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PubMed1976

Influence of dietary selenite on the binding characteristics of rat serum proteins to mercurial compounds.

The effect of dietary selenite on the binding characteristics of serum proteins was investigated with rats. In the control serum, the maximal binding of phenylmercuric acetate (PMA) and methylmercuric chloride (MMC) to rat serum protein was approx. 18 and 9 nmoles per mg protein, respectively. The binding of Hg2+ was biphasic and it did not reach a maximum at the concentrations used. Selenite treatment caused a reduction in binding capacity of serum proteins to Hg2+ and PMA, and an increase in the binding affinities. However, there were no such changes for the binding of MMC. Selenite protection from mercury toxicity, therefore, acts not only via a change in tissue distribution and a change in the formation of seleno-proteins but, also, via a change in the binding characteristics to some mercury compounds. In the case of methylmercury, a different mechanism of protection must exist as the modification of tissue distribution, its binding to subcellular and soluble proteins and the binding characteristics remained equivocal.

Administration, Oral↗
PubMed1976

Differential effects of mercurial compounds on excitable tissues.

Sarcoplasmic reticulum (SR), Ca2+ plus Mg2+-ATPase, and Ca2+-ionophore were obtained from white rabbit skeletal muscles. Methylmercury inhibited the Ca2+ plus Mg2+-ATPase and Ca2+-transport but had no effect on the Ca2+-ionophore. Mercuric chloride inhibited all three functions (i.e., ATPase, transport and ionophoric activity). The mechanism of HgCl2 inhibition of the Ca2+-ionophore was by competition with Ca2+ for Ca2+-ionophoric site whereas its inhibition of the enzyme and Ca2+-transport was due to the blockage of essential sulfhydryl (--SH) groups. Ca2+ plus Mg2+-ATPase and Ca2+-transport were more sensitive to methylmercury than to HgCl2. Acetylcholine receptor (AChR) was obtained for the electric organ of T. californica. Methylmercury inhibited the ACh binding to AChR WITH Ki = 5.7 - 10(-6) M. This effect was not due to mercuric ion alone since mercuric chloride up to 10(-4) M did not affect ACh binding to AChR. It is concluded that: the Ca2+ plus Mg2+-ATPase and Ca2+-transport contain --SH groups essential for their activity, and that the two functions are tightly coupled; the Ca2+-ionophore contains no --SH groups essential for its activity; CH3HgCl inhibition of Ca2+ plus Mg2+-ATPase and Ca2+-transport is partly due to its reactivity with --SH groups in hydrophobic environment; the Ca2+-transport is inhibited by HgCl2 through two processes, one which is the blockage of --SH groups and another which is the inhibition of the Ca2+-ionophoric site; and the inhibition of ACh binding to AChR is due to the blockage of --SH groups in hydrophobic environment, which is inaccessible to Hg2+. Our data present for the first time a molecular basis for the myopathy associated with mercurial compounds toxicity.

Acetylcholine↗
PubMed1990

Na+/K(+)-ATPase in rat brain and erythrocytes as a possible target and marker, respectively, for neurotoxicity: studies with chlordecone, organotins and mercury compounds.

Due to the inaccessibility of human nerve tissue for direct biochemical evaluation, there appears to be a need to identify peripheral markers which will reflect toxicity to the central nervous system by relatively non-invasive means. The aim of this study was to investigate whether the enzyme Na+/K(+)-ATPase in erythrocytes could be used as a marker for effects on the same enzyme in brain tissue. The compounds chosen to test this hypothesis were the pesticide chlordecone, the organotin compounds triethyltin and tributyltin, mercuric chloride and methyl mercury. All compounds were found to inhibit in vitro Na+/K(+)-ATPase activity in rat brain (IC50s = 0.9-56 microM) and in rat erythrocytes (IC50s = 1.2-66 microM) with similar potencies. However, administration of these compounds in vivo at high doses produced no significant inhibition of either brain or erythrocyte Na+/K(+)-ATPase activity, despite observed symptoms of neurotoxicity. Dialysis experiments indicated that dissociation of the compounds by dilution during tissue preparation was not responsible for the lack of detectable in vivo inhibition. Measurements of metal concentrations in brain by atomic absorption spectrometry after in vivo administration of triethyltin, mercuric chloride and methyl mercury indicated that levels of these compounds were too low to inhibit significantly NA+/K(+)-ATPase activity. These results suggest that inhibition of Na+/K(+)-ATPase activity might not represent the mechanism responsible for the neurotoxicity of these compounds, and that erythrocyte Na+/K(+)-ATPase activity is not a useful marker for neurotoxicity following acute exposures.

Animals↗
PubMed1993

Comparative studies on cytotoxic and genotoxic effects of two organic mercury compounds in lymphocytes and gastric mucosa cells of Sprague-Dawley rats.

Human lymphocytes (HL) as well as lymphocytes (RL), hepatocytes (RH), and gastric mucosa cells (GM) of Sprague-Dawley rats were treated in vitro for 1 h with methylmercury chloride (MMC, 0.5-4 micrograms/ml) and dimethylmercury (DMM, 5-40 micrograms/ml). The cytotoxicity of the two organic mercury compounds was assessed by dye exclusion, and the extent of induced DNA fragmentation was measured with a single-cell microgel electrophoresis assay. Both MMC and DMM induced DNA damage and cytotoxicity in a dose-related manner in HL, RL, and GM. MMC was more effective in causing a significant increase in median DNA migration than DMM at doses yielding approximately the same degree of cytotoxicity. In rat hepatocytes the MMC-induced DNA damage was, however, lower than in the other cells. An analysis of repair kinetics following exposure to 2 micrograms/ml MMC was carried out in human lymphocytes obtained from an adult male donor. The bulk of DNA repair occurred 90 min after in vitro exposure, and it was about complete by 120 min following cessation of exposure. Finally, in order to have a basis for extrapolating to the human situation, in vivo studies were performed with Sprague-Dawley rats, also assessing the DNA damage and cytotoxicity in the lymphocytes and gastric mucosa cells. These in vivo results after oral exposure may be directly compared to the in vitro data obtained in the same cells.

Adult↗
PubMed1989

Effect of mercurial compounds on net water transport and intramembrane particle aggregates in ADH-treated frog urinary bladder.

It has been suggested that during the oxytocin-induced hydrosmotic response, water crosses the luminal membrane of urinary bladder epithelium cells through membrane-spanning proteins. Although specific inhibitors of osmotic water transport have not been found, certain sulfhydryl reagents such as mercurial compounds may help to identify the proteins involved in this permeation process. We tested the effects of p-chloromercuribenzene sulfonate (PCMBS) and of fluorescein-mercuric acetate (FMA) on the net water flux, the microtubule and microfilament structures of the frog urinary bladder, and the distribution of intramembrane particle aggregates in the luminal membrane. We observed that: (i) 5 mM PCMBS at pH 5 and 0.5 mM FMA at pH 8 added to the mucosal bath at the maximum of the response to oxytocin partially inhibited the net water flux. Inhibition then increased progressively when the preparation was repeatedly or continuously stimulated, until it reached a maximal inhibition at 120 min. This inhibition was not reversed even when cystein was added in the mucosal bath. PCMBS and FMA effects were also observed when cyclic AMP (3',5' cyclic adenosine monophosphate) was used to increase water permeability, (ii) PCMBS mucosal pretreatment did not modify the basal water flux but potentiated the inhibitory effect of PCMBS or FMA on the hydrosmotic response to oxytocin. (iii) Microtubule and microfilament network, visualized in target cells by immunofluorescence, was not affected by PCMBS. (iv) The maximal PCMBS or FMA inhibition was not associated with a reduction of aggregate surface area in the apical membrane. The persistence of the intramembrane particle aggregates associated with the oxytocin-induced hydrosmotic response during the net water flux inhibition by PCMBS, suggests that the PCMBS effect occurs possibly at the level of sulfhydryl groups of the water channel itself.

4-Chloromercuribenzenesulfonate↗
PubMed1988

Enhancement of eicosanoid synthesis in mouse peritoneal macrophages by the organic mercury compound thimerosal.

Availability of the common precursor arachidonic acid represents the fundamental prerequisite of the cellular eicosanoid synthesis. The amount of free arachidonic acid is regulated not only by phospholipases, which liberate this polyunsaturated fatty acid from lipid pools, but also by the reacylating enzyme acylCoA:lysophosphatide acyltransferase. We have previously shown (Goppelt-Strübe, G., C.-F. Körner, G. Hausmann, D. Gemsa, and K. Resch. Control of Prostanoid Synthesis: Role of Reincorporation of Released Precursor Fatty Acids. Prostaglandins 32:373. 1986.) that the organic mercury compound thimerosal in murine peritoneal macrophages inhibits arachidonic acid reincorporation into cellular lipids, thereby leading to an enhanced prostanoid synthesis. In this report we show that the production of leukotriene C4 was also increased after the addition of thimerosal to mouse peritoneal macrophages in a time and dose dependent manner. Concomitantly, thimerosal led to a significant rise of the intracellular calcium concentration as measured by fura-2 fluorescence. Simultaneous addition of thimerosal and indomethacin or exogeneous arachidonic acid to the cells resulted in a synergistic enhancement of leukotriene C4 synthesis. On the other hand, another sulfhydryl group blocking agent, ethacrynic acid, was found to be ineffective in increasing leukotriene C4 levels even in combination with exogeneous arachidonic acid. Thimerosal therefore provides a helpful tool in studying the basic regulatory mechanisms of the cellular leukotriene synthesis.

Animals↗