REDOX CHAINS IN CHLOROPLAST ENVELOPE MEMBRANES - SPECTROSCOPIC EVIDENCE FOR THE PRESENCE OF ELECTRON CARRIERS, INCLUDING IRON-SULFUR CENTERS

We have shown that envelope membranes from spinach chloroplasts contai n (i) semiquinone and flavosemiquinone radicals, (ii) a series of iron -containing electron-transfer centers, and (iii) flavins (mostly FAD) loosely associated with proteins, In contrast, we were unable to detec t any cytochrome in spinach chloroplast envelope membranes, In additio n to a high spin [1Fe](3+) type protein associated with an EPR signal at g = 4.3, we observed two iron-sulfur centers, a [4Fe.4S](1+) and a [2Fe-2S](1+), associated with features? respectively, at g = 1.921 and g = 1.935, which were detected after reduction by NADPH and NADH, res pectively, The [4Fe-4S] center, but not the [2Fe-2S] center, was also reduced by dithionite or 5-deazaflavin/oxalate. An unusual Fe-S center , named X, associated with a signal at g = 2.057, was also detected, w hich was reduced by dithionite but not by NADH or NADPH, Extremely fas t spin-relaxation rates of flavin- and quinone-free radicals suggest t heir close proximity to the [4Fe-4S] cluster or the high-spin [1Fe](3) center, Envelope membranes probably contain enzymatic activities inv olved in the formation and reduction of semiquinone radicals (quinol o xidase, NADPH-quinone, and NADPH-semiquinone reductases), The physiolo gical significance of our results is discussed with respect to (i) the presence of desaturase activities in envelope membranes and (ii) the mechanisms involved in the export of protons to the cytosol, which par tially regulate the stromal pH during photosynthesis, The characteriza tion of such a wide variety of electron carriers in envelope membranes opens new fields of research on the functions of this membrane system within the plant cell.