Ferricyanide Reduction in Photosystem II of Spinach Chloroplasts

Ferricyanide can be reduced in Photosystem II of spinach chloroplasts at 2 separate sites, both of which are sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, but only one of which is sensitive to dibromothymoquinone. Data presented in this paper emphasize ferricyanide site II of Photosystem II, which is sensitive to thiol inhibition and may reflect a cyclic pathway around Photosystem II. Ferricyanide reduction sites 1 and 2 also differ from each other in fractions isolated from discontinuous sucrose gradients, from fragmented chloroplasts, and upon trypsin treatment. Sucrose density gradient centrifugation shows that ferricyanide reduction site 1 activity at pH 6 decreases from 30 to 50% in various isolated fractions, while the dibromothymoquinone-insensitive activity at pH 8 (site 2) is stimulated from 15 to 35%.Fragmentation of chloroplasts also stimulates ferricyanide site II activity, but trypsin treatment destroys ferricyanide reduction site II completely in 6 minutes. Ferricyanide reduction site 1 still retains 50% activity at this point. The meaning of these differences is discussed in terms of the physical location of these two sites on the thylakoid membrane.