Spectroscopic properties of zinc-metallothionein

Metallothioneins in Aquatic Organisms: Fish, Crustaceans, Molluscs, and Echinoderms

Metallothioneins (MTs) have been described in a wide range of organisms, from bacteria to mammals, thus representing an interesting example of evolutionary molecular adaptation. If the moderate variability of MTs across phylogenetically distant organisms reflects their highly conserved function, the specific environmental requirements may explain the multiplicity of isoforms also in the same organism. The MT polymorphism is particularly important in invertebrates with respect to vertebrates. This review is an attempt to summarize the knowledge about MTs from aquatic animals, both vertebrates and invertebrates, to gain new insights into the structure-function relationship of this class of proteins. The large and increasing literature on MTs indicates that MTs from aquatic vertebrates are rather similar to mammalian counterparts, whereas a variety of structures have been described in invertebrates. Although the prototypical αβ-domain organization of vertebrate MTs has been observed in most invertebrate isoforms, some invertebrate MTs display alternative structures in which the canonical organization has been modified, such as the ββ-domain, the αββ-domain, and the multiple α-domain structures of oyster MTs, and the inverted βα-domain organization of sea urchin MTs. In this review we emphasize three major taxa of aquatic invertebrates, the molluscs, the crustaceans and the echinoderms, although some data have been reported for other invertebrates.

Metallothionein: an exceptional metal thiolate protein

Metallothioneins are unusual, low molecular weight proteins of extremely high sulphur and metabl content. They occur in substantial quantity and in multiple variant forms in parenchymatous tissues (liver, kidney, intestines) of vertebrates and certain microorganisms (Neurospora crassa, yeast). They are though to play a central role in the cellular metabolism of metals such as zinc, copper and cadmium. All mammalian forms studied are single chains with 20 cysteinyl residues among a total of 61 amino acid residues and highly characteristic amino acid sequences. Their most conspicuous common features are seven -Cys-X-Cys- sequences where X stands for an alphatic residue other than Cys. Together with additional cysteinyl residues located elsewhere in the chain and brought into juxtaposition by appropriate chain folding, these dithiol sequences are believed to form the basis of the trithiolate chelating structures typical of most of the six or seven metal-binding sites of the mammalian cadium- and/or zinc-containing metallothioneins. The positions of the cysteinyl residues are preserved in evolution: the copper-containing metallothionein from Neurospora crassa, containing only 25 amino acid residues, has a distribution of metal-binding cysteinyl residues identical to that of the N-terminal portion of the mammalian chains. The detailed physiological role of metallothionein remains to be clarified but its biosynthesis is known to be modulated by nutritional and endocrine factors. Recent evidence suggests that metallothionein is a critical determinant in the homeostasis of zinc.

Use of stable isotopically enriched proteins and directly coupled high-performance liquid chromatography inductively coupled plasma mass spectrometry for quantitatively monitoring the transfer of metals between proteins

Studies have shown that metallothionein (MT) may play an important role in modulating the activity of certain Zn-regulated enzymes under various oxidoreductive conditions by either donating or removing Zn. To better determine the role of MT in interprotein metal transfer, we describe a procedure that uses stable isotopically enriched (67)Zn(7) metallothionein 2 ((67)Zn(7)-MT-2) to quantitatively determine the stoichiometry of transfer of Zn from the protein to a recipient apo-metalloenzyme, apo-carbonic anhydrase (apo-CA) by directly coupled ion exchange high-performance liquid chromatography inductively coupled plasma mass spectrometry. Quantitatively, the transfer of (67)Zn was consistent with the enzymatic activation of the apo-enzyme as judged by its esterase activity and ability to cleave p-nitrophenyl acetate. Maximum enzyme activation occurred at an MT-2:apo-CA molar ratio of 1, implying the release of a single atom of Zn from MT-2. Preincubation of (67)Zn(7)-MT-2 with an excess of oxidized glutathione (GSSG) increased metal donation fourfold, whereas reduced glutathione (GSH) inhibited donation by approximately 50%. By using multiple recipient and donor proteins having different stable isotopic signatures, the technique has the potential for quantitatively studying the kinetic and thermodynamic aspects of Zn transfer between numerous competing ligands in vitro, an important first step toward understanding the regulatory role of this metal in protein functioning and cellular metabolism in vivo.

Molecular characterization and function analysis of MT-10 and MT-20 metallothionein isoforms from Mytilus galloprovincialis

Structure and function of molluscan metallothioneins (MTs) are still poorly understood. The sea mussel Mytilus galloprovincialis displays two MT isoforms which differ in both primary sequences and physiological functions. MT-10 is the constitutive isoform, whereas MT-20 is mainly induced by cadmium (Cd). Both MTs were produced as recombinant proteins and showed identical Cd content and similar Cd-binding properties. Conversely, circular dichroism disclosed marked differences in the secondary conformations of the two Cd(7)-MTs. The possible relapses of these structural differences on protein stability and function were assessed. MT-10 presented a higher thermal stability and a more compact structure than MT-20, as it was inferred by absorption and emission spectroscopy studies. Moreover, the kinetics of Cd-release clearly indicated that MT-10 is much more sensitive to oxidation than is MT-20. The observed differences between MT-10 and MT-20 are discussed in terms of the different physiological roles exerted by the two isoforms in mussel.

Expression, purification and preliminary characterization of mussel (Mytilus galloprovincialis) metallothionein MT20

Metallothioneins are rather ubiquitous metal-binding proteins induced by stressing or physiological stimuli. Two major metallothionein isoforms have been identified in mussel: MT10 and MT20. Nevertheless the high sequence homology, the two isoforms exhibit different expression and inducibility in vivo. We cloned and produced in Escherichia coli the MT20 isoform from Mytilus galloprovincilis. cDNA was subcloned into pGEX-6P.1 vector, in frame with a sequence encoding a glutathione-S-transferase (GST) tail. Recombinant protein was purified to electrophoretic homogeneity by affinity chromatography. After enzymatic cleavage of the GST tail the MT moiety was recovered with a final yield of about 5 mg of protein per litre of bacterial culture. The metal-binding ability of MT20 was assessed by absorption spectroscopy upon addition of cadmium equivalents and the metal release was checked as a function of the environment pH. Moreover the protein was analysed for the propensity to polymerization, typical of this class of protein, before and after exposure to reducing and alkylating agents.

Fish and molluscan metallothioneins

Metallothioneins (MTs) are noncatalytic peptides involved in storage of essential ions, detoxification of nonessential metals, and scavenging of oxyradicals. They exhibit an unusual primary sequence and unique 3D arrangement. Whereas vertebrate MTs are characterized by the well-known dumbbell shape, with a beta domain that binds three bivalent metal ions and an alpha domain that binds four ions, molluscan MT structure is still poorly understood. For this reason we compared two MTs from aquatic organisms that differ markedly in primary structure: MT 10 from the invertebrate Mytilus galloprovincialis and MT A from Oncorhyncus mykiss. Both proteins were overexpressed in Escherichia coli as glutathione S-transferase fusion proteins, and the MT moiety was recovered after protease cleavage. The MTs were analyzed by gel electrophoresis and tested for their differential reactivity with alkylating and reducing agents. Although they show an identical cadmium content and a similar metal-binding ability, spectropolarimetric analysis disclosed significant differences in the Cd7-MT secondary conformation. These structural differences reflect the thermal stability and metal transport of the two proteins. When metal transfer from Cd7-MT to 4-(2-pyridylazo)resorcinol was measured, the mussel MT was more reactive than the fish protein. This confirms that the differences in the primary sequence of MT 10 give rise to peculiar secondary conformation, which in turn reflects its reactivity and stability. The functional differences between the two MTs are due to specific structural properties and may be related to the different lifestyles of the two organisms.

Expression, purification, and characterization of metallothionein-A from rainbow trout

Recombinant metallothionein A (MT-A) from rainbow trout has been successfully produced in milligram quantities in Escherichia coli. cDNA has been subcloned into pGEX-6P.1 vector, in-frame with a sequence encoding an N-terminal glutathione-S-transferase (GST) tail. Purification to electrophoretic homogeneity has been obtained by affinity chromatography using GSH-Sepharose. After enzymatic cleavage of GST tail, the MT-A moiety shows a molecular weight, corresponding to the expected one (6630 Da). The final yield of the entire expression and purification process was about 5 mg of pure metallothionein per liter of bacterial culture. The effects of different reducing and alkylating agents have been evaluated at the level of the formation of higher molecular weight aggregates. To investigate the metal-binding ability of the recombinant MT-A, we carried out a spectrophotometrical titration with cadmium ions. Finally, we checked the metal dissociation by recording the UV absorbance of the protein as a function of the environmental pH.

Purification and characterization of recombinant Caenorhabditis elegans metallothionein

The roundworm Caenorhabditis elegans adapted for survival at high concentrations of Cd(II) expresses two isoforms of metallothionein, CeMT-I and CeMT-II. To characterize one of these proteins CeMT-II was prepared as its Cd containing form by expressing its cDNA heterologously in Escherichia coli. The purified 63-amino-acid protein was identified as the desired product by ion-spray mass spectrometry and was found to resemble in most of its chemical and spectroscopic features the metallothioneins of other animal phyla. The recombinant protein contains a total of 18 cysteine residues and, as documented by electrophoresis and mass spectrometry, binds firmly six Cd ions through the cysteine's side chains. The (113)Cd NMR spectrum features six (113)Cd resonances. Their chemical shift positions between 615 and 675 ppm denote the existence of clusters of tetrahedrally coordinated cadmium thiolate complexes. The metal thiolate coordination dominates also the electronic far-UV absorption spectrum. It is characterized by a massive absorption profile with Cd thiolate shoulders at 255 and 235 nm. Upon replacement of Cd by Zn the profile was blue-shifted by 30 nm.

Evidence for a high molecular weight cytosolic factor that binds brain and liver metallothionein

When brain extracts were fractionated in a Sephadex G-75 chromatography and MT levels were assayed by RIA or ELISA using polyclonal antibodies specific for the MT-I and MT-II isoforms, it was found that MT mostly eluted in the high molecular weight (HMW) peak even in reducing or anaerobic conditions. This was also the case for the liver extracts of control rats; in stressed animals MT immunoreactivity in the HMW peak (> 80 Kd) was increased compared with undisturbed animals, but the major amount of the newly induced MT eluted, as expected from the current literature, in the low molecular weight (LMW) peak, around 10 Kd. The addition of purified MT to brain extracts precluded its binding to a DEAE-Sephadex column. Furthermore, immunoblot results of native PAGE showed that MT changed its electrophoretic mobility in the presence of HMW proteins from brain cytosol. Altogether, these results suggest that a cytosolic factor binds MT in a saturable manner, which may have strong physiological implications.

Glutathione-mediated transfer of Cu(I) into phytochelatins

Room temperature luminescence attributable to Cu(I)-thiolate clusters has been used to probe the transfer of Cu(I) from Cu(I)-glutathione complex to rabbit liver thionein-II and plant metal-binding peptides phytochelatins (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly and (gamma-Glu-Cys)4Gly. Reconstitutions were also performed using CuC1. The Cu(I)-binding stoichiometry of metallothionein or phytochelatins was generally independent of the Cu(I) donor. However, the luminescence of the reconstituted metallothionein or phytochelatins was higher when Cu(I)-GSH was the donor. This higher luminescence is presumably due to the stabilizing effect of GSH on Cu(I)-thiolate clusters. As expected, 12 Cu(I) ions were bound per molecule of metallothionein. The Cu(I) binding to phytochelatins depended on their chain length; the binding stoichiometries being 1.25, 2.0 and 2.5 for (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly and (gamma-Glu-Cys)4Gly respectively at neutral pH. A reduced stoichiometry for the longer phytochelatins was observed at alkaline pH. No GSH was found to associate with phytochelatins by a gel-filtration assay. The Cu(I) binding to (gamma-Glu-Cys)2Gly and (gamma-Glu-Cys)3Gly occurred in a biphasic manner in the sense that the relative luminescence increased approximately linearly with the amount of Cu(I) up to a certain molar ratio whereafter luminescence increased dramatically upon the binding of additional Cu(I). The luminescence intensity declined once the metal-binding sites were saturated. In analogy with the studies on metallothioneins, biphasic luminescence suggests the formation of two types of Cu(I) clusters in phytochelatins.

Purification and characterisation of recombinant sea urchin metallothionein expressed in Escherichia coli

Metallothioneins (MT) are metalloproteins expressed tissue specifically during the development of the sea urchin, Strongylocentrotus pururatus. To explore their structural and functional features and to compare them with those of the evolutionary distant mammalian MTs, one isoform (MTA) was obtained as the cadmium-containing form, from synthetic cDNA heterologously expressed in Escherichia coli. The purified protein was identified as the desired product by a combination of peptide-map analysis, amino acid sequence analysis and ion-spray mass spectroscopy. The existence of seven 113Cd NMR resonances revealed that the recombinant protein binds seven Cd ions/molecule. The position of the NMR resonances (605-695 ppm) and the electronic absorption features suggest that the sea urchin MTA, like the mammalian MTs, possesses tetrahedrally coordinated cadmium-thiolate clusters. With its large Stokes' radius, sea urchin MTA resembles the mammalian forms, suggesting a comparable elongated molecular shape. Measurements by spectrophotometric pH titration of cadmium binding by the recombinant protein suggest that it possesses two metal-thiolate clusters of distinctly different stability. At pH 7 the average apparent association constant for Cd2+ in the clusters is about 20-times weaker in sea urchin MTA than in rabbit MT-2.

Mössbauer and magnetic susceptibility studies on iron(II) metallothionein from rabbit liver. Evidence for the existence of an unusual type of [M3(CysS)9]3- cluster

The magnetic properties of the Fe(II)-binding sites in Fe(II)7-metallothionein (MT) have been studied using Mössbauer spectroscopy and magnetic-susceptibility measurements. In agreement our previous results, simulation of the Mössbauer spectra showed the presence of paramagnetic and diamagnetic subspectra in the ratio 3:4. By comparison with Mössbauer spectra of the inorganic adamantane-like (Et4N)2[Fe4(SEt)10] model compound, the diamagnetic component in Fe(II)7-MT has been assigned to a four-metal cluster in which there is antiferromagnetic coupling between the high-spin Fe(II) ions. It is suggested that the organization of this cluster is similar to that determined in the three-dimensional structure of the protein, containing diamagnetic Zn(II) and/or Cd(II) ions. From magnetic-susceptibility studies, an average magnetic moment of approximately 8.5 microB was obtained for the three remaining bound Fe(II) ions, responsible for the paramagnetic component observed in the Mössbauer studies. This value is slightly lower than that for three completely uncoupled Fe(II) ions, suggesting the existence of a three-metal cluster within which there is weak exchange coupling between adjacent Fe(II) ions. The spin-Hamiltonian formalism including, besides zero-field and Zeeman interaction, also exchange interaction among the three Fe(II) ions in the three-metal cluster, H = -J12 (S1.S2)-J23 (S2.S3)-J13 (S1.S3), was applied to simulate both magnetic-Mössbauer and magnetic-susceptibility data. Reasonable fits were achieved only with values magnitude of J12 = magnitude of J23 = magnitude of J13 = magnitude of J < 1 cm-1. Such a situation could not be reconciled with the chair-like geometry of the [M3(CysS)9]3- cluster determined with paramagnetic metal ions, where significantly stronger coupling would be anticipated (magnitude of J = 50-70 cm-1). However, modest exchange-coupling properties have been reported for a number of crystallographically characterized trinuclear [Fe3(SR)3X6]3- clusters (X = Cl, Br; R = Phe, p-tolyl, 2,6-Me2C6H3) distinguished by the preferential formation of a planar Fe3(mu 2-SR)3 ring [Whitener, M. A., Bashkin, J. A., Hagen, K. S., Girerd, J.-J., Gamp, E. Edelstein, N. & Holm, R. H. (1986) J. Amer. Chem. Soc. 108, 5607-5620]. It is therefore more likely that a pseudo-planar geometry rather than a chair-like geometry is present in the Fe3 cluster of Fe(II)7-MT. This would represent the first example of structural differences on binding divalent metal ions to this protein.

Development of a competitive double antibody radioimmunoassay for rat metallothionein

A competitive double antibody radioimmunoassay (RIA) for rat metallothionein (MT) has been developed that has a detection limit of 100 pg and a range of 100 to 100000 pg. The antibody was raised in rabbits against rat MT-2 but it crossreacts equally with MT-1 and MT-2. However, when the assay is done in the presence of 2-mercaptoethanol the antibody is more specific for MT-2. Zn- and Cd- saturated MTs have similar responses in the assay. Addition of Cu(II) to Zn-MT (more than 6 mol Cu/mol MT) in non-reducing conditions modifies the response of the antibody, probably because of Cu(II) oxidation and later MT polymerization. Standard curves developed in the presence of cytosols from brain cortex, hypothalamus or liver did not differ from the standard curve, indicating the absence of interfering substances in the assay. Furthermore, serial dilutions of those cytosols paralleled the response of the standard curve, indicating that the response of the antibody was specific. For comparison, MT levels in some brain areas measured with the present RIA were compared with those measured with an established RIA. In addition, the expected effect of dexamethasone and stress on liver MT levels was clearly identified by this RIA. The results suggest that the present RIA can be used for quantitation of metallothionein.

Copper(I) transfer into metallothionein mediated by glutathione

Rabbit liver metallothionein depleted of Cd(II) and Zn(II) was fully reconstituted using a Cu(I)-GSH complex under strictly anaerobic conditions. Anaerobic fluorescence titration, using an emission band at 625 nm which is diagnostic of the correct insertion of Cu(I) into the thiolate clusters of metallothionein, showed that the fluorescence maximum was obtained on addition of as many Cu(I) equivalents as the available Cu(I)-binding sites in the protein (i.e. 12). Binding was nearly complete within 1 min, and Cu(I)-GSH was much more efficient than Cu(I)-thiourea or Cu(I)-acetonitrile in metallothionein reconstitution. In air, full reconstitution was obtained with stoichiometric copper only when an excess of GSH was present in the reaction mixture. Cu(I)-GSH was also able to displace Zn(II) and Cd(II) from natural metallized thionein. It is concluded that: (a) Cu(I)-GSH is a potential physiological Cu(I) carrier, not only for Cu2+/Zn2+ superoxide dismutase [Ciriolo, Desideri, Paci and Rotilio (1990) J. Biol. Chem. 265, 11030-11034] but also for metallothionein; (b) in the case of metallothionein, physiological concentrations of GSH protect the protein from autoxidation in air and facilitate Cu(I)-thiolate exchange; (c) the natural metal composition of metallothionein may be related to metal bioavailability rather than to evolutionary changes in protein structure.

Metallothionein detoxification function is impaired by replacement of both conserved lysines with glutamines in the hinge between the two domains

Mammalian metallothioneins (MTs) possess eight highly conserved lysine residues, two of which constitute the hinge between two metal binding domains. By site-directed mutagenesis and recombinant DNA techniques, we replaced the interdomain lysines in Chinese hamster ovary MT2 with all possible combinations of glutamic acid and/or glutamine. The resultant MTs were expressed and assayed for detoxification function in a transformed yeast system. Results showed that these mutant MTs, like the native protein, bound seven atoms of divalent metal per molecule and conferred cadmium resistance to a metal-sensitive yeast host. Replacement of one or both of the lysines in the interdomain region was inconsequential to the structure and function of MT, unless both substituted residues were uncharged. When both lysines were replaced by glutamine (K30,31Q), a reduction in the ability of MT to protect yeast transformants against otherwise toxic levels of cadmium was observed. This diminished metal detoxification capacity was due to a decrease in the steady-state level of MT. These results suggest that at least one charged amino acid must be present in the hinge for the proper expression of MT.

Formation and spectroscopic characterization of a novel monomeric cadmium- and phosphate-containing form of metallothionein

The occurrence of a new monomeric cadmium and phosphate containing metallothionein (MT) form, i.e., Cd13-(Pi)2-MT, is reported. The preferential formation of this MT form from Cd7-MT has been shown to occur in the presence of phosphate and free cadmium at low protein concentration and elevated temperatures. This form displays in gel filtration an apparent molecular weight of 8,900 as opposed to 12,000 for Cd7-MT, suggesting the presence of a more globular structure. This new MT form was characterized by electronic absorption, by circular dichroism (CD), and by 1H, 31P, and 113Cd NMR spectroscopy. The Cd13-(Pi)2-MT form displays at least 24 113Cd signals between 240 and 520 ppm indicating (i) the absence of the original cluster structure of Cd7-MT, (ii) the participation of oxygen and/or nitrogen ligands besides thiolates in metal coordination, and (iii) the presence of more than one stable MT form in the sample. From homonuclear 113Cd COSY and CD studies, evidence for the existence of a cluster structure was obtained. It has been demonstrated that in the Cd13-(Pi)2-MT form two phosphate molecules are bound noncovalently and shown that a very slow exchange with [32P]phosphate in solution (half-life of approximately 56 h) takes place. In the 31P NMR studies, three 31P signals from protein-bound phosphate between 3.5 and 6 ppm have been observed. The 31P signal at 5.7 ppm displayed a heteronuclear 31P-113Cd coupling (J2 = 56 Hz) which provides evidence for direct metal-phosphate coordination. The structural and NMR features of this new MT form are discussed.

Spectroscopic studies on metal distribution in Co(II)/Zn(II) mixed-metal clusters in rabbit liver metallothionein 2

Metal selectivity of metal-thiolate clusters in rabbit liver metallothionein (MT) 2 has been studied by examining the metal distribution of two similarly sized divalent metal ions, cobalt and zinc, which have different thiolate affinity. The forms of mixed-metal cluster species in (Co/Zn)7-MT generated with different ratios of both metal ions offered to the metal-free protein were investigated using EPR, ultraviolet/visible absorption and MCD spectroscopy. The results demonstrated that the distribution of these metals between the two metal-thiolate clusters is not random. Thus, the EPR absorption intensities of the bound Co(II) ions in the Zn-cluster matrix increased linearly up to a ratio of Co(II)/Zn(II) equivalents of 3:4, with the final EPR intensity of three non-interacting Co(II)-binding sites. This EPR behaviour is consistent with a binding scheme in which one Co(II) ion occupies a metal-binding site within the three-metal cluster and the remaining two Co(II) ions occupy two distinctly separate sites in the four-metal cluster. With four or more Co(II) ions in the cluster matrix, magnetic coupling between adjacent, sulphur-bridged Co(II) ions was observed. In previous studies on mixed-metal clusters in MT formed with Co(II)/Cd(II), Zn(II)/Cd(II) and Cd(II)/Fe(II), changes in the respective cluster volumes were shown to be a significant factor dictating the widely differing metal distributions in these systems. Based on the results of the current study, it is suggested that both the sizes of the two metal ions and their relative affinities towards the cysteine-thiolate ligands are important in the formation of mixed-metal clusters in MT.

Binding of inorganic phosphate to the cadmium-induced dimeric form of metallothionein from rabbit liver

Recently we have demonstrated that the exposure of monomeric Cd7-metallothionein (MT) to Cd(II) ions in potassium phosphate buffer results in the nonoxidative formation of MT dimers containing approximately two additional Cd(II) ions/monomer subunit [Palumaa, P., Mackey, E. and Vasák, M. (1992) Biochemistry 31, 2181-2186]. In this study, we demonstrate that inorganic phosphate participates in the Cd-induced dimerization of MT. In the absence of phosphate, Cd-induced oligomerization of MT still takes place, but a substantially lower apparent yield of the dimeric form and an additional peak of MT tetramers were detected in gel-filtration experiments. Arsenate exhibits a similar effect to that of phosphate, whereas a number of other anions, i.e. F-, NO3-, SO4(2-), ClO4-, BO3-, SCN-, HCOO- and CH3COO- had no effect on Cd-induced oligomerization of MT. Studies on the pH dependence of MT dimerization indicate that the dianionic form of phosphate is involved in this process. Equilibrium-dialysis experiments using potassium [32P]phosphate established binding of two molecules of phosphate to the dimeric MT form with a dissociation constant, Kd, of 23 +/- 3 microM (20 mM Tris/HCl and 0.1 M KCl, pH 8.0 at 25 degrees C), whereas binding of phosphate was not observed with the monomeric Cd7-MT. The noncovalent nature of phosphate binding to the Cd-induced MT dimers has been demonstrated. The presented data provide the first evidence for the binding of a nonmetallic cellular component to MT.

Metal selectivity of clusters in rabbit liver metallothionein

In mammalian metallothioneins the metals are organized in two adamantane-type clusters with three and four metal ions which are tetrahedrally coordinated by thiolate ligands. The metal selectivity of the metal-thiolate clusters in rabbit liver metallothionein has been studied by offering two ions, i.e. Co(II)/Cd(II), Zn(II)/Cd(II) or Co(II)/Zn(II), to the metal-free protein. The heterogeneous metal complexes thus formed were characterized by electronic absorption, magnetic circular dichroism. 113Cd-NMR and EPR spectroscopy. In the case of Co/Cd-metallothionein, homometallic cluster occupation occurs, with the Cd(II) ions bound exclusively to the four-metal cluster. In contrast, heterometallic clusters were formed for both Zn/Cd- and Co/Zn-metallothionein. Based on evidence from corresponding inorganic structures of adamantane metal-thiolate cages, it is suggested that the major factor governing the cluster type is the protein structure perturbation due to the cluster volume variations. Thus, while metal thiolate affinities are important in the folding process, size-match selectivity is the dominant factor in the metal-loaded protein.

Presence of a metallothionein-like protein in the bovine pineal gland

The high concentration of zinc in the bovine pineal gland prompted us to investigate the existence of a zinc-binding protein in this organ. In this study, we report that the subcellular distribution of zinc in the bovine pineal gland is nonuniform, with the crude nuclear, mitochondrial, microsomal, and supernatant fractions having 0.264 +/- 0.038, 0.160 +/- 0.019, 0.130 +/- 0.016, and 0.287 +/- 0.010 micrograms zinc/mg protein, respectively. Furthermore, gel filtration studies using Sephadex G-75 and a 105,000 g supernatant fraction revealed two zinc binding protein peaks that bind 1.7 and 3.7 micrograms Zn++/mg protein, respectively. Furthermore, purification of the protein peak with an elution volume (ve/vo) of 2.06 on anion exchange chromatography (DEAE-A25) yielded a single protein peak which binds 10 micrograms zinc/mg protein. The comparative high performance liquid chromatographic (HPLC) profiles of the zinc-induced hepatic metallothionein isoform I (retention time = 17.39 min) and of the bovine pineal metallothionein-like protein isoform I (retention time = 17.49 min) are similar. Since zinc is a potent inhibitor of sulfhydryl-containing enzymes and receptor sites, we investigated the effects of zinc and found that it inhibited the binding of [3H]glutamate (IC 50 = 80 microM) and of [3H]spiroperidol (IC 50 = 0.6 mM) to the pineal membranes. The results of these studies are interpreted to indicate that the bovine pineal gland possesses an active and dynamic zinc homeostatic mechanism, whose precise function(s) remain(s) to be delineated.

Comparative 113Cd-n.m.r. studies on rabbit 113Cd7-, (Zn1,Cd6)- and partially metal-depleted 113Cd6-metallothionein-2a

Rabbit 113Cd7-metallothionein-2a (MT) contains two metal-thiolate clusters of three (cluster B) and four (cluster A) metal ions. The 113Cd-n.m.r. spectrum of 113Cd6-MT, isolated from 113Cd7-MT upon treatment with EDTA, is similar to that of 113Cd7-MT, but the cluster B resonances are lower in intensity, suggesting its co-operative metal depletion. (Zn1,113Cd6)-MT, formed upon addition of the Zn(II) ions to 113Cd6-MT, shows 113Cd-n.m.r. features characteristic of cluster B populations containing both Cd(II) and Zn(II) ions. The overall intensity gain of the mixed cluster B resonances per Cd as to those in 113Cd6- and 113Cd7-MT suggests a stabilization effect of the bound Zn(II) ions upon the previously established intramolecular 113Cd exchange within this cluster.

Primary- and secondary-structural analysis of a unique prokaryotic metallothionein from a Synechococcus sp. cyanobacterium

Biochemical and physiological studies of Synechococcus cyanobacteria have indicated the presence of a low-Mr heavy-metal-binding protein with marked similarity to eukaryotic metallothioneins (MTs). We report here the characterization of a Synechococcus prokaryotic MT isolated by gel-permeation and reverse-phase chromatography. The large number of variants of this molecule found during chromatographic separation could not be attributed to the presence of major isoproteins as assessed by amino acid analysis and amino acid sequencing of isoforms. Two of the latter were shown to have identical primary structures that differed substantially from the well-described eukaryotic MTs. In addition to six long-chain aliphatic residues, two aromatic residues were found adjacent to one another near the centre of the molecule, making this the most hydrophobic MT to be described. Other unusual features included a pair of histidine residues located in repeating Gly-His-Thr-Gly sequences near the C-terminus and a complete lack of association of hydroxylated residues with cysteine residues, as is commonly found in eukaryotes. Similarly, aside from a single lysine residue, no basic amino acid residues were found adjacent to cysteine residues in the sequence. Most importantly, sequence alignment analyses with mammalian, invertebrate and fungal MT sequences showed no statistically significant homology aside from the presence of Cys-Xaa-Cys structures common to all MTs. On the other hand, like other MTs, the prokaryotic molecule appears to be free of alpha-helical structure but has a considerable amount of beta-structure, as predicted by both c.d. measurements and the Chou & Fasman empirical relations. Considered together, these data suggested that some similarity between the metal-thiolate clusters of the prokaryote and eukaryote MTs may exist.

Spectroscopic and chemical approaches to the study of metal-thiolate clusters in metallothionein (MT)

Structural properties of the metal-thiolate clusters in mammalian and vertebrate MTs were studied by employing various spectroscopic techniques. The stepwise probing of the metal-binding sites of the four-metal cluster domain (alpha-fragment) of rabbit MT-2 by Co(II) ions monitoring the V2[4A2----4T1(P)] ligand-field transition at 1260 nm and the EPR intensity of the high spin Co(II) complexes revealed spectral changes suggestive of a cluster structure with one metal-binding site of different symmetry. These results are in contrast with our previous findings on rabbit Co(II)7-MT-2 where a tetrahedral tetrathiolate Co(II) coordination at all stages of filling has been demonstrated. Similar titration studies on MT isolated from the crab Cancer pagurus revealed that all six metal-binding sites are bound in tetrahedral type of symmetry as documented by the development of the d-d absorption profile of crab Co(II)-MT as a function of increasing Co(II)/apoMT ratios. Evidence for clusters in the latter species is provided by the manifestation of magnetic exchange coupling in the EPR spectra when the last two Co(II) ions are bound. Unexpected spectral features in the d-d region of Co(II)6-MT and in the CD-profile of the Cd(II)6-MT seen after the addition of an excess of Co(II) or Cd(II), respectively, suggest the existence of a seventh metal-binding site of different symmetry in crab MT. Finally, the isolation of the metal-deficient rabbit Cd(II)6-MT-2 after exposure of the fully metal-occupied Cd(II)7-MT-2 to EDTA is reported. A combination of UV, CD and 113Cd NMR data obtained with both Cd(II)6- and Cd(II)7-MT-2 supports the occurrence of a labile metal-binding site in the three-metal cluster domain of mammalian MT.

Sequence-specific 1H-NMR assignments in rabbit-liver metallothionein-2

The complete sequence-specific assignment of the 1H nuclear magnetic resonance spectrum of a major subform of rabbit liver metallothionein-2 is presented. The sequential assignment procedures revealed a number of differences with regard to results obtained by earlier partial chemical sequencing of a preparation now known to be microheterogeneous. In particular, the present data indicate a polypeptide chain length of 62 amino acid residues as compared to the occurrence of 61 amino acids in all other known mammalian metallothioneins. In the new sequence, which was also fully confirmed by chemical means, the additional amino acid residue was identified as Ala8' inserted between Ala8 and Ala9 of the standard amino acid numeration. In addition to the predominant protein species all preparations contained a minor component, for which the two-dimensional 1H-nuclear magnetic resonance features are compatible with a chemically different, homologous metallothionein.

Systematic application of high-resolution, phase-sensitive two-dimensional 1H-NMR techniques for the identification of the amino-acid-proton spin systems in proteins. Rabbit metallothionein-2

Novel strategies for elucidation and classification of amino acid 1H-NMR spin systems in proteins were developed exploiting recently introduced two-dimensional NMR techniques such as phase-sensitive double-quantum-filtered correlated spectroscopy, relayed coherence transfer spectroscopy, double quantum spectroscopy and nuclear Overhauser spectroscopy. Due to the improved resolution in phase-sensitive spectra, the fine structure of cross peaks could be exploited as a powerful source of information for establishing 1H-1H connectivities. Principles for the interpretation of multiplet structures of absorption mode cross peaks are discussed. With these methods the 1H spin systems of rabbit liver metallothionein-2 were elucidated and classified according to amino acid types. Despite the intrinsically difficult situation arising from the unusual amino acid composition of this protein, a more complete characterization of the 1H spin systems prior to the step of sequential resonance assignments was achieved with the presently introduced methodology than was possible in earlier studies of proteins of similar size.

113CD-1H spin-spin couplings in homonuclear 1H correlated spectroscopy of metallothionein. Identification of the cysteine 1H spin systems

Heteronuclear spin-spin couplings between 113Cd and C beta protons of the metal-bound cysteines were observed in phase-sensitive, double-quantum filtered, homonuclear two-dimensional correlated (COSY) 1H NMR spectra of 113Cd-metallothionein-2 from rabbit liver. Comparison of 113Cd- and 112Cd-metallothionein-2 spectra revealed that 19 1H spin systems show heteronuclear couplings to at least one 113Cd and were thus identified as 19 of the 20 cysteines in this protein. From a detailed analysis of the manifestations of heteronuclear couplings in the homonuclear 1H COSY spectra, two cysteines could be identified as 'bridging cysteines', with spin-spin couplings to two different 113Cd nuclei. The observed 113Cd-1H coupling constants vary between less than or equal to 5 Hz and 80 Hz.

Antigenicity of metallothionein

The antigenic determinants of vertebrate metallothionein have been determined by a competitive-binding double-antibody radioimmunoassay to consist of two immunologically dominant regions in the NH2-terminal domain (residues 1-29). The COOH-terminal domain (residues 30-61) exhibited trivial immunoreactivity in competitive binding assays. The tryptic peptide encompassing residues 1-25 of the molecule competed with 125I-labeled metallothionein as effectively as the native protein. The crossreactivity was unaffected whether the protein was native or denatured. The antigenicity is thus independent of the degree of folding of the protein and, although all antigenic sites depend to some degree on conformation or topography, this favors a sequential (or continuous) rather than a discontinuous nature of the determinants. The two regions in metallothionein that appear to be important in the interaction of the molecule with the antisera include the NH2-terminal acetylated methionine and the cluster of lysines in the sequence from residues 20-25. These regions are homologous in the various vertebrate metallothioneins known to crossreact with rabbit anti-rat metallothionein antiserum. This implies that the induction in rabbits of antiserum to rat metallothionein is an autoimmune phenomenon. The results support the two-cluster model of metallothionein and are compatible with predictions of sites of antigenicity based on regions of high hydrophilicity.

Metal thiolate clusters in cobalt(II)-metallothionein

Rabbit liver metallothionein-1 in which all seven metal-binding sites are occupied by cobalt(II) exhibits spectral features typical of tetrathiolate coordination with approximate Td microsymmetry [Vasák, M. (1980) J. Am. Chem. Soc. 102, 3953-3955]. With a total of 20 cysteine residues per molecule, this mode of metal binding implies that some of the thiolate ligands are shared by neighboring Co(II) ions, resulting in clustered structures. In this study, evidence for the existence of thiolate-linked Co(II) clusters is presented and their mode of formation is explored by comparing the optical and magnetic properties of forms of Co(II)-metallothionein containing 1-7 equivalents of Co(II). Preparations with up to 4 Co(II) equivalents display electronic spectra in the d-d and charge-transfer regions that resemble those of isolated tetrahedral Co(II)-tetrathiolate complexes. Upon binding of more than four Co(II) ions, however, the spectrum changes progressively and approaches in the fully saturated Co(II)-metallothionein an absorption profile similar to that of crystallographically defined model (Co)II-tetrathiolate clusters [Dance, I. G. (1979) J. Am. Chem. Soc. 101, 6264-6273]. These effects are closely paralleled by changes in the ESR spectrum. Above 4 Co(II) equivalents per thionein, the ESR signal at gx approximately 5.9 measured at 4 K decreases progressively in intensity, until in the fully occupied protein the complex is nearly diamagnetic. These changes, which were confirmed by measurements of paramagnetic susceptibility, establish the existence of Co(II) thiolate clusters in Co(II)-metallothionein. The loss of paramagnetism reflects most likely antiferromagnetic coupling of neighboring Co(II) ions brought about by a superexchange mechanism via the thiolate bridging ligands.