Multiple light-induced reactions of cytochromes b and c in Rhodopseudomonas spheroides

Abstract

Illumination of chromatophore preparations from Rhodopseudomonas spheroides causes the oxidation of a cytochrome c and a slight oxidation of a cytochrome b with a maximum at 560nm. When illuminated in the presence of antimycin A the oxidation of cytochrome c was more pronounced and cytochrome b560 was reduced; the dark oxidation of cytochrome b560 was biphasic in the presence of succinate, but not in the presence of NADH, a less effective reductant. Split-beam spectroscopy showed that, in addition to the reduction of cytochrome b560, another pigment with maxima at 565 and 537nm. was reduced and was more rapidly oxidized in the dark than cytochrome b560. This pigment, tentatively identified as cytochrome b565, was also detected in spectra at 77°k, after brief illumination at room temperature; the maxima at 77°k were at 562 and 536nm. In the absence of antimycin A, light caused a transient reduction of cytochrome b565 and an oxidation of cytochrome b560. Dark oxidation of b565 was rapid, even in the presence of antimycin A and succinate. Difference spectra, at 77°k, of ascorbate-reduced minus succinate-reduced chromatophores or of anaerobic succinate-reduced minus aerobic succinate-reduced chromatophores suggested that two cytochromes c were present, with maxima at 547 and 549nm. When chromatophores frozen at 77°k were illuminated both these cytochromes c were oxidized, indicating a close association with the photochemical reaction centre. A scheme involving two reaction centres is proposed to explain these results.