Effect of digitonin concentration on electron transport, phosphorylation, and proton uptake by subchloroplast particles.

The relative activity of photosystem I subchloroplast particles was dependent on the digitonin concentration used during incubation. At low digitonin concentrations (2 mg digitonin per mg chlorophyll), the particles were greatly enriched in photosystem I activities and showed high rates of cyclic phosphorylation and appreciable light-induced proton uptake. Increasing the concentration of digitonin increased the yield of photosystem I particles but decreased their specific activity of electron transport and of cyclic phosphorylation. The decrease in activity was not related to the degree of enrichment of the pigments associated with photosystem I nor to the degree of separation of the photosystems. Release of plastocyanin was dependent on the digitonin concentration. As the digitonin concentration was increased, the plastocyanin content of the photosystem I particles decreased. The decrease in photochemical activity with increasing digitonin concentration was attributed to the loss of plastocyanin during isolation.