The effect of thylakoid lipids on an oxygen-evolving Photosystem II preparation

The lipid content of a Photosystem II preparation, derived by Triton X-100 fractionation of spinach thylakoid membranes, which retained high rates of oxygen evolution was examined. It was found that the detergent treatment resulted in a preferential loss of digalactosyldiacylglycerol. The effect on this preparation of exogenously added lipids, or lipid mixtures, purified from thylakoids was investigated. Addition of total polar lipid extract stimulated the rates of oxygen evolution, the same effect being achieved with isolated digalactosyldiacylglycerol or phosphatidylcholine alone. The stimulation of oxygen evolution was dependent on the degree of unsaturation of the lipids used and on the relative amounts of acidic lipids present in the mixtures. Addition of isolated acidic lipids alone resulted in a complete inhibition of oxygen evolution. The results are discussed in terms of the possible involvement of thylakoid lipids in the molecular organisation of the Photosystem II oxygen-evolving system.