Three forms of cytochrome b 559 and their relation to the photosynthetic activity of chloroplasts

Spinach chloroplasts were found to contain three forms of cytochrome b(559) that have the same alpha-peak at 559 nm, but are distinguished from one another by their oxidation-reduction potentials. The high-potential (H) form (E(m) about 330-350 mV) is reducible by hydroquinone, the middle-potential (M) form (E(m) about 50-80 mV) is reducible by ascorbate but not by hydroquinone, and the low-potential (L) form is reducible by dithionite but not by ascorbate. The H form was the predominant one in freshly prepared chloroplasts and was strongly correlated with Photosystem II activity. Chloroplast treatments such as aging, sonication, and mild heating, singly or in combination, brought about a shift of the H to the M form. More drastic treatments resulted in shifts of the H and M forms to the L form. The role of cytochrome b(559) as an electron carrier in System II of plant photosynthesis is discussed.