Evidence that cytochrome b(559) is involved in superoxide production in photosystem II: effect of synthetic short-chain plastoquinones in a cytochrome b(559) tobacco mutant

摘要

Light-induced production of superoxide (O-2(center dot-)) in spinach PSII (photosystem 11) 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(center dot-) production in the following order: PQ-1 > PQ-2 PQ-9. In PSII membrane particles isolated from the tobacco cyt (cytochrome) b(559) mutant which carries a single-point mutation in the beta-subunit and also has a decreased amount of the alpha-subunit, the effect of PQ-1 was less than in the wild-type. The increase in LP (low-potential) cyt b(559) content, induced by the incubation of spinach PSII membrane particles at low pH, resulted in a significant increase in O-2(center dot-) formation in the presence of PQ-1, whereas it had little effect on O2(center dot-) production in the absence of PQ-1. The enhancement of O-2(center dot-) 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 b(559) increased, as pH was decreased. The presence of molecular oxygen significantly enhanced dark oxidation of LP cyt b(599). Based on these findings it is suggested that short-chain PQs stimulate O-2(center dot-) production via a mechanism that involves electron transfer from Pheo(-) (pheophytin) to LP cyt b(559) and subsequent auto-oxidation of LP cyt b(559).