title: Cytochrome b6/f complex as an indigenous photodynamic generator of singlet oxygen in thylakoid membranes.
abstract: Possible association of photodynamic sensitization by cytochrome b6/f complex (cyt b6/f) via singlet oxygen (1O2) 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 b6/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 b6/f in samples resulted in a marked exacerbation of the photoinactivation, particularly in UV-A and blue light. Such effect of cyt b6/f was suppressed by azide and enhanced by the medium deuteration. Photogeneration of 1O2 from cyt b6/f was confirmed by ESR and spectrophotometry, chemically trapping 1O2. Action spectra for both 1O2 photoproduction and PS II core photoinactivation by cyt b6/f bore a close resemblance to each other, seemingly carrying the absorption characteristics of the Rieske Fe-S protein. A complex deficient in the Rieske protein prepared from intact cyt b6/f showed virtually no generation of 1O2 in light, whereas an efficient photoformation of 1O2 was seen in the Rieske protein preparation. The results suggest that cyt b6/f, rather specifically the Rieske center, may play a prominent role in photoinhibition processes through type II photosensitization in thylakoids.
authors.affiliation: Department of Agricultural Chemistry, College of Agriculture and Life Sciences, Seoul National University, Suwon, Korea.
authors.forename: H J
authors.initials: HJ
authors.lastname: Suh
authors.forename: C S
authors.initials: CS
authors.lastname: Kim
authors.forename: J
authors.initials: J
authors.lastname: Jung
mesh_terms: D003573:Cytochrome b Group / Q000378:metabolism*; D045346:Cytochrome b6f Complex; D010100:Oxygen; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D026082:Singlet Oxygen; D018724:Spinacia oleracea; D020524:Thylakoids / Q000378:metabolism*
chemical_list: D003573:Cytochrome b Group; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D026082:Singlet Oxygen; D045346:Cytochrome b6f Complex; D010100:Oxygen