Chloroplast cytochrome b-563, hydrophobic environment and lack of direct reaction with ferredoxin.

1. The reaction of electron donors with cytochrome b-563 in chloroplasts was studied by investigating their effects on the rate of its reduction in the presence of dithionite, which reacts only slowly with the cytochrome. The relative effects of 9,10-anthraquinone and 9,10-anthraquinone 2-sulphonate in the presence of dithionite suggested that the site of attack of redox reagents was protected behind a hydrophobic barrier from the external medium. 2. Ferredoxin had no measurable effect on the rate of reduction of the cytochrome in the presence of dithionite. 3. The reduction of pigment P700 in the dark after illumination in the presence of the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea was stimulated by a combination of NADPH and ferredoxin, although NADPH alone had little effect. The same combination was unable to reduce cytochrome b-563 at a measurable rate. 4. It is concluded that the cytochrome is unlikely to be part of a linear pathway of electron flow between ferredoxin and pigment P700.