Cytochrome b(6)/f complex as an indigenous photodynamic generator of singlet oxygen in thylakoid membranes

Possible association of photodynamic sensitization by cytochrome b(6)/f complex (cyt b6/f) via singlet oxygen (O-1(2)) mechanism with photoinhibition damage to photosystem II (PS II) was studied using such subthylakoid preparations as photosystem I(PS I) particles, PS II core complex and cyt b(6)/f from spinach leaves. Upon exposure to bright light, PS II core complex lost photosynthetic electron transport activity to a certain extent, whose spectral dependence implied that pheophytin a is likely involved in photoinactivation of PS II core complex in itself. The presence of PS I particles exerted virtually no effect on PS II core photoinactivation, However, the inclusion of cyt b(6)/f in samples resulted in a marked exacerbation of the photoinactivation, particularly in UV-A and blue light. Such effect of cyt b(6)/f was suppressed by azide and enhanced by the medium deuteration. Photogeneration of O-1(2) from cyt b(6)/f was confirmed by ESR and spectrophotometry, chemically trapping O-1(2). Action spectra for both O-1(2) photoproduction and PS II core photoinactivation by cyt b(6)/f bore a close resemblance to each other, seemingly carrying the absorption characteristics of the Rieske FeS protein. A complex deficient in the Rieske protein prepared from intact cyt b(6)/f showed virtually no generation of O-1(2) in light, whereas an efficient photoformation of O-1(2) was seen in the Rieske protein preparation, The results suggest that cyt b(6)/f, rather specifically the Rieske center, may play a prominent role in photoinhibition processes through type II photosensitization in thylakoids.