Alkaloids as inhibitors of photophosphorylation in spinach chloroplasts

A group of 12 alkaloids were tested as inhibitors of photophosphorylation in spinach chloroplasts. Ajmaline, a dihydroindole alkaloid, was found to be the strongest inhibitor of both cyclic and non-cyclic photophosphorylation. Low concentrations of ajmaline also inhibited the dark and light ATPases, and the coupled electron flow from water to ferricyanide, measured either as ferrocyanide formed or as oxygen evolved, but not the uncoupled electron transport or the pH rise of illuminated unbuffered suspensions of chloroplasts. Higher concentrations of ajmaline stimulated, instead of inhibiting, photosynthetic electron transport or oxygen evolution and decreased the pH rise, thus behaving as an uncoupler, such as ammonia. Photophosphorylation was partially inhibited by 100 microM dihydrosanguinarine, 100 microM dihydrochelerythrine (benzophenanthridine alkaloids); 500 microM O,O'-dimethylmagnoflorine, 500 microM N-methylcorydine (aporphine alkaloids) and 1 mM julocrotine. They also inhibited coupled oxygen evolution and only partially (dihydrosanguinarine and dihydrochelerythrine) or not at all (the other alkaloids) uncoupled oxygen evolution. Spegazzinine (dihydroindole alkaloid), magnoflorine, N-methylisocorydine, coryneine (aporphine alkaloids), candicine and ribalinium chloride were without effect on photophosphorylation at 500 microM.