Superoxide generated in photosystem I is scavenged by plastoquinol and other natural prenyllipids in thylakoid membranes

Abstract

Photosytem I (PS I) is the main site of superoxide generation in thylakoid membranes. Within this photosystem, superoxide is produced in the aprotic, hydrophobic interior of the membrane at Fe-S centers: X and A/B. The superoxide, because of its extended lifetime in the aprotic interior of the membrane, can cause directly deleterious effects on the membrane components or other toxic effects in a form of hydrogen peroxide after dismutation at the polar membrane surface. We have examined the reactivity of different prenyllipids occurring in thylakoid membranes, i.e., plastoquinone (PQ-9), a -tocoquinone (a -TQ), their reduced forms and a -tocopherol (a -Toc) with superoxide, measuring oxygen consumption in hexane-extracted and DCMU-inhibited thylakoids. The obtained results show that all the investigated prenyllipids inhibited the measured oxygen consumption by PS I. The most effective in this respect was plastoquinol (PQH2-9) followed by PQ-9, a -TQH2 and a -Toc. a -TQ showed the lowest activity in the investigated reaction. The PQH2-9 formed in thylakoids by enzymatic reduction of PQ-9 with ferredoxin-PQ reductase was even more active in the inhibition of oxygen consumption than the externally added PQH2-9. Our results indicate that the PQH2-9/PQ-9 couple, as well as other natural membrane prenyllipids, such as a -Toc or a -TQH2, play an important role in scavenging superoxide formed in PS I. This reaction reduces the level of superoxide diffusing towards membrane surface and the formation of the toxic hydrogen peroxide in chloroplasts.