Evidence for α-Tocopherol Function in the Electron Transport Chain of Chloroplasts

The effect of three different stable radicals-2,2-diphenyl-1-picrylhydrazyl, 1,3,5-triphenyl-verdazyl, and galvinoxyl-was studied in photosystem II of spinach (Spinacia oleracea) chloroplasts. Inhibition by the three was noted on dimethylbenzoquinone reduction in presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and on silicomolybdate reduction in presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) in photosystem II and on the H(2)O --> methylviologen reaction encompassing both photosystems. Inhibition of all photosystem II reactions except silicomolybdate reduction could be partially restored by alpha-tocopherol or by 9-ethoxy-alpha-tocopherone but not by other quinones or radical chasers. On this basis, a functional role for alpha-tocopherol in the electron transport chain of spinach chloroplasts between the DCMU and DBMIB inhibition sites is postulated.