Interaction of alpha-tocopherol quinone, alpha-tocopherol and other prenyllipids with photosystem II

We have found that plastoquinone-A (PQ-A) and alpha-tocopherol (alpha-Toc) increased the reduction level of the high-potential form of cytochrome b-55 9 (cyt, b-559 HP) and alpha-tocopherol quinone (alpha-TQ) decreased the lev el of this cytochrome form in Scenedesmus obliquus wild-type, while the inv estigated prenyllipids were not active in the restoration of the cyt. b-559 HP form in Scenedesmus PS28 mutant and Synechococcus 6301 (Anacystis nidul ans) where the cyt. b-559 HP form is naturally not present. Among the teste d prenyllipids, alpha-TQ quenched fluorescence in thylakoids of S. obliquus wild-type, the PS28 mutant and tobacco to the highest extent, while PQ-A w as less effective in this respect. alpha-Tocopherol showed the opposite eff ect to alpha-TQ and it was rather small. The fluorescence quenching measure ments of thylakoids in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-dim ethylurea) showed that the alpha-Toc and FCCP (carbonylcyanide-p-trifluorom ethoxyphenyl-hydrazone) did not quench non-photochemically chlorophyll fluo rescence while PQ-9 and alpha-TQ were effective fluorescence quenchers at h igher concentrations (> 15 mu M). However, at the lower alpha-TQ concentrat ions where its effective fluorescence quenching was found in DCMU-free samp les, there was nearly no quenching effect by alpha-TQ observed in DCMU-trea ted thylakoids. This suggested a specific, not non-photochemical, DCMU sens itive, fluorescence quenching of photosystem II (PSII) at low alpha-TQ conc entrations which is probably connected with the cyclic electron transport a round PSII and might have a function of excess light energy dissipation. Th e effects of alpha-TQ on PSII resembled those of FCCP under many respects w hich might suggest similar mechanism of action of these compounds on PSII, i.e. the catalytic deprotonation and/or redox changes of some components of PSII such as the water splitting system, tyrosine D, Chl(z) or cytochrome b-559. (C) 2000 Editions scientifiques et medicales Elsevier SAS.