Spinach chloroplasts scavenge hydrogen peroxide on illumination

Intact spinach chloroplasts isolated by the modified silica sol density centrifugation scavenged HzC>2 on illumination at a rate about 3-fold that of bicarbonate-dependent C>2-evolution. Accompanying the disappearance of 1 mole of H2O2 is the evolution of a half mole of O2. The photoscavenging of H2O2 was inhibited by 3(3,4-dichloro-phenyl)-l,l-dimethylurea, cyanide and azide. These results indicate that in chloro-plasts H2O2 is reduced to H2O by a cyanide and azide-sensitive peroxidase using a photoreductant as an electron donor. Key words: Active oxygen — Hydrogen peroxide — Intact chloroplasts — Per-oxidase — Scavenger for hydrogen peroxide. Photoreduction of molecular oxygen by chloroplasts was found by Mehler in 1951 (17). Recent studies have shown that molecular oxygen is photoreduced univalently by the primary electron acceptor in photosystem I and that the superox-ide anion radical (O2~-) is the primary product of this photoreduction (/, 3). The photoreduction rate is saturated at a low concentration of O2; the apparent Km for O2 is —2.5 /*M (2, 15). Most of the C>2~- produced is disproportionated catalyticall