Partial Resolution of the Enzymes Catalyzing Photophosphorylation

Exposure of chloroplasts to sonic oscillation in a medium of low osmolarity rapidly inactivated the Hill reaction, photophosphorylation, and photooxidation of ascorbate in the presence of dichlorophenyl-1, 1-dimethylurea. Concurrently with this inactivation plastocyanin was liberated from the chloroplasts. The addition of plastocyanin during sonic oscillation prevented net loss of plastocyanin from the chloroplasts and preserved the Hill reaction, cyclic and noncyclic photophosphorylation, and ascorbate photooxidation. These effects were not reversed by washing the particles. In contrast, the addition of plastocyanin to the assay mixtures after sonication did not enhance photophosphorylation even though it stimulated electron flow. This stimulation by external plastocyanin was abolished by washing. 2. A specific antibody against spinach plastocyanin had no effect on electron transport or photophosphorylation in chloroplasts and did not agglutinate chloroplasts. However, if chloroplasts were sonicated in the presence of the antibody, a more pronounced inhibition of the Hill reaction, of ascorbate photooxidation, and of photophosphorylation was observed than when sonic oscillation was performed in the presence of γ-globulins from nonimmunized rabbits. 3. From these findings we conclude that plastocyanin functions in noncyclic electron flow between the two photosystems as well as in cyclic photophosphorylation. In contrast to the chloroplast coupling factor (CF1) and ferredoxin-NADP+ reductase, plastocyanin in situ is not accessible to antibody added to the chloroplasts. Plastocyanin added to chloroplast particles can induce an artificial electron flow through Photosystem I. This process is not coupled to photophosphorylation but is sensitive to external antibody. The similarity between the function of plastocyanin in the inner chloroplast membrane and cytochrome c in the inner mitochondrial membrane is discussed.