Content of mercury in chromatin and level of metallothionein proteins in kidneys and liver of rats

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.

Mercuric ion inhibition of eukaryotic transcription factor binding to DNA

Mercury has harmful effects in both rodents and humans. In rodent tissue culture cells exposed to HgCl(2), the metal ions were observed to concentrate in cell nuclei and to associate with chromatin. Thus, transcription factors and other proteins associated with chromatin are possible targets of mercuric ion toxicity. In this study, mercuric ions were found to inhibit the DNA binding activity of the Cys(2)His(2) zinc finger proteins transcription factor IIIA (TFIIIA) and Sp1. These factors are prototypes of the largest eukaryotic protein superfamily. Neither the presence of excess zinc ions nor beta-mercaptoethanol prevented inhibition by mercuric ions. Mercuric ions also inhibited DNA binding by the non-zinc finger protein AP2. Zinc finger-DNA binding was inhibited when both TFIIIA/5S RNA complex and TFIIIA alone were preincubated with concentrations as low as 15 microM mercuric ion. Inhibition occurred in less than 1 min and was not readily reversible. Mercuric ions also inhibited the digestion of DNA by the restriction enzymes BamHI or EcoRI. Inhibition of transcription factors as well as potentially other DNA binding proteins by micromolar concentrations of mercuric ion suggests additional biochemical mechanisms for mercury toxicity in promoting disease via alterations in gene transcription patterns.

Preliminary studies on the differential removal of products formed in the DNA of various rat organs after chronic administration of a low dose of zinc

Metals can bind to various sites on the bases, the phosphate groups and/or sugars in DNA, depending on the physico-chemical characteristics of the metal ion. Up till now most studies concerned with the interaction of metal ions with DNA and polynucleotides have been carried out in vitro. In the present study, 23 ppm Zn2+ was administered chronically to rats in the drinking water for periods up to 1 week, after which the DNA was isolated from liver, kidney, ileum, colon and brain. The DNA was subsequently hydrolysed and the purine bases separated on Sephadex G-10. Three products of metalation were eluted. There were differences in the overall levels of metalation and in the capacity of the different organs to remove the major product of metalation from the DNA: after 7 days the Zn2+ content of this adduct in brain and kidney was 2 and 4 times respectively that of the controls, but in colon and ileum it had returned to control values, despite the continued administration of Zn2+.

Changes in the intracellular accumulation and distribution of metallothionein in rat liver and kidney during postnatal development

Metallothionein (MT) bound to zinc and copper was detected in high concentration in fetal and newborn rat livers by a cadmium saturation method. The levels of both hepatic zinc and MT remained high for the first 14 days after birth and decreased to adult levels by 24 days of age. There was a direct linear relationship between hepatic metallothionein and zinc concentrations during the first 31 days after birth. The ratio of MT to zinc levels also decreased with age suggesting a rapid degradation of MT during postnatal development. Immunohistochemical localization of MT by peroxidase-antiperoxidase technique, using a specific antibody to MT, showed intense intranuclear staining for MT in fetal and newborn rat liver which persisted until Day 9. The nuclear MT staining decreased with age; at 11 days it was equal both in nucleus and cytoplasm and at 14 days, MT was localized mainly in the cytoplasm, similar to adult rat liver pattern. The intranuclear localization of MT in neonates could be considered as a typical fetal-neonatal morphological pattern and its subsequent presence in the cytoplasm, an adult pattern.

Immunochemical detection of metallothionein in specific epithelial cells of rat organs

The distribution of a heavy metal binding protein, metallothionein, was studied immunocytochemically by using antimetallothionein antibody and the immunoperoxidase staining technique on histological sections of liver, kidney, intestine, lung, and testis from cadmium-treated rats. These tissues either accumulate heavy metals (e.g., liver, kidney, and testis) or are exposed to metal by ingestion or inhalation (intestine and lung). Staining for metallothionein was observed intracellularly in epithelial parenchymal cells of the liver and kidney; all hepatocytes and most renal tubular cells stained for the protein. Accumulation of metallothionein was not seen in connective tissue cells surrounding either blood vessels or renal tubules. Extracellular localization of metallothionein was also observed in the liver sinusoids and within the lumina of the renal tubules, suggesting a metal transport or excretory function for this protein. Surface columnar epithelial cells of the intestinal villi indicated the presence of metallothionein but connective tissue cells of the lamina propria were negative for the protein. The granular secretory Paneth cells of the small intestine also stained strongly for metallothionein as did respiratory epithelial cells of the lung. In the testis, metallothionein was detected in the Sertoli cells and interstitial cells but not within the spermatogonia. Sertoli cells are closely associated with the developing spermatogonia and appear to serve a nutritive role in spermatogenesis. Because of the secretory, absorptive, or nutritive function of the metallothionein-localizing cells in the organs studied, we suggest that metallothionein may be involved in metal storage or transport in addition to its commonly proposed detoxification role.