Metal constituents of chromatin. Interaction of mercury in vivo

An Extremely Stable Interprotein Tetrahedral Hg(Cys)4 Core Forms in the Zinc Hook Domain of Rad50 Protein at Physiological pH

In nature, thiolate-based systems are the primary targets of divalent mercury (Hg^II ) toxicity. The formation of Hg(Cys)_x cores in catalytic and structural protein centers mediates mercury's toxic effects and ultimately leads to cellular damage. Multiple studies have revealed distinct Hg^II -thiolate coordination preferences, among which linear Hg^II complexes are the most commonly observed in solution at physiological pH. Trigonal or tetrahedral geometries are formed at basic pH or in tight intraprotein Cys-rich metal sites. So far, no interprotein tetrahedral Hg^II complex formed at neutral pH has been reported. Rad50 protein is a part of the multiprotein MRN complex, a major player in DNA damage-repair processes. Its central region consists of a conserved CXXC motif that enables dimerization of two Rad50 molecules by coordinating Zn^II . Dimerized motifs form a unique interprotein zinc hook domain (Hk) that is critical for the biological activity of the MRN. Using a series of length-differentiated peptide models of the Pyrococcus furiosus zinc hook domain, we investigated its interaction with Hg^II . Using UV-Vis, CD, PAC, and ¹⁹⁹ Hg NMR spectroscopies as well as anisotropy decay, we discovered that all Rad50 fragments preferentially form homodimeric Hg(Hk)₂ species with a distorted tetrahedral HgS₄ coordination environment at physiological pH; this is the first example of an interprotein mercury site displaying tetrahedral geometry in solution. At higher Hg^II content, monomeric HgHk complexes with linear geometry are formed. The Hg(Cys)₄ core of Rad50 is extremely stable and does not compete with cyanides, NAC, or DTT. Applying ITC, we found that the stability constant of the Rad50 Hg(Hk)₂ complex is approximately three orders of magnitude higher than those of the strongest Hg^II complexes known to date.

A zinc-finger like metal binding site in the nucleosome

UV spectroscopy demonstrated that chicken mononucleosomes bind Co(II) and Zn(II) ions at submicromolar concentrations in a tetrahedral mode, at a conserved zinc finger-like site, composed of Cys110 and His113 residues of both H3 molecules. Neither of these metal ions substituted for another, indicating a limited binding reversibility. Molecular modeling indicated that the tetrahedral site is formed by unhindered rotations around Calpha-Cbeta bonds in the side chains of the zinc binding residues. The resulting local rearrangement of the protein structure shields the bound metal ion from the solvent, explaining the observed lack of reversibility of the binding. Consequences of these findings for zinc homeostasis, metal toxicology and nucleosomal regulation are discussed.

The mechanism of Hg2+ toxicity in cultured human oral fibroblasts: the involvement of cellular thiols

To study amalgam-related toxicity in a primary target cell type, human oral fibroblasts were grown in a low-serum medium containing 1.25% fetal bovine serum and exposed to Hg2+, a corrosion product of amalgam. A 1-h exposure to various concentrations of Hg2+ resulted in a dose-dependent loss of colony forming efficiency. Removal of the low-molecular-weight thiol cysteine from the medium increased the toxicity of Hg2+ almost 50-fold in comparison with complete medium or medium without fetal bovine serum. Accordingly, fetal bovine serum was not found to contain detectable levels of low-molecular-weight thiols. The levels of cellular free protein thiols were shown to be depleted Hg2+ at significantly lower concentrations of the metal ion than those required to decrease the levels of the major cellular low-molecular weight thiol glutathione. These decreases were dependent on the exposure conditions, i.e. the presence of serum and thiols, in a manner similar to the effect on colony forming efficiency. Other functions commonly related to cell viability, including the accumulation of the vital dye neutral red, the cytosolic retention of deoxyglucose and the mitochondrial reduction of tetrazolium were also inhibited by Hg2+, albeit at higher concentrations. Finally, the depletion of cellular glutathione, by pre-exposure of the cells to the glutathione synthesis inhibitor buthionine sulfoximine, somewhat increased the toxicity of Hg2+ and potentiated the depletion of protein thiols. Taken together, the toxicity of Hg2+ in human oral fibroblasts was demonstrated in several assays of which colony forming efficiency was the most sensitive, cell killing by this agent was related to its high affinity for protein thiols, whereas glutathione showed a significant, but limited, ability to protect the cells from Hg2+ toxicity.

Effect of molybdenum on the acute toxicity of mercuric chloride. IV. Effect of molybdenum on mercury-mediated metallothionein mRNA induction

In order to elucidate the mechanism of the stimulative effect of molybdenum on mercury-mediated renal metallothionein induction, the levels of translatable metallothionein mRNA (MT mRNA) in the kidneys of rats treated with saline or Na2MoO4 or HgCl2 or Na2MoO4 and HgCl2 were measured by translation experiments in cell-free protein synthesizing systems. The time course of accumulation of mercury in renal nuclei of rats given HgCl2 with or without Na2MoO4-pretreatment was also investigated. Molybdenum, itself, did not elevate levels of MT mRNA compared to saline controls at all time points (0, 6 and 14 h after exposure to HgCl2) but rapidly elevated the levels of the mRNA more than Hg-dosed rats when HgCl2 was also administered. On the other hand, the time course study in renal nuclei showed that the mercury content of nuclei was consistently lower in Mo-Hg-dosed rats than in Hg-dosed rats at all time points (4, 8 and 24 h after exposure to HgCl2). These results suggest that the stimulative effect of molybdenum on mercury-mediated metallothionein induction is coupled with an increase of the mRNA coding for the low molecular weight protein and that such an increase in the levels of translatable MT mRNA is not due to the difference in uptake of mercury into renal nuclei.

Enhancement of ribonucleic acid synthesis by chromium(III) in mouse liver

The effect of Cr(III) administration on hepatic RNA synthesis in mice was studied. It was found that Cr accumulated in mouse liver. Forty-eight hours after intraperitoneal injection of CrCl3 (0.005-5 mg Cr/kg body weight) approximately 10% of the administered dose per g of tissue remained. The accumulated Cr was still retained 64 days after administration (5 mg Cr/kg) with only a slight decrease. Approximately 20% of the hepatic Cr was detected in the nuclei. By administering CrCl3. RNA synthesis in mouse liver was markedly enhanced without altering the pool size of nucleotides. This enhancement was dose-dependent and statistically significant at doses of 0.05 (p less than 0.05), 0.5 (p less than 0.01), and 5 mg Cr/kg (p less than 0.01), and remained so for at least 16 days after administration of 5 mg Cr/kg. The synthesis of DNA and protein in mouse liver were not significantly changed by CrCl3 administration. On the other hand, Cr(VI) administration did not enhance but rather inhibited RNA synthesis in mouse liver. These results suggest that Cr(III) specifically enhances RNA synthesis in mouse liver.

Selenite prevents the induction of sister-chromatid exchanges by methyl mercury and mercuric chloride in human whole-blood cultures

The protective effect of sodium selenite (Na2SeO3) against the cytogenetic toxicity of methyl mercury (CH3HgCl) and mercuric chloride (HgCl2) were investigated on human whole-blood cultures in relation to induction of sister-chromatid exchange (SCE). Both mercurials caused a dose-dependent increase in SCEs, methyl mercury being about 5 times more potent than mercuric chloride. Sodium selenite also induced SCEs. However, the simultaneous addition of selenite (1 x 10(-7) -3 x 10(-5) M) to cell cultures containing either methyl mercury (3 x 10(-6) M) or mercuric chloride (1 x 10(-5) M) prevented the induction of SCEs by the mercurial in a clear dose-related manner. When selenite and mercurial were simultaneously added at a molar ratio of 1:2 Na2SeO3:CH3HgCl, or 1:1 Na2SeO3:HgCl2, cells from treated cultures showed no increase in the SCE frequency. These results indicate that selenite and mercury mutually antagonize their ability to cause DNA damage leading to the formation of SCEs. The formation of bis(methylmercuric)selenide, (CH3Hg)2Se, from Na2SeO3 and CH3HgCl, or a high molecular complex consisting of glutathione-Se-Hg from Na2SeO3 and HgCl2 involving the participation of glutathione in RBCs might play a key role in this antagonism between mercury and selenium.

Mutagenicity study on mice given mercuric chloride

The mutagenic activity of different concentrations of mercuric chloride (0, 2, 4 and 6 mg HgCl2 per kg body wt) has been analysed in Swiss Albino mice at several periods (12, 24, 36 and 48 h) after intraperitoneal injection. No increased frequency of chromosomal aberrations was observed in bone marrow cells or in spermatogonia.

Alterations in ribonucleic acid synthesis by chromium (III)

The effect of Cr(III) on in vitro RNA synthesis directed by DNA and chromatin isolated from mouse liver was investigated in comparison with other inorganic metals. At 1 mM, CrCl3 significantly stimulated RNA synthesis when incubated with DNA or chromatin prior to the addition of Escherichia coli RNA polymerase, while other metals inhibited it. This stimulation by Cr(III) was caused even at 1 muM CrCl3 on either DNA- or chromatin- directed RNA synthesis. The Cr(III)-complexes of DNA and chromatin also showed significantly enhanced template activities. On the other hand, when RNA polymerase was preincubated with Cr(III), a remarkable inhibition was observed in RNA synthesis directed by both DNA and chromatin. In isolated mouse liver nuclei with endogenous RNA polymerases, Cr(III) stimulated Mg2+-activated RNA synthesis but not Mn2r-(NH4)2SO4-activated synthesis. These results suggest that Cr(III) may alter or regulate gene expressions in mammals.

Regulation of metallothionein synthesis in HeLa cells by heavy metals and glucocorticoids

Metallothioneins (MTs) are low molecular weight, cysteine-rich proteins that bind heavy metals. MT induction occurs in liver in response to either heavy metal (Zn++ or Cd++) administration or stress. The synthesis of MT can also be induced by either heavy metals or glucocorticoid hormones in HeLa cells cultured in serum-free medium. Induction of MT by zinc is subject to "desensitization." In contrast, dexamethasone (dex) induction results in a continued elevation in the rate of MT synthesis. The stability of MT is dependent on the availability of metal; consequently, MT induced by dex is degraded much more rapidly (half-life of 11 to 12 hours) than MT induced by elevated zinc levels (half-life of 36 to 38 hours). Removal of either inducer results in biphasic degradation curves, as apothionein and zinc come into balance. In contrast, deinduction kinetics for MT synthesis following removal of the two inducers (zinc and dex) are the same, with a half-life of two and one-half hours. Inhibition of RNA synthesis blocks deinduction after removal of inducer. Induction of MT occurs in a wide variety of species, from blue-green algae to man. This system should provide an excellent model for the comparative biochemistry of regulation of gene expression.

Intranuclear localization of mercury in vivo

Purified nuclei isolated from mice challenged with nonlethal levels of mercury chloride (10-(3)M) in drinking water for 4 to 7 weeks (experimental) and from animals given deionized water (control) were fractionated and the subsequent fractions were analyzed for mercury by flameless atomic absorption. Control (active) euchromatin contained 1.75 +/- 0.53 micrograms of mercury per milligram of DNA. There was a 12- to 15-fold enrichment of mercury in the euchromatin fraction of challenged animals. Mercury was not detected in control (inactive) heterochromatin, and only trace levels (parts per billion) appeared in experimental heterochromatin. It seems likely that mercury can be incorporated into chromatin as a metal-protein complex, but the possibility of protein-mercury-DNA or mercury-DNA complexes within euchromatin cannot be excluded.