Protective role of CaCl2 against Pb2+ inhibition in Photosystem II.

The Pb2+ treatment of Photosystem II (PS II) membrane fragments, either intact or depleted in 17 and 23 kDa extrinsic polypeptides, inhibits PS II activity. When CaCl2 was present in the assay, the Pb2+ inhibition was significantly reduced in both types of PS II membranes, suggesting a protective role of CaCl2 against Pb2+ inhibition. However, in either case, the degree of PS II inhibition by Pb2+ was higher in the protein depleted than in intact PS II. It showed that the loss of endogenous Ca2+ induced by polypeptide depletion causes the PS II to be more susceptible to Pb2+. The interaction of Pb2+ with CaCl2 in protein-depleted PS II was competitive. Our results suggest that Pb2+ competes for binding to the Ca2(+)- and Cl- active sites in the water-splitting complex. Since Pb2+ inhibition of PS II activity cannot be reversed by CaCl2 but can be reversed by diphenylcarbazide, we conclude that Pb2+ induced inhibition of PS II activity was mediated via the water-splitting system.