Metallothionein-null mice absorb less Zn from an egg-white diet, but a similar amount from solutions, although with altered intertissue Zn distribution.
The influence of metallothionein (MT) on Zn transfer into non-gut tissues was investigated in MT-null (MT-/-) and normal (MT+/+) mice 4 h after oral gavage of aqueous 65ZnSO4solution at doses of 154, 385, 770 and 1540 nmol Zn per mouse. Zn transfer was not significantly different between MT+/+ and MT-/- mice and was directly proportional to the oral dose (slope = 0.127, r = 0.991; 0. 146, r = 0.994, respectively). Blood 65Zn and plasma Zn concentrations increased progressively in MT-/- mice at doses >154 nmol Zn, reaching levels of 2.4% of oral dose and 60 micromol/L, respectively, at the 1540 nmol Zn dose. The corresponding values for MT+/+ mice were approximately half, 1.0% and 29 micromol/L. Intergenotypic differences were found in tissue distribution of 65Zn within the body; MT-/- mice had higher 65Zn levels in muscle, skin, heart and brain, whereas MT+/+ mice retained progressively more Zn in the liver, in conjunction with a linear increase in hepatic MT up to the highest Zn dose. MT induction in the small intestine reached its maximum at an oral dose of 385 nmol Zn and did not differ at higher doses. Absorption of a 770 nmol 65Zn dose from a solid egg-white diet was only one fourth (MT+/+) and one eighth (MT-/-) of the Zn absorption from the same dose of 65Zn in aqueous solution. MT+/+ mice had greater (P < 0.05) Zn absorption from the egg-white diet than did MT-/- mice, indicating that gut MT confers an absorptive advantage, but only when Zn is incorporated into solid food. (+info)
Cadmium-mediated activation of the metal response element in human neuroblastoma cells lacking functional metal response element-binding transcription factor-1.
Metal response element-binding transcription factor-1 (MTF-1) binds specifically to metal response elements (MREs) and transactivates metallothionein (MT) gene expression in response to zinc and cadmium. This investigation contrasts the mechanism of mouse MT gene (mMT-I) promoter activation by cadmium and zinc in IMR-32 human neuroblastoma cells to determine whether MTF-1 binding to the MRE is necessary for activation by these metals. Cadmium activated a mMT-1 promoter (-150 base pairs) luciferase reporter 20-25-fold through a MRE-dependent mechanism. In contrast, zinc had little effect on the mMT-1 luciferase reporter. IMR-32 cells lacked MRE binding activity, and treatment with zinc in vitro or in vivo did not generate a MTF-1. MRE complex, suggesting that IMR-32 cells lack functional MTF-1. Overexpression of mMTF-1 regenerated a zinc-mediated induction of the MRE without affecting cadmium activation. Because no other transition metals tested activated the MRE, this effect appeared to be cadmium-specific. These data demonstrate that in IMR-32 human neuroblastoma cells, zinc and cadmium can use independent mechanisms for activation of the mMT-I promoter and cadmium-mediated MRE activation is independent of MTF-1 and zinc. (+info)
Clinical and immunochemical study of the serum IgG fraction not precipitated in a zinc-sodium salicylate reagent.
A reagent made of zinc sulphate (0-08 M) in a 0-4 M sodium salicylate solution at pH 7-3 precipitated most of the IgG when a small volume of human serum was added. Sera with normal IgG levels or polyclonal hyperglobulinaemia showed a close correlation between total IgG and zinc-precipitated IgG (r = + 0-95). In clinical material, not including IgG myeloma, zinc-soluble IgG varied between 0 and 6 mg/ml and was independent of the IgG serum concentration. In 31 normal subjects the average IgG concentration, as determined by the Technicon immunonephelometric method, was 10-2 +/- 1-7 mg/ml for total IgG and 2-2 +/- 1-0 mg/ml for the soluble fraction. Among 173 sera, including 24 from cord blood, 16 from pregnant women, and 133 from patients with miscellaneous diseases, no pathological conditions except three cases of IgG myeloma were found with a zinc-soluble IgG definitely above the normal values; zinc-soluble IgG levels were often low in patients with hyperglobulinaemia, and the difference was highly significant in liver disease. kappa and gamma light chains as well as the four IgG-Hp chain subclasses were found in both zinc-soluble fractions of normal IgG. A study of myeloma monoclonal IgG showed that globulins of classes 1, 3, and 4 could be either soluble or insoluble in the zinc reagent. One, G2, was mainly insoluble. Hexose and antistreptolysin contents per milligram normal IgG were not significantly different in either fraction. It is suggested that zinc-soluble IgG consists of the recently synthesized molecules, the zinc-solubility of which has not yet been decreased by protein association, lipid interaction, antigen binding, or enzymatic denaturation. Within this hypothesis, a low level of soluble IgG would mean either an increased precatabolic protein or a decreased synthesis. (+info)
Enhanced bioaccumulation of heavy metal ions by bacterial cells due to surface display of short metal binding peptides.
Metal binding peptides of sequences Gly-His-His-Pro-His-Gly (named HP) and Gly-Cys-Gly-Cys-Pro-Cys-Gly-Cys-Gly (named CP) were genetically engineered into LamB protein and expressed in Escherichia coli. The Cd2+-to-HP and Cd2+-to-CP stoichiometries of peptides were 1:1 and 3:1, respectively. Hybrid LamB proteins were found to be properly folded in the outer membrane of E. coli. Isolated cell envelopes of E. coli bearing newly added metal binding peptides showed an up to 1.8-fold increase in Cd2+ binding capacity. The bioaccumulation of Cd2+, Cu2+, and Zn2+ by E. coli was evaluated. Surface display of CP multiplied the ability of E. coli to bind Cd2+ from growth medium fourfold. Display of HP peptide did not contribute to an increase in the accumulation of Cu2+ and Zn2+. However, Cu2+ ceased contribution of HP for Cd2+ accumulation, probably due to the strong binding of Cu2+ to HP. Thus, considering the cooperation of cell structures with inserted peptides, the relative affinities of metal binding peptide and, for example, the cell wall to metal ion should be taken into account in the rational design of peptide sequences possessing specificity for a particular metal. (+info)
Postnatal development of hippocampal dentate granule cell gamma-aminobutyric acidA receptor pharmacological properties.
Postnatal development of hippocampal dentate granule cell gamma-aminobutyric acidA (GABAA) receptor pharmacological properties was studied. Granule cells were acutely isolated from hippocampi of 7- to 14- and 45- to 52-day-old rats, and whole cell patch-clamp recordings were obtained. The sensitivity of GABAA receptors to GABA and modulation of GABAA receptor currents by benzodiazepines (BZ), zinc, furosemide, and loreclezole was studied. Multiple changes in the pharmacological properties of dentate granule-cell GABAA receptors occurred during the first 52 days of postnatal development: GABA-evoked maximal current increased with postnatal age; GABAA receptors changed from BZ type 3 in young rats to BZ type 1 in adult rats; furosemide and zinc inhibited GABAA receptor currents in young rats but not in adult rats; the fraction of cells that expressed loreclezole-sensitive GABAA receptors increased with postnatal age. These findings suggest that dentate granule cells in young and adult animals express pharmacologically distinct GABAA receptors and that the postnatal development of these receptors is prolonged, lasting at least 45 days. Comparison with the previously reported pharmacological properties of GABAA receptors on dentate granule cells acutely isolated from hippocampi of 28- to 35-day-old rats suggests that receptors expressed at that age have properties intermediate between young and adult rats. (+info)
Selenium redox biochemistry of zinc-sulfur coordination sites in proteins and enzymes.
Selenium has been increasingly recognized as an essential element in biology and medicine. Its biochemistry resembles that of sulfur, yet differs from it by virtue of both redox potentials and stabilities of its oxidation states. Selenium can substitute for the more ubiquitous sulfur of cysteine and as such plays an important role in more than a dozen selenoproteins. We have chosen to examine zinc-sulfur centers as possible targets of selenium redox biochemistry. Selenium compounds release zinc from zinc/thiolate-coordination environments, thereby affecting the cellular thiol redox state and the distribution of zinc and likely of other metal ions. Aromatic selenium compounds are excellent spectroscopic probes of the otherwise relatively unstable functional selenium groups. Zinc-coordinated thiolates, e.g., metallothionein (MT), and uncoordinated thiolates, e.g., glutathione, react with benzeneseleninic acid (oxidation state +2), benzeneselenenyl chloride (oxidation state 0) and selenocystamine (oxidation state -1). Benzeneseleninic acid and benzeneselenenyl chloride react very rapidly with MT and titrate substoichiometrically and with a 1:1 stoichiometry, respectively. Selenium compounds also catalyze the release of zinc from MT in peroxidation and thiol/disulfide-interchange reactions. The selenoenzyme glutathione peroxidase catalytically oxidizes MT and releases zinc in the presence of t-butyl hydroperoxide, suggesting that this type of redox chemistry may be employed in biology for the control of metal metabolism. Moreover, selenium compounds are likely targets for zinc/thiolate coordination centers in vivo, because the reactions are only partially suppressed by excess glutathione. This specificity and the potential to undergo catalytic reactions at low concentrations suggests that zinc release is a significant aspect of the therapeutic antioxidant actions of selenium compounds in antiinflammatory and anticarcinogenic agents. (+info)
Inhibitory sites in enzymes: zinc removal and reactivation by thionein.
Thionein (T) has not been isolated previously from biological material. However, it is generated transiently in situ by removal of zinc from metallothionein under oxidoreductive conditions, particularly in the presence of selenium compounds. T very rapidly activates a group of enzymes in which zinc is bound at an inhibitory site. The reaction is selective, as is apparent from the fact that T does not remove zinc from the catalytic sites of zinc metalloenzymes. T instantaneously reverses the zinc inhibition with a stoichiometry commensurate with its known capacity to bind seven zinc atoms in the form of clusters in metallothionein. The zinc inhibition is much more pronounced than was previously reported, with dissociation constants in the low nanomolar range. Thus, T is an effective, endogenous chelating agent, suggesting the existence of a hitherto unknown and unrecognized biological regulatory system. T removes the metal from an inhibitory zinc-specific enzymatic site with a resultant marked increase of activity. The potential significance of this system is supported by the demonstration of its operations in enzymes involved in glycolysis and signal transduction. (+info)
Synaptically released Zn2+ can enter and cause injury to postsynaptic neurons. Microfluorimetric studies using the Zn2+-sensitive probe, Newport green, examined levels of [Zn2+]i attained in cultured cortical neurons on exposure to N-methyl-D-asparte, kainate, or high K+ (to activate voltage-sensitive Ca2+ channels) in the presence of 300 microM Zn2+. Indicating particularly high permeability through Ca2+-permeable alpha-amino3-hydroxy-5-methyl-4-isoxazolepropionic-acid/kainate (Ca-A/K) channels, micromolar [Zn2+]i rises were observed only after kainate exposures and only in neurons expressing these channels [Ca-A/K(+) neurons]. Further studies using the oxidation-sensitive dye, hydroethidine, revealed Zn2+-dependent reactive oxygen species (ROS) generation that paralleled the [Zn2+]i rises, with rapid oxidation observed only in the case of Zn2+ entry through Ca-A/K channels. Indicating a mitochondrial source of this ROS generation, hydroethidine oxidation was inhibited by the mitochondrial electron transport blocker, rotenone. Additional evidence for a direct interaction between Zn2+ and mitochondria was provided by the observation that the Zn2+ entry through Ca-A/K channels triggered rapid mitochondrial depolarization, as assessed by using the potential-sensitive dye tetramethylrhodamine ethylester. Whereas Ca2+ influx through Ca-A/K channels also triggers ROS production, the [Zn2+]i rises and subsequent ROS production are of more prolonged duration. (+info)