Cytochrome c is reduced mainly by plastoquinol and not by superoxide in thylakoid membranes at low and medium light intensities: its specific interaction with thylakoid membrane lipids.

We have found that, at low light intensity (5-10 micromol photons x m(-2) x s(-1)), photoreduction of cyt (cytochrome) c by isolated thylakoids was not inhibited by dinitrophenylether of iodonitrothymol, an inhibitor of the cyt b6- f complex, and the inhibition was only partial at medium light intensity (50-200 micromol photons x m(-2) x s(-1)). The photoreduction was not significantly influenced by superoxide dismutase. The conclusion that cyt c could be reduced directly by the plastoquinone pool was confirmed by the observation that plastoquinol-9 reduced cyt c efficiently when it was incorporated into liposome membranes prepared from thylakoid membrane lipids. It was shown that the cyt is specifically bound to thylakoid lipid liposomes owing to the presence of negatively charged lipids, phosphatidylglycerol and sulphoquinovosyldiacylglycerol, and the reduction was stimulated by the presence of monogalactosyldiacylglycerol, an inverted micelles-forming lipid, in the membranes, where the cyt c reduction by plastoquinol probably takes place. The results obtained are also discussed in terms of reliability of the method of cyt c photoreduction for determining superoxide production by illuminated thylakoids.