Effects of metal binding on protein structure

The Early Phase of β2-Microglobulin Aggregation: Perspectives From Molecular Simulations

Protein β2-microglobulin is the causing agent of two amyloidosis, dialysis related amyloidosis (DRA), affecting the bones and cartilages of individuals with chronic renal failure undergoing long-term hemodialysis, and a systemic amyloidosis, found in one French family, which impairs visceral organs. The protein’s small size and its biomedical significance attracted the attention of theoretical scientists, and there are now several studies addressing its aggregation mechanism in the context of molecular simulations. Here, we review the early phase of β2-microglobulin aggregation, by focusing on the identification and structural characterization of monomers with the ability to trigger aggregation, and initial small oligomers (dimers, tetramers, hexamers etc.) formed in the so-called nucleation phase. We focus our analysis on results from molecular simulations and integrate our views with those coming from in vitro experiments to provide a broader perspective of this interesting field of research. We also outline directions for future computer simulation studies.

Chromium and leather: a review on the chemistry of relevance for allergic contact dermatitis to chromium

Abstract

As other causes decline in importance, chromium-tanned leather has become a more important source for chromium allergy, which affects around 1% of the general population. The aim of this review is to give suggestions on how to minimize the risk of leather-related allergic contact dermatitis, which can be elicited in chromium-allergic persons by hexavalent and trivalent chromium released from leather. Hexavalent chromium is the more potent chromium form and requires a lower skin dose to elicit allergic reactions. It is formed on the surface of some, antioxidant-free, leathers at dry conditions (< 35% relative humidity) and is influenced by the tanning process and other conditions, such as UV irradiation, contact with alkaline solutions, and leather age. Trivalent chromium is the dominant form released from chromium-tanned leather and its released amount is sufficient to elicit allergic reactions in some chromium-allergic individuals when they are exposed repetitively and over longer time (days – months). A low initial test result (< 3 mg/kg) for hexavalent chromium with the current standard test (ISO 17075) does not guarantee a low release of chromium from the leather or a low release of hexavalent chromium under typical exposure conditions during the service life of the leather. Information, labels, and certificates regarding leather products are often insufficient to protect chromium-allergic individuals. Correct labelling and information on the possible content of different allergens, as well as different tanning alternatives for certain leather products, are crucial.

Graphical abstract

Synergistic effects of metal-induced aggregation of human serum albumin

Exposure to cobalt (Co), chromium (Cr), and nickel (Ni) occurs often via skin contact and from different dental and orthopedic implants. The metal ions bind to proteins, which may induce structural changes and aggregation, with different medical consequences. We investigated human serum albumin (HSA) aggregation in the presence of Co^II, Cr^III, and/or Ni^II ions and/or their nanoparticle precipitates by using scattering, spectroscopic, and imaging techniques, at simulated physiological conditions (phosphate buffered saline - PBS, pH 7.3) using metal salts that did not affect the pH, and at HSA:metal molar ratios of up to 1:8. Co ions formed some solid nanoparticles in PBS at the investigated conditions, as determined by nanoparticle tracking analysis, but the Cr^III anions and Ni^II ions remained fully soluble. It was found that all metal ions induced HSA aggregation, and this effect was significantly enhanced when a mixture of all three metal ions was present instead of any single type of ion. Thus, the metal ions induce aggregation synergistically. HSA aggregates formed linear structures on a mica surface in the presence of Cr^III ions. A clear tendency of aggregation and linearly aligned aggregates was seen in the presence of all three metal ions. Spectroscopic investigations indicated that the majority of the HSA molecules maintained their alpha helical secondary structure and conformation. This study highlights the importance of synergistic effects of metal ions and/or their precipitates on protein aggregation, which are highly relevant for implant materials and common exposures to metals.

Proteomic identification of divalent metal cation binding proteins in plant mitochondria

Divalent metal binding proteins in the Arabidopsis mitochondrial proteome were analysed by mobility shifts in the presence of divalent cations during two-dimensional diagonal sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Tandem mass spectrometry and searches of the predicted Arabidopsis protein dataset were used in an attempt to identify 34 of the proteins which shifted. This analysis identified a total of 23 distinct protein spots as the products of at least 11 different Arabidopsis genes. A series of proteins known to be divalent cation-binding proteins, or to catalyse divalent cation-dependent reactions, were identified. These included: succinyl CoA ligase beta subunit, Mn-superoxide dismutase (SOD), an Fe-S centred component of complex I and the REISKE iron-sulphur protein of the b/c(1) complex. A further set of four proteins of known function but without known divalent binding properties were also identified: the Vb subunit of cytochrome c oxidase, a subunit of ATP synthase (orfB), the acyl carrier protein, and the translocase of the outer membrane (TOM20). Three other proteins, of unknown function, were also found to shift in the presence of divalent cations. This approach has broad application for the identification of sub-proteomes based on the metal interaction of polypeptides.

Modulation of GABA-mediated synaptic transmission by endogenous zinc in the immature rat hippocampus in vitro

1. Intracellular recordings from postnatal 2- to 12-day-old (P2-12) rat hippocampal CA3 pyramidal neurones exhibited spontaneous synaptic potentials mediated by GABAA receptors. These potentials can be separated on the basis of amplitude into two classes which are referred to as small and large. 2. The large depolarizing potentials were reversibly inhibited by the Zn2+ chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94). The small inhibitory postsynaptic potentials. (IPSPs) were apparently unaffected. 3. Stimulation of the mossy fibre pathway evoked composite excitatory postsynaptic potentials (EPSPs) and IPSPs. Threshold stimulus-evoked synaptic potentials were mediated by GABAA receptors and were reversibly blocked by CP94. The responses evoked by suprathreshold stimulation and persisting in the presence of bicuculline or CP94 were partially inhibited by 2-amino-5-phosphonopropionic acid (AP5) and were completely blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. L-Histidine, which preferentially forms complexes with Cu2+ > Zn2+ > Fe2+ > Mn2+, inhibited both naturally occurring spontaneous and evoked GABAA-mediated large synaptic potentials without affecting the neuronal resting membrane properties. Exogenously applied Zn2+ induced large spontaneous synaptic potentials and prolonged the duration of the evoked potentials. These effects were reversibly blocked by histidine. 5. The metal chelating agent diethyldithiocarbamate had little effect on the large amplitude synaptic potentials. 6. The transition metal divalent cations Fe2+ and Mn2+ did not initiate large synaptic potentials in CA3 neurones; however, Cu2+ depolarized the membrane and enhanced both excitatory and inhibitory synaptic transmission, resulting in a transient increase in the frequency of the large amplitude events. In comparison, zinc increased the frequency of the large potentials and also induced such events in neurons (P4-21) where innate potentials were absent. The postsynaptic response to ionophoretically applied GABA was either unaffected or slightly enhanced by Zn2+. 7. Under conditions favouring the activation of non-NMDA receptors, excitatory synaptic transmission was unaffected by CP94 but was depressed by Zn2+. Responses to ionophoretically applied glutamate were not inhibited by Zn2+, indicating that Zn2+ affects excitatory synaptic transmission via a presynaptic mechanism. 8. We conclude that the naturally occurring large synaptic potentials in young CA3 neurones are apparently induced by endogenous Zn2+ which can promote or synchronize the release of GABA in the immature hippocampus.

A novel modulatory binding site for zinc on the GABAA receptor complex in cultured rat neurones

1. The properties of gamma-aminobutyric acidA (GABAA) receptor-ion channel complexes and the interaction with the transition metal zinc, were studied on rat sympathetic and cerebellar neurones in dissociated culture using patch clamp recording techniques. 2. The antagonism of GABA-induced membrane currents by zinc on sympathetic neurones was subject to developmental influence. Using embryonic sympathetic neurones acutely cultured for 24-72 h, GABA responses were more depressed by zinc when compared to responses evoked on adult neurones cultured for the same period. For neurones developing in vivo, the percentage inhibition of GABA responses produced by zinc in embryonic neurones was estimated to decline by 50% after 48.2 days following birth. 3. Embryonic sympathetic neurones maintained in culture for prolonged periods (40-50 days in vitro, DIV) became less sensitive to zinc when compared to neurones cultured for shorter periods (10-20 DIV). The decrease in the zinc inhibition for neurones maintained in vitro proceeded at an apparent rate of 0.55% per day. 4. Activation of the GABA receptor by muscimol (0.2-2 microM) was also antagonized by zinc (50-100 microM). 5. Lowering the pH of the perfusing Krebs solution did not affect the inhibition of GABA responses by zinc on sympathetic neurones. 6. Modulation of the GABAA receptor by some benzodiazepines, a barbiturate, a steroid based on pregnanolone, or antagonists bicuculline and picrotoxinin, did not interfere with the antagonism exerted by zinc on sympathetic neurones. A novel binding site for zinc on the GABAA receptor is proposed. 7. Analysis of the GABA-activated current noise on sympathetic neurones revealed two kinetic components to the power spectra requiring a double Lorentzian fit. The time constant describing the fast component (tau 2, 2.1 ms) was unaffected by zinc, whereas the slow component time constant (tau 1, 21.7 ms) was slightly reduced to 17.1 ms. 8. The apparent single-channel conductance for GABA-activated ion channels was determined from the power spectra (gamma s = 22.7 pS) and also from the relationship between the mean GABA-induced inward current and the variance of the current (gamma v = 24 pS). Zinc (25-100 microM) did not affect the single-channel conductance. 9. Single GABA-activated ion channels were recorded from outside-out patches taken from the soma of large cerebellar neurones. Single GABA channels were capable of activation to multiple current amplitudes which were assessed into the following conductance levels: 8, 18, 23, 29 and 34 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

A high-affinity zinc-binding plasma protein in channel catfish (Ictalurus punctatus)

1. Channel catfish (Ictalurus punctatus) have a remarkably high concentration of zinc (Zn) in their blood serum, about 20 micrograms/ml. However, compared to mammals, the concentrations of Zn in their tissues are not remarkable. The serum Zn is dialyzable against a solution containing 1 mM EDTA. 2. Following separation of serum proteins by gel-filtration most of the Zn was recovered in a fraction with the same peak volume of elution for the Zn and protein concentrations and having a molecular weight similar to bovine serum albumin. 3. Binding of Zn to such sites was not changed by Cu2+, Cd2+, Ca2+, or La3+. N-ethylemaleimide (NEM) appeared to decrease binding slightly. 4. Equilibrium dialysis with a Scatchard plot analysis of these data suggested that a single set of binding sites was present on the protein(s) with KD of 2 x 10(-5) M. There were binding sites for 35 x 10(-8) M Zn/mg protein. 5. Parallel equilibrium dialysis measurements using human, rabbit and chicken albumins indicated that the catfish serum protein(s) had a much higher affinity and binding capacity for Zn than the albumins in these species. 6. The catfish Zn serum-binding protein may be an albumin. The possible physiological significance of such a serum protein in freshwater fish is discussed.

Increased acidophilia of eosinophil granules after EDTA treatment

The acidophilic reaction of eosinophil leucocyte granules from human, pig and horse blood smears was investigated by using May-Grünwald-Giemsa staining after previous treatment with EDTA and sodium citrate solutions. The same peak at 530 nm, but absorption values considerably higher than those of controls, were found in eosinophil granules after application of chelating agents, indicating that removal of metal cations could unmask basic groups in these structures.

A novel effect of zinc on the lobster muscle GABA receptor

The effect of zinc (and copper) was investigated on the lobster muscle gamma-aminobutyric acid (GABA) receptor. Zinc (10 microns-1 mM) depressed the GABA-evoked conductance increase in a fully reversible manner by possibly binding to an imidazole group, suggested from pH titration studies on the evoked-chloride conductance. Other transition metal (period 4) divalent cations (up to 500 microM) were inactive in antagonizing GABA responses. Variation of external chloride or anion substitution did not perturb the zinc antagonism; however, decreasing the pH markedly decreased the potency of zinc. A possible explanation for these results is discussed. Although the zinc antagonism resembled that produced by picrotoxinin, combination of these two agents depressed the GABA dose--conductance curve in a manner expected for two antagonists acting on independent sites. The zinc binding site was also discrete from the GABA recognition site; the results are interpreted in terms of a distinct binding site for zinc and H+. The distortion of an agonist dose--response curve by formation of an inactive agonist-divalent cation complex is discussed; however, complexation of GABA did not explain the observed antagonism by zinc. By comparison, zinc had no effect on the GABA responses of rat ganglionic neurons. It is concluded that the zinc binding site, on lobster muscle, may be an important modulatory site for the GABA-evoked chloride conductance.

Cytosolic levels and distribution of cadmium, copper and zinc in pretumorous livers from diethylnitrosamine-exposed mice and in non-cancerous kidneys cancer patients

Exposure of mice to diethylnitrosamine (DEN) in their drinking water, resulted in decreases of Cu and Zn in cytosol of both pretumorous and tumorous liver tissue. Copper was reduced 28% and zinc 44.5% in cytosol of tumorous tissue at the time of death. Analysis of Sephadex G-75 gel chromatography profiles revealed that these decreases were mainly from the high molecular weight protein pool, with smaller decreases from metallothionein. Exposure of mice to Cd in their drinking water resulted in the accumulation of most Cd on metallothionein with a smaller accumulation in the high molecular weight protein pool; Cu and Zn levels were increased in both of these cytosolic pools. When DEN was administered with Cd, there were smaller increases of Cu and Zn, and more Cd accumulated in the high molecular weight protein pool. In non-cancerous kidney of postmortem cancer patients, most Cd appeared in the high molecular weight protein pool, whereas in controls, most appeared on metallothionein.

A novel enzymatic activity of phenylalanyl transfer ribonucleic acid synthetase from baker's yeast: zinc ion induced transfer ribonucleic acid independent hydrolysis of adenosine triphosphate

Phenylalanyl-tRNA synthetase from baker's yeast in the presence of phenylalanine or other amino acids misactivated by the enzyme, ATP, and low concentrations of Zn2+ is able to hydrolyze ATP to AMP and PPi very efficiently. After dialysis of the enzyme against ethylenediaminetetraacetic acid (EDTA), this amino acid dependent but tRNAPhe-independent hydrolysis is suppressed to negligible levels. The ATP hydrolysis can be restored by the addition of Zn2+ to the EDTA-dialyzed enzyme. During aminoacylation of tRNAPhe the Zn2+-induced ATP hydrolysis parallels the aminoacylation reaction, leading to nonstoichiometric production of AMP. Mechanistically, we conclude that Zn2+ can be bound to phenylalanyl-tRNA synthetase and can influence the stability of ATP if an activatable amino acid is present. The influence of Zn2+, if any, on the aminoacylation of tRNAPhe is not known. In practice, this side reaction is of the utmost importance in all cases in which the fate of ATP during aminoacylation is followed, especially if the stoichiometry of ATP consumption in relation to Phe-tRNAPhe formation has to be determined.

Interactions between cadmium and zinc in cytoplasm of duck liver and kidney

Ducks were collected from a marine environment known to be polluted with heavy metals. Gel elution profiles were determined for both liver and kidney tissue using Sephadex G-75 gel. Zn increased linearly on the high molecular weight protein pool until this pool was apparently Zn saturated; saturation levels were 0.06 mumole cytoplasmic Zn/g kidney (wet wt.) and 0.14 mumole cytoplasmic Zn/g liver (wet wt.). Seventy-five percent of cytoplasmic Cd was found on the high molecular weight protein pool unless it was Zn saturated. If this pool was Zn saturated, then excesses of Zn and up to 75% of cytoplasmic Cd appeared on metallothionein. Results are discussed in terms of a competition of Cd and Zn for Zn binding sites on metalloenzymes. Cu increased linearly on both the high molecular weight protein pool and metallothionein, with increases of Cu in tissue.

Thermodynamic analysis of ion effects on the binding and conformational equilibria of proteins and nucleic acids: the roles of ion association or release, screening, and ion effects on water activity

The purpose of this review is to examine the various effects of low- molecular-weight electrolytes on the associations and interactions of proteins and nucleic acids. Our primary interest is in general electrostatic effects, rather than chemical effects (specific interactions) of particular ions (e.g. transition metals, protons). We consider those interactions in which a variation in salt concentration has a significant effect on the macromolecular equilibrium, and analyse the effects of salt in these situations in terms of (i) direct participation of ions in the biopolymer reaction, (ii) Debye—Hückel screening by salt ions of the charge interactions on the biopolymers, and (iii) the reduction in water activity brought about at high salt concentrations.

Plastocyanin: possible significance of quaternary structure

Some properties of the blue copper protein plastocyanin from the green alga Scenedesmus have been investigated and compared with that from spinach, including amino acid composition, isoelectric point and copper content. The protein from Scenedesmus contains two, that from spinach four copper atoms per molecular weight of 40000. A combination of sodium dodecylsulfate/polyacrylamide gel electrophoresis and quantification of sulfhydryl groups indicates a strong preference for a species composed of 4 polypeptide chains of identical amino acid composition representing the enzymically active entity. Due to various treatments the subunits of both plastocyanins are detected as either monomer species alone or as monomer and dimer in a molar ratio of 2 : 1 on sodium dodecylsulfate gels. The four -SH groups per molecule are of different reactivity: two -SH groups can be detected after destruction of the chromophore; two more (forming an S-S bridge in the dimer) become evident after appropriate reduction. A KCN treatment for production of apoprotein is reported and the use of electrodialysis to improve incomplete apoprotein formation. These studies lend support to the proposal of a quaternary structure. Apoproteins were subjected to dodecylsulfate gel analysis, which proved to be an effective means of estimating both the extent of apoprotein formation and its reconstitution to the haloprotein.

The effect of zinc on the late displacement current in squid giant axons

1. Displacement currents produced by single depolarizing or hyperpolarizing voltage-clamp pulses (Idepol and Ihyperpol) were recorded from intracellularly perfused squid giant axons treated with tetrodotoxin and tetraethylammonium chloride. The effect of internal Zn on the slow part of the displacement current was studied at different holding potentials. 2. Internal Zn in a concentration of 3-3 mM markedly reduced the slow charge displacement associated with depolarizing and hyperpolarizing pulses. 3. At a holding potential more negative than -60 mV Idepol is normally larger than Ihyperpol if measured with pulses of equal height. The asymmetry Idepol greater than Ihyperpol (which possibly reflects the movement of gating charges) was abolished by Zn. 4. The reversed asymmetry Ihyperpol greater than Idepol which is normally seen at holding potentials less negative than -60 mV was not blocked by Zn. This suggests that the underlying mechanism is different from that of the asymmetry Idepol greater than Ihyperpol. 5. The Zn-sensitive slow charge displacement during single depolarizing pulses was strongly reduced by lowering the holding potential from about -90 to about -30 mV. 6. The observations with single clamp pulses were confirmed by averaging and summing the currents associated with an equal number of depolarizing and hyperpolarizing pulses. 7. The effect of internal Zn on the charge displacement is thought to be due to a reaction with mobile charges in the membrane dielectric. Internal Zn in a concentration of 0-5-1 mM did not significantly shift the Na inactivation curve, indicating that it does not react with surface charges at the inner side of the membrane.