Evidence that cytochrome b₅₅₉ is involved in superoxide production in photosystem II: effect of synthetic short-chain plastoquinones in a cytochrome b₅₅₉ tobacco mutant Available to Purchase

Light-induced production of superoxide (O₂^•−) in spinach PSII (photosystem II) membrane particles was studied using EPR spin-trapping spectroscopy. The presence of exogenous PQs (plastoquinones) with a different side-chain length (PQ-n, n isoprenoid units in the side-chain) enhanced O₂^•− production in the following order: PQ-1>PQ-2≫PQ-9. In PSII membrane particles isolated from the tobacco cyt (cytochrome) b₅₅₉ mutant which carries a single-point mutation in the β-subunit and also has a decreased amount of the α-subunit, the effect of PQ-1 was less than in the wild-type. The increase in LP (low-potential) cyt b₅₅₉ content, induced by the incubation of spinach PSII membrane particles at low pH, resulted in a significant increase in O₂^•− formation in the presence of PQ-1, whereas it had little effect on O₂^•− production in the absence of PQ-1. The enhancement of O₂^•− formation induced by PQ-1 was not abolished by DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Under anaerobic conditions, dark oxidation of LP cyt b₅₅₉ increased, as pH was decreased. The presence of molecular oxygen significantly enhanced dark oxidation of LP cyt b₅₅₉. Based on these findings it is suggested that short-chain PQs stimulate O₂^•− production via a mechanism that involves electron transfer from Pheo⁻ (pheophytin) to LP cyt b₅₅₉ and subsequent auto-oxidation of LP cyt b₅₅₉.