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1 January 2000 Cytochrome b6/f Complex as an Indigenous Photodynamic Generator of Singlet Oxygen in Thylakoid Membranes
Hwa-Jin Suh, Chang Sook Kim, Jin Jung
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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.

Hwa-Jin Suh, Chang Sook Kim, and Jin Jung "Cytochrome b6/f Complex as an Indigenous Photodynamic Generator of Singlet Oxygen in Thylakoid Membranes," Photochemistry and Photobiology 71(1), 103-109, (1 January 2000).<0103:CBFCAA>2.0.CO;2
Received: 30 July 1999; Accepted: 1 October 1999; Published: 1 January 2000

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Cytochrome, b6/f
Fe-S center
Pheophytin a
Photosystem II
Rieske center
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