Calcium content of oxygen-evolving Photosystem II preparations from higher plants. Effects of NaCl treatment

The abundance of bound Ca²⁺ in four Photosystem II oxygen-evolving preparations from spinach and rice were determined after removal of contaminating Ca²⁺ with chelex 100 (Kashino, Y., Satoh, K. and Katoh, S. (1986) FEBS Lett. 205, 150-154). (1) Spinach membrane preparations which evolved oxygen at rates of 350-500 μmol per mg chlorophyll (Chl) per h contained, on average, 1.8 Ca²⁺ per 200 Chl, whereas a mean value of 2.1 Ca²⁺ per 200 Chl was obtained with more active spinach preparations, showing oxygen-evolving rates of 500-800 μmol per mg Chl per h. The most active membrane preparations, which evolved oxygen at rates of 750-1050 μmol per mg Chl per h, were isolated from rice seedlings. The rice preparations also contained 2.1 Ca²⁺ per 200 Chl. Thus, different activities of the three preparations cannot be related to difference in their Ca²⁺ abundance and the maximum number of Ca²⁺ needed to promote high rates of oxygen evolution is 2 Ca²⁺ per 200 Chl in higher plants. (2) Highly purified spinach oxygen-evolving complexes (Ikeuchi, M., Yuasa, M. and Inoue, Y. (1985) FEBS Lett. 185, 316-323) contained 1 Ca²⁺ per 50 Chl. (3) Treatment of the spinach membrane preparations with high concentrations of NaCl, either in the dark or light, caused no significant loss of the bound Ca²⁺, although the treatment strongly inhibited oxygen evolution and the inhibition was considerably reversed by addition of 5 mM CaCl₂. Exposure of the NaCl-washed membranes to EGTA failed to reduce the amount of bound Ca²⁺. Thus, inactivation of oxygen evolution by the salt wash is not related to release of Ca²⁺. (4) The abundance of Ca²⁺ in spinach membrane preparations was not, or only slightly, affected by treatments with 1 mM NH₂OH, 1 M MgCl₂ or 0.8 M Tris, which solubilized either Mn or the three extrinsic proteins associated with the membranes, or both. It is concluded that the three proteins are not involved in the Ca²⁺ binding. The number of Ca²⁺ functioning in Photosystem II electron transport of higher plants is discussed.