low anion buffer. The pH dependence of inhibition of the Hill reaction by ADP, of stimulation by associated phosphorylation and of the stoichiometry between ferricyanide photoreduction and phosphate consumption was also examined. It was found that at every pH, the “extra” ferricyanide reduced when phosphate was added to the ADP-inhibited system, was equal, on a molar basis, to the amount of phosphate consumed. The pH dependence of ferricyanide photoreduction stimulated by trichlorophenol indophenol dye and NH4Cl was examined. Taking into consideration the variation in the concentration of NH3 in the presence of NH4Cl at various pH values, it is concluded that the Hill reaction as stimulated by both dye and NH4Cl is relatively independent of pH. However, the degree of stimulation induced by either agent was strongly dependent upon pH, and was greatest at low pH, where the unstimulated reaction (and phosphorylation) is most sluggish. Concentrations of dye which have little effect upon the ferricyanide Hill reaction were found to relieve or prevent the marked inhibition of ferricyanide photoreduction induced by ADP, and to stimulate ferricyanide photoreduction in the presence of ADP and suboptimal concentrations of phosphate. In such concentrations of dye its uncoupling effects are thus seen to resemble those of arsenate. These results are discussed in terms of the possible participation of an activated intermediate which is unstable at alkaline pH. Recent studies on the formation of activated intermediates during photophosphorylation are discussed and the results of such studies are shown to be consistent with a novel scheme for the Hill reaction.