Affinity for Oxygen in Photoreduction of Molecular Oxygen and Scavenging of Hydrogen Peroxide in Spinach Chloroplasts

The apparent K m for O 2 in the photoreduction of molecular oxygen by spinach class II chloroplasts and photosystem I subchloroplast fragments was determined. In both cases, a value of 2 ∼ 3 μ M O 2 was obtained. The reaction rate constant between O 2 and P‐430, the primary electron acceptor of PS I, is estimated to be ∼ 1.5 × 10 7 M ‐1 s ‐1 and the factors affecting the production of superoxide by the photoreduction of O 2 in chloroplasts are discussed. Preliminary evidence is presented indicating the occurrence of an azide‐insensitive scavenging system for H 2 O 2 in chloroplast stroma.