Effect of Pb2+ ions on photosynthetic apparatus.

Using model lead compounds Pb(NO3)2 and Pb(CH3CHOO)2, the mechanism and the site of action of Pb2+ ions in the photosynthetic apparatus of spinach chloroplasts were studied. Both compounds inhibited photosynthetic electron transport (PET) through photosystem 1 (PS1) and photosystem 2 (PS2), while Pb(NO3)2 was found to be more effective PET inhibitor. Using EPR spectroscopy the following sites of Pb2+ action in the photosynthetic apparatus were determined: the water-splitting complex and the Z•/D• intermediates on the donor side of PS2 and probably also the ferredoxin on the acceptor side of PS1, because cyclic electron flow in chloroplasts was impaired by treatment with Pb2+ ions. Study of chlorophyll fluorescence in suspension of spinach chloroplasts in the presence of Pb2+ ions confirmed their site of action in PS2. Using fluorescence spectroscopy also formation of complexes between Pb2+ and amino acid residues in photosynthetic proteins was confirmed and constants of complex formation among Pb2+ and aromatic amino acids were calculated for both studied lead compounds.