Univalent reduction of molecular oxygen by spinach chloroplasts on illumination.

Spinach chloroplasts induced the photoreduction of cytochrome c and photooxidation of epinephrine, both of which depended on oxygen and were inhibited by superoxide dismutase. This confirmed the univalent reduction of molecular oxygen in illuminated chloroplasts. In the presence of cytochrome c, hydrogen peroxide formation by chloroplasts was inhibited completely. Rates for the photoreduction of cytochrome c and the formation of hydrogen peroxide were equivalent, indicating that the univalent reduction of oxygen is a part of the Mehler reaction and the divalent reduction of oxygen is absent in chloroplasts. Spinach ferredoxin, cytochrome c-reducing substance, and phenazine methosulfate did not stimulate or decrease the superoxide dismutase-inhibited rates of the photoreduction of cytochrome c and photo-oxidation of epinephrine. Chloroplasts treated with histone, which blocked Photosystem I, could not photooxidize epinephrine. Thus, we propose that the univalent reducing site of oxygen is a primary electron acceptor in Photosystem I.