Methyl mercury interactions with phospholipid membranes as reported by fluorescence, P-31 and Hg-199 NMR

Methylmercury (CH3Hg(II)) interactions with multilamellar vesicles of dimyristoyl(DM)- and dipalmitoyl(DP)-phosphatidylcholine (PC), -phosphatidic acid (PA), -phosphatidylglycerol (PG), -phosphatidylserine (PS) and -phosphatidylethanolamine (PE) have been investigated from the metal viewpoint by solution Hg-199-NMR and from the membrane side by diphenylhexatriene fluorescence polarization and solid state P-31-NMR. Results can be summarized as follows: (1) CH3Hg(II) strong binding to membranes results in a progressive decrease of the free CH3HgOH Hg-199-NMR isotropic signal and because of a slow exchange, in the NMR time scale, between free and bound methylmercury pools the lipid/water partition coefficients, K-lw, of the CH3HgOH species can be determined in the lamellar gel (fluid) phase. It is found: K-lw(DMPC) approximate to 2+/-2 (2+/-2); K-lw(DMPE) approximate to 7+/-3 (16+/-3); K-lw(DMPG)=170+/-10 (110+/-10); K-lw(DMPS) = 930+/-50 (1250+/-60); K-lw(DMPA) = 1250+/-60 (300+/-20). CH3Hg(II) interactions with membrane phospholipids are therefore electrostatic in nature and the phosphate moiety is proposed as a potential binding site. (2) The presence of CH3HgOH stabilizes the PG gel phase and destabilizes that of PS. No effect is observed on PC, PA and PE thermotropism. (3) methylmercury promotes the formation of isotropic P-31-NMR lines with PG, PA and PE systems suggesting the presence of non-bilayer phases and hence membrane reorganization. The above effects are compared to those of inorganic mercury Hg(II) and discussed in the context of cell toxicity.