Weak Light Dependent Coupling of the Photoreduction of Oxygen and the Photoreactivation of Oxygen Evolving Center

Abstract

Tris-washed chloroplasts are extracted of Mn and inhibited at the oxygen evolving center. The inactivated chloroplasts recovered the activity by treating them with reduced DCPIP (DCPIP-treatment) and then by photoreactivation. The photoreactivation is incubating chloroplasts with Mn²⁺, Ca²⁺, Cl⁻ and DTT under weak light (300 lux, 0.7 W/m²) at 20 °C for 20 min. It has been unknown why weak light and DTT are necessary for the reactivation of oxygen evolving center. Recently we found that oxygen was another essential factor for the photoreactivation because anaerobic condition inhibited the reactivation, and that NADP⁺, NAD⁺ and FMN, but not NADPH and NADH, substituted for oxygen in the anaerobic photoreactivation. This was an important suggestion for us that photoreactivation was dependent on the weak light stimulated electron transport to oxygen or NADP⁺. Next we tried to find what should be the electron donor for the electron transport in the photoreactivation and found DTT and many other reductants useful as the electron donor. The following results suggest that weak light stimulate the electron transport from DTT to oxygen or NADP⁺ and that the electron transport coupled to the photoreactivation of oxygen evolving center because omission of the electron donor and/or the acceptor inhibited the reactivation.