In chloroplasts of Spinacea oleracea L., Hg2+ ions interact with some sites in the photosynthetic electron transport chain: (l) with the intermediates Z+/D+ situated in the D1 and D2 proteins and with the manganese cluster in the oxygen evolving complex which are located on the donor side of photosystem (PS) 2, (2) with the chlorophyll a dimer in the core of PS1 (P700). P700 is oxidized in the dark by HgCl2. The Hg2+ ions form organometallic complexes with amino acids contained in chloroplast proteins. and F. Šeršeň, K. Král'ová, A. Bumbálová.
Five potential /3-adrenoceptor blocking (BAB) compounds, alkylesters of 4-[(2-hydroxy-3-alkylamino)propoxy] phenylcarbamic acid, and eight calcium channel blockers (CB), i.e. nifedipine, nimodipine, niludipine, nitrendipine, verapamil, gallopamil, mepamil and diltiazem, were compared as to their inhibitory effect on thrombin induced aggregation of washed rat platelets and their effect on dynamies/disorder of liposomal membranes prepared from platelet lipids, studied by EPR spectroscopy of a lipid spin probe. The anti-aggregatory potency of the BAB and CB drugs was effective within the concentration range of 0.01-1 mmol/1. The antiaggregatory potency of BAB increased in the order BL- 143< BL-243< BL-343 < BL-443 < BL-543 and among the CB, nifedipine and diltiazem were the least potent, whereas nitrendipine and mepamil were the most potent drugs. The potency of the other CB tested was intermediate. The BAB drugs increased the dynamies/disorder of the liposomes in the same order as they inhibited platelet aggregation, whereas there was no relationship between antiaggregatory effect of CB and their influence on dynamies/disorder of the liposomes. Nifedipine, nimodipine, niludipine and nitrendipine had a minor perturbation effect on the liposomes, whereas verapamil, mepamil, gallopamil and diltiazem pronouncedly increased the dynamies/disorder of the hydro- phobic part of the liposomes. The results indicate that the anti-aggregatory activity of BAB drugs may be mediated, at least partially, through their perturbation effect on the lipid part of biological membranes.
Using EPR spectroscopy it was found that CdCl2 and HgCl2 interact (1) with the intermediates Z./D., i.e. with the tyrosine radicals on the donor side of photosystem (PS) 2 situated in the 161st position in D1 and D2 proteins; (2) with the primary donor of PS1 (P700) whereby the oxidation of chlorophyll (Chl) a dimer in the reaction centre of PS1 occurs yet in the dark; (3) with the manganese cluster which is situated in the oxygen evolving complex. Due to these interactions of investigated metal chlorides with the photosynthetic apparatus, the interruption of the photosynthetic electron transport through photosynthetic centres occurs. Monitoring of time dependence of EPR signal I of chloroplasts treated with CdCl2 or HgCl2 after switching off the light suggests that all mechanisms, i.e. direct, cyclic, and non-cyclic reductions of P700+ are damaged. The formation of complexes between mercury or cadmium ions and amino acid residues constituting photosynthetic peptides was suggested as possible mechanism of their inhibitory action. The higher HgCl2 efficiency in comparison with that of CdCl2 was explained by higher ability of mercury ions to form complexes with amino acids, what was demonstrated by their apparent binding constants: K = 10 200 M-1 for Hg2+ ions, and K = 3 700 M-1 for Cd2+ ions. and F. Šršeň, K. Kráľová.