Scavenging of superoxide generated in photosystem I by plastoquinol and other prenyllipids in thylakoid membranes

We have examined scavenging of a superoxide by various prenyllipids occurring in thylakoid membranes, such as plastoquinone-9, alpha-tocopherolquinone, their reduced forms, and alpha-tocopherol, measuring oxygen uptake in hexane-extracted and untreated spinach thylakoids with a fast oxygen electrode under flash-light illumination. The obtained results demonstrated that all the investigated prenyllipids showed the superoxide scavenging properties, and plastoquinol-9 was the most active in this respect. Plastoquinol-9 formed in thylakoids as a result of enzymatic reduction of plastoquinone-9 by ferredoxin-plastoquinone reductase was even more active than the externally added plastoquinol-9 in the investigated reaction. Scavenging of superoxide by plastoquinol-9 and other prenyllipids could be important for protecting membrane components against the toxic action of superoxide. Moreover, our results indicate that vitamin K-1 is probably the most active redox component of photosystem I in the generation of superoxide within thylakoid membranes.