Gram scale purification and preparation of rabbit liver zinc metallothionein

Thermodynamic origin of the affinity, selectivity, and domain specificity of metallothionein for essential and toxic metal ions

The small Cys-rich protein metallothionein (MT) binds several metal ions in clusters within two domains. While the affinity of MT for both toxic and essential metals has been well studied, the thermodynamics of this binding has not. We have used isothermal titration calorimetry measurements to quantify the change in enthalpy (ΔH) and change in entropy (ΔS) when metal ions bind to the two ubiquitous isoforms of MT. The seven Zn2+ that bind sequentially at pH 7.4 do so in two populations with different coordination thermodynamics, an initial four that bind randomly with individual tetra-thiolate coordination and a subsequent three that bind with bridging thiolate coordination to assemble the metal clusters. The high affinity of MT for both populations is due to a very favourable binding entropy that far outweighs an unfavourable binding enthalpy. This originates from a net enthalpic penalty for Zn2+ displacement of protons from the Cys thiols and a favourable entropic contribution from the displaced protons. The thermodynamics of other metal ions binding to MT were determined by their displacement of Zn2+ from Zn7MT and subtraction of the Zn2+-binding thermodynamics. Toxic Cd2+, Pb2+, and Ag+, and essential Cu+, also bind to MT with a very favourable binding entropy but a net binding enthalpy that becomes increasingly favourable as the metal ion becomes a softer Lewis acid. These thermodynamics are the origin of the high affinity, selectivity, and domain specificity of MT for these metal ions and the molecular basis for their in vivo binding competition.

Responses to cadmium intoxication in the liver of the wall lizard Podarcis sicula

This study examined the cytological and molecular effects of cadmium, a toxic heavy metal, in the liver of the Italian wall lizard Podarcis sicula. Cadmium was administered in single dose, by diet, to induce a concentration comparable with that measured in animals living in contaminated sites. For comparison, cadmium was also administered in multiple doses by food (chronic) or in a single dose intraperitoneally (i.p.); the effects were followed at regular time intervals up to 30 days post treatments. Atomic absorption spectrometry analysis demonstrated cadmium ion uptake and accumulation in the parenchyma with an estimated half-life of approximately 8 days. Cytological analyses revealed that the metal induced oedema, activated metallothionein expression in Kupffer cells and extracellular matrix production in fat storing cells. It also caused swelling and alteration in lipid and sugar metabolism in hepatocytes. In conclusion, in the wall lizard cadmium is toxic to the liver even at very low concentrations, the response is not strictly dose and time dependent and almost no recovery occurs in short (30 days) time periods.

Evidence for oligomerization of metallothioneins in their functional state

Capillary zone electrophoresis in the polyacrylamide-coated capillary was used to study metallothionein (MT) isoforms at physiological pH in horse kidney and rabbit liver MT preparations produced commercially by Sigma. Evidence is put forward that MT develops oligomers or aggregates in its metal binding situation at these pH values in both species. For the horse kidney preparation two forms were found for both the MT-IA and the MT-IB forms, for the rabbit liver three forms could be seen for the MT-I form and two for the MT-II form. At pH values above the physiological range (pH 8-10) up to four forms could be seen for the MT-I form (MT-IA in the horse) in both preparations. Compared to the MTCd-II form, the rabbit liver MTZn-II form not only behaved electrophoretically identical, but also showed a corresponding oligomerization behaviour. Our results indicate that the oligomerized MT-I form in the rabbit liver and the MT-IA form in the horse kidney bind more Cd atoms than the expected number of 7 per monomer.

Binding properties and stoichiometries of a palladium(II) complex to metallothioneins in vivo and in vitro

This paper will be the first to discuss the in vivo and in vitro properties of a Pd(II) complex, K2PdCl4, interacting with metallothioneins (MTs). In vivo experiments revealed that intraperitoneal injections of K2PdCl4 into rabbits led to the simultaneous synthesis of Pd-MT in the kidney and Zn7MT in the liver. The renal Pd-MT complex contains 3.6 +/- 0.3 Pd, 2.1 +/- 0.2 Zn, and 1.0 +/- 0.1 Cu per mole protein. It was found that pre-treatment with Zn(NO3)2 before K2PdCl4 injections significantly enhanced renal Pd-MT level. The same pre-treatment also increases hepatic Zn-MT levels. These results strongly suggest that Pd(II) ions can be bound in vivo by MT existing in the rabbit kidneys to form Pd-MT. Gel-filtration chromatographic studies after the incubation of either native Cd5Zn2MT2 or Zn7MT2 with K2PdCl4 in vitro demonstrate that Pd(II) ions promote the non-oxidative oligomerization of native MTs. Increasing the level of Pd(II) relative to MT led to a concomitant increase in the apparent yield of MT oligomers. At relatively low Pd-MT ratio, Pd(II) is found predominantly in the oligomers while the monomeric products are chiefly composed of the reactants, Cd5Zn2MT2 or Zn7MT2. Based on our experimental data, the mechanisms of the reactions between Pd(II) and MTs in vivo and in vitro are discussed.

Reaction of a platinum(IV) complex with native Cd,Zn-metallothionein in vitro

The first observation of a redox process following a substitution reaction between a platinum(IV) complex K2PtCl6 with rabbit liver native Cd,Zn-MT is presented. The reaction features and products are studied by UV-visible and circular dichroism spectroscopy, chromatography, and X-ray photoelectron spectroscopic measurements. It is a significant complicated reaction comprising redox and substitution reactions. The reaction generates monomeric and dimeric products, and higher oligomers precipitate with intra- or intra- and intermolecular CyS-SCy linkages. Pt(IV) is reduced to Pt(II), which then binds to the monomeric and dimeric products, and may also bind to higher oligomers. The beta-cluster is more reactive than the alpha-cluster, and reacts first with K2PtCl6. Cd5Pt2 and Cd4Pt4 were found when native Cd,Zn-MT reacted with 2 and 4 molar equivalents of Pt(IV) for 2 h in which four Cd ions were located in the alpha-cluster. The amounts of Cd and Pt ions decreased in both monomeric and dimeric products when the reaction was prolonged and intramolecular CyS-SCy linkages increased. Besides the oligomers which precipitated, only dimeric products were formed when the reaction molar ratio of Pt(IV) to MT was more than 10:1. Cd3Pt6 and Cd1Pt8 were obtained when the reaction occurred for 2 and 72 h, respectively. The structure of the clusters may exist when native Cd, Zn-MT reacts with substoichiometric quantities of K2PtCl6 (< 0.5 K2PtCl6 per MT thiolate) for a short time (2 h), but may be partly disrupted with stoichiometric or excess quantities of K2PtCl6 (> or = 0.5 K2PtCl6 per MT thiolate) for a long time. The disruption of the cluster structures results in an increase of the nonbridge thiolate and an increase of the binding sites to Pt ions. The mechanism of the antitumor activity and developing drug resistance of Pt(IV) complex drugs is discussed.

Interaction of sodium chloroplatinate and iproplatin with metallothionein in vivo

The abilities of platinum(IV) complexes to induce the biosynthesis of metallothionein (MT) were investigated in rabbits given injections s.c. of sodium chloroplatinate (Na2PtCl6) and iproplatin (cis-dichloro-bis-isopropylamine-trans-dihydroxylplatinum IV). It is revealed for the first time that both complexes can induce MT synthesis in the liver and the kidney, but the induction ability was weaker compared to Zn2+ compounds. The induced MT was purified and identified. The hepatic MT resulting from Na2PtCl6 injection only contained Zn, whereas the hepatic MT from iproplatin injection and the renal MT from injection of both complexes contained 4-5 Zn and 1-2 Pt per mole of protein, and the renal MT also contained 1-2 Cu per mole of protein. The oxidation state of platinum in the MT is +2 as determined by X-ray photoelectron spectroscopic measurements. Pretreatment with Zn(NO3)2 elevated the levels of MT, but the binding of Pt to MT was significantly less compared to that without Zn(NO3)2 pretreatment. The data obtained from the amino acid composition analysis were consistent with the theoretical values. Upon these bases, the role of MT in relation to its involvement in the metabolism of Pt(IV) complexes and the mechanism of drug resistance to the Pt(IV) complexes as antitumor agents are discussed.

Fluorometric study of the isoform difference of mammalian metallothionein

A fluorometric method for studying the isoform difference of mammalian metallothionein (MT), which lacks aromatic amino-acid residues, is reported. Cadmium-induced rabbit and hedgehog liver MT exhibited a strong luminescence signal in the range of 335 to 340 nm when excited at 285 nm in aqueous solution. The differences in emission intensity of the two major isoforms of MTs are significant. When titrated with chloride acid, which is believed to proton the metal-thiolate coordination bound to the -SH group in MT, a 10-nm red-shift property of the emission spectrum was observed, and the red-shift properties of the isoforms varied with the species. The observed fluorescence property of MT was considered to be the result of its polypeptide chains, which was confirmed by comparing the luminescence and absorption spectra during the titration of MT with diethylenetriaminepentaacetic acid. The new luminescence property of MT should be useful in studying the isoform and function difference of MT.

Interaction of cis- and trans-diamminedichloroplatinum with metallothionein in vivo

The properties of platinum (II) complexes to induce the biosynthesis of metallothionein (MT) were investigated in rabbits following injections of K2PtCl4, cis and trans isomers of DDP (diammine-dichloroplatinum). It was demonstrated that cis-DDP has an ability to induce MT specifically in the liver, whereas trans-DDP appears to be unable to stimulate the biosynthesis of MT in either the liver or the kidneys. In contrast, K2PtCl4 is effective to elevate the MT level in both tissues. However, all of these platinum complexes are rather poor stimulators for MT biosynthesis compared to cadmium and zinc compounds. Preinjection with Zn(NO3)2 significantly enhances the amount of Pt associated with the MT fractions compared to that resulting from injections with either cis- or trans-DDP without Zn(NO3)2 pretreatment. Metallothionein containing Pt was purified and identified from the liver and kidneys of rabbits after preinjections with Zn(NO3)2 followed by repeated injections of cis-DDP and trans-DDP, respectively. It was found for the first time that a relatively higher degree of Pt was associated with MT fractions in the case of trans-DDP treatment than that of cis-DDP injection. On this basis, the role of MT was discussed in relation to its involvement in the metabolism of cis-DDP, and the difference of the antitumor activity and toxicity between cis- and trans-DDP.

Platinum binding to metallothionein. Analysis of circular dichroism spectra of complexes formed between metallothionein and platinum from cis- and trans-diamminedichloroplatinum

The first observation of the circular dichroism (CD) spectral features of platinum-saturated metallothionein, Pt7MT, is presented. It is characterized by two intensive bands with maxima at 235 nm (-) and 255 nm (+). Properties of the reactions of cis- and trans-diamminedichloroplatinum (DDP) with rabbit liver MT have been studied by monitoring the CD spectral changes. The overall rate of the reaction between trans-DDP and MT is determined to be significantly faster than that for cis-DDP and MT. The stepwise incorporation of Pt into Zn7MT resulted in a nonsystemic changes in the CD spectral envelope. These data argue that Pt(II) replaces Zn(II) in ZnMT in a nonspecific, "distributed" manner across both domains, because the onset of clustering was observed after 5 mol equiv of Pt was bound. A dynamic mechanism has been proposed in which the reaction proceeds via the formation of an intermediate, RS-Pt (NH3)2-SR. As it is converted to the product, the ammine ligands are lost very slowly, especially in the case of cis-DDP, because of the strong requirement of Pt for square planar coordination, which comprises the rate-limiting step.

Application of a polyamine-coated capillary to the separation of metallothionein isoforms by capillary zone electrophoresis

In this study a fused-silica capillary treated internally with a polyamine coating which reverses electroosmotic flow in the direction of the anode was evaluated for its ability to resolve metallothionein (MT) isoforms. Analysis of different MTs purified from liver and kidney tissue revealed the following numbers of putative isoform peaks resolved: rabbit (3-6); horse (3-5); rat (2-3), chicken (1); human MT-1 (5-6); sheep (4-5) and pig (4-5). The greater degree of MT isoform heterogeneity detected in this study using the polyamine-coated capillary suggested a higher resolving capacity for capillary zone electrophoresis conducted with this capillary compared to an uncoated one. Using the single isoform of chicken MT (cMT) as a reference standard, relative standard deviations of 2.53, 1.85 and 2.21% for peak migration time, area and height, respectively, were observed for eight consecutive runs. A standard curve for cMT established linearity (r2 = 0.99) for integrated peak area over three log units of cMT concentration with a lower limit of detection estimated to be < or = 5 micrograms/ml. Acetonitrile extracts of chick liver tissue homogenates were successfully analyzed for the presence of MT isoforms from both control and zinc-injected animals. Based on our initial evaluation, capillary zone electrophoresis using the polyamine-coated capillary appears to be a very useful analytical method for the separation and quantification of individual MT isoforms.