Photosynthetic oxygen evolution from hydrogen peroxide.

Abstract A study was made of the interactions of flash-illuminated chloroplasts with hydrogen peroxide. We conclude: 1. 1. The oxygen precursor system can be reduced beyond the S 0 state to an S −1 state, which can be oxidized to S 0 by a single flash. 2. 2. In the dark, a two-electron donation by H 2 O 2 takes place which reduces S 2 to S 0 and S 1 to S −1 . 3. 3. At the same time, two-electron oxidations by H 2 O 2 re-form S 2 from S 0 and S 1 from S −1 . 4. 4. The catalase-like activity due to this cyclic oxidation and reduction of the S enzyme is higher with the S 2 ag S 0 couple than with the S 1 ag S −1 couple. Another process, however, is responsible for most of the O 2 evolution from H 2 O 2 in the light. Our evidence indicates that this process: (1) is independent of the S states and insensitive to Tris washing, (2) turns over rapidly in high concentrations of peroxide, (3) yields 1 O 2 per electron passing through system II; (4) dismutates two H 2 O 2 molecules, so that there is no net consumption of ‘holes’.