Metallothionein-3 attenuates the effect of Cu2+ ions on actin filaments

Metallothionein 3 (MT-3) is a cysteine-rich metal-binding protein that is expressed in the mammalian central nervous system and kidney. Various reports have posited a role for MT-3 in regulating the actin cytoskeleton by promoting the assembly of actin filaments. We generated purified, recombinant mouse MT-3 of known metal compositions, either with zinc (Zn), lead (Pb), or copper/zinc (Cu/Zn) bound. None of these forms of MT-3 accelerated actin filament polymerization in vitro, either with or without the actin binding protein profilin. Furthermore, using a co-sedimentation assay, we did not observe Zn-bound MT-3 in complex with actin filaments. Cu²⁺ ions on their own induced rapid actin polymerization, an effect that we attribute to filament fragmentation. This effect of Cu²⁺ is reversed by adding either EGTA or Zn-bound MT-3, indicating that either molecule can chelate Cu²⁺ from actin. Altogether, our data indicate that purified recombinant MT-3 does not directly bind actin but it does attenuate the Cu-induced fragmentation of actin filaments.

Metal binding and interdomain thermodynamics of mammalian metallothionein-3: enthalpically favoured Cu+ supplants entropically favoured Zn2+ to form Cu4 + clusters under physiological conditions

Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d¹⁰ metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn²⁺ and Cu⁺ binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn²⁺ binding was measured by chelation titrations of Zn₇MT-3, while Cu⁺ binding was measured by Zn²⁺ displacement from Zn₇MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled a detailed analysis that yielded condition-independent values for the association constant (K) and the change in enthalpy (ΔH) and entropy (ΔS) for these metal ions binding to MT-3. Zn²⁺ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn₇MT-2 with Cu⁺ revealed that both MT isoforms have similar Cu⁺ affinities and binding thermodynamics, indicating that ΔH and ΔS are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS) analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu₄ ⁺-thiolate clusters when Cu⁺ displaces Zn²⁺ under physiological conditions. Comparison of the Zn²⁺ and Cu⁺ binding thermodynamics reveal that enthalpically-favoured Cu⁺, which forms Cu₄ ⁺-thiolate clusters, displaces the entropically-favoured Zn²⁺. These results provide a detailed thermodynamic analysis of d¹⁰ metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.

Shift from Entropic Cu2+ Binding to Enthalpic Cu+ Binding Determines the Reduction Thermodynamics of Blue Copper Proteins

The enthalpic and entropic components of Cu²⁺ and Cu⁺ binding to the blue copper protein azurin have been quantified with isothermal titration calorimetry (ITC) measurements and analysis, providing the first such experimental values for Cu⁺ binding to a protein. The high affinity of azurin for Cu²⁺ is entirely due to a very favorable binding entropy, while its even higher affinity for Cu⁺ is due to a favorable binding enthalpy and entropy. The binding thermodynamics provide insight into bond enthalpies at the blue copper site and entropic contributions from desolvation and proton displacement. These values were used in thermodynamic cycles to determine the enthalpic and entropic contributions to the free energy of reduction and thus the reduction potential. The reduction thermodynamics obtained with this method are in good agreement with previous results from temperature-dependent electrochemical measurements. The calorimetry method, however, provides new insight into contributions from the initial (oxidized) and final (reduced) states of the reduction. Since ITC measurements quantify the protons that are displaced upon metal binding, the proton transfer that is coupled with electron transfer is also determined with this method. Preliminary results for Cu²⁺ and Cu⁺ binding to the Phe114Pro variant of azurin demonstrate the insight about protein tuning of the reduction potential that is provided by the binding thermodynamics of each metal oxidation state.

Photo-transformation of aqueous nitroguanidine and 3-nitro-1,2,4-triazol-5-one: Emerging munitions compounds

Two major components of insensitive munition formulations, nitroguanidine (NQ) and 3-nitro-1,2,4-triazol-5-one (NTO), are highly water soluble and therefore likely to photo-transform while in solution in the environment. The ecotoxicities of NQ and NTO solutions are known to increase with UV exposure, but a detailed accounting of aqueous degradation rates, products, and pathways under different exposure wavelengths is currently lacking. Here, we irradiated aqueous solutions of NQ and NTO over a 32-h period at three ultraviolet wavelengths (254 nm, 300 nm, and 350 nm) and analyzed their degradation rates and transformation products. NQ was completely degraded by 30 min at 254 nm and by 4 h at 300 nm, but it was only 10% degraded after 32 h at 350 nm. Mass recoveries of NQ and its transformation products were ≥80% for all three wavelengths, and consisted of large amounts of guanidine, nitrate, and nitrite, and smaller amounts of cyanamide, cyanoguanidine, urea, and ammonium. NTO degradation was greatest at 300 nm with 3% remaining after 32 h, followed by 254 nm (7% remaining) and 350 nm (20% remaining). Mass recoveries of NTO and its transformation products were high for the first 8 h but decreased to 22-48% by 32 h, with the major aqueous products identified as ammonium, nitrate, nitrite, and a urazole intermediate. Environmental half-lives of NQ and NTO in pure water were estimated as 4 and 6 days, respectively. We propose photo-degradation pathways for NQ and NTO supported by observed and quantified degradation products and changes in solution pH.

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

Structural and Biochemical Characterization of Organotin and Organolead Compounds Binding to the Organomercurial Lyase MerB Provide New Insights into Its Mechanism of Carbon-Metal Bond Cleavage

The organomercurial lyase MerB has the unique ability to cleave carbon-Hg bonds, and structural studies indicate that three residues in the active site (C96, D99, and C159 in E. coli MerB) play important roles in the carbon-Hg bond cleavage. However, the role of each residue in carbon-metal bond cleavage has not been well-defined. To do so, we have structurally and biophysically characterized the interaction of MerB with a series of organotin and organolead compounds. Studies with two known inhibitors of MerB, dimethyltin (DMT) and triethyltin (TET), reveal that they inhibit by different mechanisms. In both cases the initial binding is to D99, but DMT subsequently binds to C96, which induces a conformation change in the active site. In contrast, diethyltin (DET) is a substrate for MerB and the Sn^IV product remains bound in the active site in a coordination similar to that of Hg^II following cleavage of organomercurial compounds. The results with analogous organolead compounds are similar in that trimethyllead (TML) is not cleaved and binds only to D99, whereas diethyllead (DEL) is a substrate and the Pb^IV product remains bound in the active site. Binding and cleavage is an exothermic reaction, while binding to D99 has negligible net heat flow. These results show that initial binding of organometallic compounds to MerB occurs at D99 followed, in some cases, by cleavage and loss of the organic moieties and binding of the metal ion product to C96, D99, and C159. The N-terminus of MerA is able to extract the bound Pb^VI but not the bound Sn^IV. These results suggest that MerB could be utilized for bioremediation applications, but certain organolead and organotin compounds may present an obstacle by inhibiting the enzyme.

Stabilization of Cu(I) for binding and calorimetric measurements in aqueous solution

Conditions have been developed for the comproportionation reaction of Cu(2+) and copper metal to prepare aqueous solutions of Cu(+) that are stabilized from disproportionation by MeCN and other Cu(+)-stabilizing ligands. These solutions were then used in ITC measurements to quantify the thermodynamics of formation of a set of Cu(+) complexes (Cu(I)(MeCN)3(+), Cu(I)Me6Trien(+), Cu(I)(BCA)2(3-), Cu(I)(BCS)2(3-)), which have stabilities ranging over 15 orders of magnitude, for their use in binding and calorimetric measurements of Cu(+) interaction with proteins and other biological macromolecules. These complexes were then used to determine the stability and thermodynamics of formation of a 1 : 1 complex of Cu(+) with the biologically important tri-peptide glutathione, GSH. These results identify Me6Trien as an attractive Cu(+)-stabilizing ligand for calorimetric experiments, and suggest that caution should be used with MeCN to stabilize Cu(+) due to its potential for participating in unquantifiable ternary interactions.

Structural and Biochemical Characterization of a Copper-Binding Mutant of the Organomercurial Lyase MerB: Insight into the Key Role of the Active Site Aspartic Acid in Hg-Carbon Bond Cleavage and Metal Binding Specificity

In bacterial resistance to mercury, the organomercurial lyase (MerB) plays a key role in the detoxification pathway through its ability to cleave Hg-carbon bonds. Two cysteines (C96 and C159; Escherichia coli MerB numbering) and an aspartic acid (D99) have been identified as the key catalytic residues, and these three residues are conserved in all but four known MerB variants, where the aspartic acid is replaced with a serine. To understand the role of the active site serine, we characterized the structure and metal binding properties of an E. coli MerB mutant with a serine substituted for D99 (MerB D99S) as well as one of the native MerB variants containing a serine residue in the active site (Bacillus megaterium MerB2). Surprisingly, the MerB D99S protein copurified with a bound metal that was determined to be Cu(II) from UV-vis absorption, inductively coupled plasma mass spectrometry, nuclear magnetic resonance, and electron paramagnetic resonance studies. X-ray structural studies revealed that the Cu(II) is bound to the active site cysteine residues of MerB D99S, but that it is displaced following the addition of either an organomercurial substrate or an ionic mercury product. In contrast, the B. megaterium MerB2 protein does not copurify with copper, but the structure of the B. megaterium MerB2-Hg complex is highly similar to the structure of the MerB D99S-Hg complexes. These results demonstrate that the active site aspartic acid is crucial for both the enzymatic activity and metal binding specificity of MerB proteins and suggest a possible functional relationship between MerB and its only known structural homologue, the copper-binding protein NosL.

Isothermal Titration Calorimetry Measurements of Metal Ions Binding to Proteins

ITC measurements involving metal ions are susceptible to a number of competing reactions (oxidation, precipitation, and hydrolysis) and coupled reactions involving the buffer and protons. Stabilization and delivery of the metal ion as a well-defined and well-characterized complex with the buffer, or a specific ligand, can suppress undesired solution chemistry and, depending on the stability of the metal complex, allow accurate measurements of higher affinity protein-binding sites. This requires, however, knowledge of the thermodynamics of formation of the metal complex and accounting for its contribution to the experimentally measured values (KITC and ΔHITC) through a post hoc analysis that provides the condition-independent binding thermodynamics (K, ΔG(o), ΔH, ΔS, and ΔCP). This analysis also quantifies the number of protons that are displaced when the metal ion binds to the protein.

Development and validation of an ex vivo electron paramagnetic resonance fingernail biodosimetric method

There is an imperative need to develop methods that can rapidly and accurately determine individual exposure to radiation for screening (triage) populations and guiding medical treatment in an emergency response to a large-scale radiological/nuclear event. To this end, a number of methods that rely on dose-dependent chemical and/or physical alterations in biomaterials or biological responses are in various stages of development. One such method, ex vivo electron paramagnetic resonance (EPR) nail dosimetry using human nail clippings, is a physical biodosimetry technique that takes advantage of a stable radiation-induced signal (RIS) in the keratin matrix of fingernails and toenails. This dosimetry method has the advantages of ubiquitous availability of the dosimetric material, easy and non-invasive sampling, and the potential for immediate and rapid dose assessment. The major challenge for ex vivo EPR nail dosimetry is the overlap of mechanically induced signals and the RIS. The difficulties of analysing the mixed EPR spectra of a clipped irradiated nail were addressed in the work described here. The following key factors lead to successful spectral analysis and dose assessment in ex vivo EPR nail dosimetry: (1) obtaining a thorough understanding of the chemical nature, the decay behaviour, and the microwave power dependence of the EPR signals, as well as the influence of variation in temperature, humidity, water content, and O₂ level; (2) control of the variability among individual samples to achieve consistent shape and kinetics of the EPR spectra; (3) use of correlations between the multiple spectral components; and (4) use of optimised modelling and fitting of the EPR spectra to improve the accuracy and precision of the dose estimates derived from the nail spectra. In the work described here, two large clipped nail datasets were used to test the procedures and the spectral fitting model of the results obtained with it. A 15-donor nail set with 90 nail samples from 15 donors was used to validate the sample handling and spectral analysis methods that have been developed but without the interference of a native background signal. Good consistency has been obtained between the actual RIS and the estimated RIS computed from spectral analysis. In addition to the success in RIS estimation, a linear dose response has also been achieved for all individuals in this study, where the radiation dose ranges from 0 to 6 Gy. A second 16-donor nail set with 96 nail samples was used to test the spectral fitting model where the background signal was included during the fitting of the clipped nail spectra data. Although the dose response for the estimated and actual RIS calculated in both donor nail sets was similar, there was an increased variability in the RIS values that was likely due to the variability in the background signal between donors. Although the current methods of sample handling and spectral analysis show good potential for estimating the RIS in the EPR spectra of nail clippings, there is a remaining degree of variability in the RIS estimate that needs to be addressed; this should be achieved by identifying and accounting for demographic sources of variability in the background nail signal and the composition of the nail matrix.

Thermodynamic contributions to the stability of the insulin hexamer

The insulin hexamer is resistant to degradation and fibrillation, which makes it an important quaternary structure for its in vivo storage in Zn(2+)- and Ca(2+)-rich vesicles in the pancreas and for pharmaceutical formulations. In addition to the two Zn(2+) ions that are required for its formation, three other species, Zn-coordinating anions (e.g., Cl(-)), Ca(2+), and phenols (e.g., resorcinol), bind to the hexamer and affect the subunit conformation and stability. The contributions of these four species to the thermodynamics of insulin unfolding have been quantified by differential scanning calorimetry and thermal unfolding measurements to determine the extent and nature of their stabilization of the insulin hexamer. Both Zn(2+) and resorcinol make a significant enthalpic contribution, while Ca(2+) primarily affects the protein heat capacity (solvation) by its interactions in the central cation-binding cavity, which is modulated by the surrounding subunit conformations. Coordinating anions have a negligible effect on the stability of the hexamer, even though subunits shift to an alternate conformation when these anions bind to the Zn(2+) ions. Finally, Zn(2+) in excess of the two that are required to form the hexamer further stabilizes the protein by additional enthalpic contributions.

Electron paramagnetic resonance dosimetry for a large-scale radiation incident

With possibilities for radiation terrorism and intensified concerns about nuclear accidents since the recent Fukushima Daiichi event, the potential exposure of large numbers of individuals to radiation that could lead to acute clinical effects has become a major concern. For the medical community to cope with such an event and avoid overwhelming the medical care system, it is essential to identify not only individuals who have received clinically significant exposures and need medical intervention but also those who do not need treatment. The ability of electron paramagnetic resonance to measure radiation-induced paramagnetic species, which persist in certain tissues (e.g., teeth, fingernails, toenails, bone, and hair), has led to this technique becoming a prominent method for screening significantly exposed individuals. Although the technical requirements needed to develop this method for effective application in a radiation event are daunting, remarkable progress has been made. In collaboration with General Electric and through funding committed by the Biomedical Advanced Research and Development Authority, electron paramagnetic resonance tooth dosimetry of the upper incisors is being developed to become a Food and Drug Administration-approved and manufacturable device designed to carry out triage for a threshold dose of 2 Gy. Significant progress has also been made in the development of electron paramagnetic resonance nail dosimetry based on measurements of nails in situ under point-of-care conditions, and in the near future this may become a second field-ready technique. Based on recent progress in measurements of nail clippings, it is anticipated that this technique may be implementable at remotely located laboratories to provide additional information when the measurements of dose on-site need to be supplemented. The authors conclude that electron paramagnetic resonance dosimetry is likely to be a useful part of triage for a large-scale radiation incident.

Thermodynamics of Zn2+ binding to Cys2His2 and Cys2HisCys zinc fingers and a Cys4 transcription factor site

The thermodynamics of Zn(2+) binding to three peptides corresponding to naturally occurring Zn-binding sequences in transcription factors have been quantified with isothermal titration calorimetry (ITC). These peptides, the third zinc finger of Sp1 (Sp1-3), the second zinc finger of myelin transcription factor 1 (MyT1-2), and the second Zn-binding sequence of the DNA-binding domain of glucocorticoid receptor (GR-2), bind Zn(2+) with Cys(2)His(2), Cys(2)HisCys, and Cys(4) coordination, respectively. Circular dichroism confirms that Sp1-3 and MyT1-2 have considerable and negligible Zn-stabilized secondary structure, respectively, and indicate only a small amount for GR-2. The pK(a)'s of the Sp1-3 cysteines and histidines were determined by NMR and used to estimate the number of protons displaced by Zn(2+) at pH 7.4. ITC was also used to determine this number, and the two methods agree. Subtraction of buffer contributions to the calorimetric data reveals that all three peptides have a similar affinity for Zn(2+), which has equal enthalpy and entropy components for Sp1-3 but is more enthalpically disfavored and entropically favored with increasing Cys ligands. The resulting enthalpy-entropy compensation originates from the Zn-Cys coordination, as subtraction of the cysteine deprotonation enthalpy results in a similar Zn(2+)-binding enthalpy for all three peptides, and the binding entropy tracks with the number of displaced protons. Metal and protein components of the binding enthalpy and entropy have been estimated. While dominated by Zn(2+) coordination to the cysteines and histidines, other residues in the sequence affect the protein contributions that modulate the stability of these motifs.

A metal switch for controlling the activity of molecular motor proteins

Kinesins are molecular motors that require a divalent metal ion (for example, Mg(2+)) to convert the energy of ATP hydrolysis into directed force production along microtubules. Here we present the crystal structure of a recombinant kinesin motor domain bound to Mn(2+) and ADP and report on a serine-to-cysteine substitution in the switch 1 motif of kinesin that allows its ATP hydrolysis activity to be controlled by adjusting the ratio of Mn(2+) to Mg(2+). This mutant kinesin binds ATP similarly in the presence of either metal ion, but its ATP hydrolysis activity is greatly diminished in the presence of Mg(2+). In human kinesin-1 and kinesin-5 as well as Drosophila melanogaster kinesin-10 and kinesin-14, this defect is rescued by Mn(2+), providing a way to control both the enzymatic activity and force-generating ability of these nanomachines.

Dosimetry based on EPR spectral analysis of fingernail clippings

Exposure of fingernails and toenails to ionizing radiation creates radicals that are stable over a relatively long period (days to weeks) and characterized by an isotropic EPR signal at g = 2.003 (so-called radiation-induced signal, RIS). This signal in readily obtained fingernail parings has the potential to be used in screening a population for exposure to radiation and determining individual dose to guide medical treatment. However, the mechanical harvesting of fingernail parings also creates radicals, and their EPR signals (so-called mechanically-induced signals, MIS) overlap the g approximately 2.0 region, interfering with efforts to quantify the RIS and, therefore, the radiation dose. Careful analysis of the time evolution and power-dependence of the EPR spectra of freshly cut fingernail parings has now resolved the MIS into three major components, including one that is described for the first time. It dominates the MIS soon after cutting, but decays within the first hour and consists of a unique doublet that can be resolved from the RIS. The MIS obtained within the first few minutes after cutting is consistent among fingernail samples and provides an opportunity to achieve the two important dosimetry objectives. First, perturbation of the initial MIS by the presence of RIS in fingernails that have received a threshold dose of radiation leads to spectral signatures that can be used for rapid screening. Second, decomposition of the EPR spectra from irradiated fingernails into MIS and RIS components can be used to isolate and thus quantify the RIS for determining individual exposure dose.

Calorimetric investigation of phosphorylated and non-phosphorylated peptide ligand binding to the human Grb7-SH2 domain

Grb7 is a member of the Grb7 family of proteins, which also includes Grb10 and Grb14. All three proteins have been found to be overexpressed in certain cancers and cancer cell lines. In particular, Grb7 (along with the receptor tyrosine kinase erbB2) is overexpressed in 20-30% of breast cancers. In general, growth factor receptor bound (Grb) proteins bind to activated membrane-bound receptor tyrosine kinases (RTKs; e.g., the epidermal growth factor receptor, EGFR) through their Src homology 2 (SH2) domains. In particular, Grb7 binds to erbB2 (a.k.a. EGFR2) and may be involved in cell signaling pathways that promote the formation of metastases and inflammatory responses. In previous studies, we reported the solution structure and the backbone relaxation behavior of the Grb7-SH2/erbB2 peptide complex. In this study, isothermal titration calorimetry studies have been completed by measuring the thermodynamic binding parameters of several phosphorylated and non-phosphorylated peptides representative of natural Grb7 receptor ligands as well as ligands developed through combinatorial peptide screening methods. The entirety of these calorimetric studies is interpreted in an effort to describe the specific ligand binding characteristics of the Grb7 protein.

Oxygen effects on the EPR signals from wood charcoals: experimental results and the development of a model

Charcoals prepared from certain tropical woods contain stable paramagnetic centers, and these have been characterized by EPR spectroscopy in the absence and presence of oxygen. The EPR-detectable spin density has been determined, as has been the temperature- and frequency-dependence of the oxygen broadening of the EPR signal, which is orders of magnitude larger than that observed with other materials, such as lithium phthalocyanine. Three Lorentzian components are required to fit the char EPR spectrum in the presence of oxygen, and the oxygen-dependence of the line width, intensity, and resonance position of the three components have been quantified. These results and the properties of porous carbonaceous materials are used to develop a model to explain the effect of oxygen on the char EPR spectral properties. The model is based on oxygen adsorption on the char surface according to a Langmuir isotherm and a dipolar interaction between the paramagnetic adsorbed gas and the charcoal spins. The three EPR components are correlated with the three known classes (sizes) of pores in charcoal, with the largest line broadening attributed to dipolar relaxation of spins in micropores, which have a larger specific surface area and a higher concentration of adsorbed oxygen. An attenuated, but similar, EPR response to oxygen by chars when they are immersed in aqueous solution is attributed to water competition with oxygen for adsorption on the char surface.

Metal-binding thermodynamics of the histidine-rich sequence from the metal-transport protein IRT1 of Arabidopsis thaliana

The widespread ZIP family of transmembrane metal-transporting proteins is characterized by a large intracellular loop that contains a histidine-rich sequence whose biological role is unknown. To provide a chemical basis for this role, we prepared and studied a peptide corresponding to this sequence from the first iron-regulated transporter (IRT1) of Arabidopsis thaliana, which transports Fe2+ as well as Mn2+, Co2+, Zn2+, and Cd2+. Isothermal titration calorimetry (ITC) measurements, which required novel experiments and data analysis, and supporting spectroscopic methods were used to quantify IRT1's metal-binding affinity and associated thermodynamics. The peptide, PHGHGHGHGP, binds metal ions with 1:1 stoichiometry and stabilities that are consistent with the Irving-Williams series. Comparison of the metal-binding thermodynamics of the peptide with those of trien provides new insight about enthalpic and entropic contributions to the stability of the metal-peptide complex. Although Fe2+ and other IRT1-transported metal ions do not bind very tightly, this His-rich sequence has a very high entropy-driven affinity for Fe3+, which may have biological significance.

Life expectancy and death from cardiomyopathy amongst carriers of Duchenne and Becker muscular dystrophy in Scotland

OBJECTIVES

To assess life expectancy and cardiovascular mortality in carriers of Duchenne and Becker muscular dystrophy.

DESIGN

Family pedigrees of individuals affected with these conditions, held by the four genetics centres in Scotland, were examined to identify a cohort of definite carriers. Electronic death registration data, held by the General Register Office for Scotland, were used to identify death certificates of carriers who had died, to obtain age at death and cause of death. Survival and mortality data were obtained for the general population for comparison.

PATIENTS

397 definite carriers in 202 pedigrees were identified from which 94 deaths were identified by record linkage to death certificates.

MAIN OUTCOME MEASURES

Observed numbers surviving to certain ages and numbers dying of cardiac causes were compared with expected numbers calculated from general population data.

RESULTS

There were no significant differences between observed and expected numbers surviving to ages 40-90. The standardised mortality ratio for the 371 carriers alive in 1974 was 0.53 (95% confidence interval 0.32 to 0.82).

CONCLUSIONS

Whereas female carriers may have clinical features of cardiomyopathy, this study does not suggest that this is associated with reduced life expectancy or increased risk of cardiac death. Routine cardiac surveillance of obligate carriers is therefore probably unnecessary.

Thermodynamics of Ni2+, Cu2+, and Zn2+ binding to the urease metallochaperone UreE

The two Ni2+ ions in the urease active site are delivered by the metallochaperone UreE, whose metal binding properties are central to the assembly of this metallocenter. Isothermal titration calorimetry (ITC) has been used to quantify the stoichiometry, affinity, and thermodynamics of Ni2+, Cu2+, and Zn2+ binding to the well-studied C-terminal truncated H144*UreE from Klebsiella aerogenes, Ni2+ binding to the wild-type K. aerogenes UreE protein, and Ni2+ and Zn2+ binding to the wild-type UreE protein from Bacillus pasteurii. The stoichiometries and affinities obtained by ITC are in good agreement with previous equilibrium dialysis results, after differences in pH and buffer competition are considered, but the concentration of H144*UreE was found to have a significant effect on metal binding stoichiometry. While two metal ions bind to the H144*UreE dimer at concentrations <10 microM, three Ni2+ or Cu2+ ions bind to 25 microM dimeric protein with ITC data indicating sequential formation of Ni/Cu(H144*UreE)4 and then (Ni/Cu)2(H144*UreE)4, or Ni/Cu(H144*UreE)2, followed by the binding of four additional metal ions per tetramer, or two per dimer. The thermodynamics indicate that the latter two metal ions bind at sites corresponding to the two binding sites observed at lower protein concentrations. Ni2+ binding to UreE from K. aerogenes is an enthalpically favored process but an entropically driven process for the B. pasteurii protein, indicating chemically different Ni2+ coordination to the two proteins. A relatively small negative value of DeltaCp is associated with Ni2+ and Cu2+ binding to H144*UreE at low protein concentrations, consistent with binding to surface sites and small changes in the protein structure.

Thermodynamic and kinetic aspects of metal binding to the histidine-rich protein, Hpn

The histidine-rich protein, Hpn, binds to essential metals Ni2+, Cu2+, Zn2+ and a therapeutic metal Bi3+ with the in vitro affinities in the order of Cu2+ > Ni2+ > Bi3+ > Zn2+. In contrast, the in vivo (in E. coli) protection by the protein is in the order of Ni2+ > Bi3+ > Cu2+ approximately Zn2+. The release of Ni2+ from the protein follows a two-step process consisting of a rapidly established equilibrium and subsequently a rate-determining step (dissociation of Hpn-Ni...EDTA to Ni-EDTA). Our work suggests the nickel storage and homeostasis in H. pylori as the primary role of Hpn.

Preferential binding of human XPA to the mitomycin C-DNA interstrand crosslink and modulation by arsenic and cadmium

The Xeroderma Pigmentosum A (XPA) protein is involved in the DNA damage recognition and repair complex formation steps of nucleotide excision repair (NER), and has been shown to preferentially bind to various forms of DNA damage including bulky lesions. DNA interstrand crosslinks are of particular interest as a form of DNA damage, since these lesions involve both strands of duplex DNA and present special challenges to the repair machinery, and mitomycin C (MMC) is one of several useful cancer chemotherapy drugs that induce these lesions. Purified XPA and the minimal DNA-binding domain of XPA are both fully capable of preferentially binding to MMC-DNA interstrand crosslinks in the absence of other proteins from the NER complex. Circular dichroism (CD) and gel shift assays were used to investigate XPA-DNA binding and to assess changes in secondary structure induced as a consequence of the interaction of XPA with model MMC-crosslinked and unmodified DNAs. These studies revealed that while XPA demonstrates only a modest increase in affinity for adducted DNA, it adopts a different conformation when bound to MMC-damaged DNA than when bound to undamaged DNA. This change in conformation may be more important in recruiting other proteins into a competent NER complex at damaged sites than preferential binding per se. Arsenic had little effect on XPA binding even at toxic concentrations, whereas cadmium reduced XPA binding to DNA to 10-15% that of Zn-XPA, and zinc addition could only partially restore activity. In addition, there was little or no change in conformation when Cd-XPA bound MMC-crosslinked DNA even though it demonstrated preferential binding, which may contribute to the mechanism by which cadmium can act as a co-mutagen and co-carcinogen.

Backbone nuclear relaxation characteristics and calorimetric investigation of the human Grb7-SH2/erbB2 peptide complex

Grb7 is a member of the Grb7 family of proteins, which also includes Grb10 and Grb14. All three proteins have been found to be overexpressed in certain cancers and cancer cell lines. In particular, Grb7 (along with the receptor tyrosine kinase erbB2) is overexpressed in 20%-30% of breast cancers. Grb7 binds to erbB2 and may be involved in cell signaling pathways that promote the formation of metastases and inflammatory responses. In a prior study, we reported the solution structure of the Grb7-SH2/erbB2 peptide complex. In this study, T(1), T(2), and steady-state NOE measurements were performed on the Grb7-SH2 domain, and the backbone relaxation behavior of the domain is discussed with respect to the potential function of an insert region present in all three members of this protein family. Isothermal titration calorimetry (ITC) studies were completed measuring the thermodynamic parameters of the binding of a 10-residue phosphorylated peptide representative of erbB2 to the SH2 domain. These measurements are compared to calorimetric studies performed on other SH2 domain/phosphorylated peptide complexes available in the literature.

Thermodynamics of the As(III)-thiol interaction: arsenite and monomethylarsenite complexes with glutathione, dihydrolipoic acid, and other thiol ligands

Colorimetric (near-UV absorption spectroscopy) and calorimetric (isothermal titration calorimetry) methods have been used to quantify the equilibrium and thermodynamics of arsenite and monomethylarsenite (MMA) coordinating to glutathione (GSH) and the dithiols dimercaptosuccinic acid (DMSA), dihydrolipoic acid (DHLA), and dithiothreitol (DTT). We found that both arsenite and MMA form moderately stable complexes (beta = 10(6)-10(7)) with GSH; that arsenite forms a particularly stable 2:3 complex (beta approximately 10(18)) with the biological cofactor DHLA; that MMA has a somewhat higher affinity than arsenite for thiol ligands; and that entropic factors modulate the overall stability of As(III) complexes with thiols, which are favored by the exothermic formation of As(III)-thiolate bonds. The implications of these results for arsenic toxicity are discussed.

Thermodynamic and spectroscopic study of Cu(II) and Ni(II) binding to bovine serum albumin

The thermodynamics of Cu(II) and Ni(II) binding to bovine serum albumin (BSA) have been studied by isothermal titration calorimetry (ITC). The Cu(II) binding affinity of the N-terminal protein site is quantitatively higher when the single free thiol, Cys-34, is reduced (mercaptalbumin), compared to when it is oxidized or derivatized with N-ethylmaleimide. This increased affinity is due predominantly to entropic factors. At higher pH (approximately 9), when the protein is in the basic (B) form, a second Cu(II) binds with high affinity to albumin with reduced Cys-34. The Cu(II) coordination has been characterized by UV-vis absorption, CD, and EPR spectroscopy, and the spectral data are consistent with thiolate coordination to a tetragonal Cu(II), indicating this is a type 2 copper site with thiolate ligation. Nickel(II) binding to the N-terminal site of BSA is also modulated by the redox/ligation state of Cys-34, with higher Ni(II) affinity for mercaptalbumin, the predominant circulating form of the protein.

Inhibition of potato polyphenol oxidase by anions and activity in various carboxylate buffers (pH 4.8) at constant ionic strength

The activity of potato polyphenol oxidase (tyrosinase) toward DL-3,4-dihydroxyphenylalanine (K(M) 5.39 mM) was studied using a variety of carboxylate buffers at a common pH and ionic strength. Enzyme activity, greatest in citrate and least in oxalate, correlated with increasing carboxyl concentration and molecular mass. The lower activity in oxalate was attributed to more effective chelation of a copper(II) form of the enzyme by the oxalate dianion. Sodium halide salts inhibited the enzyme. Although there was little difference in inhibition between sodium and potassium salts, the degree and type of inhibition was anion dependent; K(is), values for NaCl and KCl, (competitive inhibitors) were 1.82 and 1.62 mM, whereas Na(2) SO(4) and K(2) SO(4) (mixed inhibitors) had K(is) and K(ii) values in the 250 to 450 mM range.

Oxidation of zinc-binding cysteine residues in transcription factor proteins

Recent results on the oxidation of cysteine residues that bind zinc in transcription factors and their analogous peptides and in related proteins and model systems are reviewed. Two classes of oxidants, the transition metals and dioxygen, hydrogen peroxide, and related species, are considered, and the role of metal ions in suppressing or enhancing Cys oxidation is a major focus. Cysteines in the zinc-bound structures of transcription factors are less susceptible to oxidation than in the metal-free form, and this appears to correlate with reduced accessibility of the thiolates to oxidants. Substitution of other metal ions for Zn(II) increases the rate of Cys oxidation, apparently through increased oxidant accessibility. Reactions that result in reversible or irreversible oxidation of these zinc-binding cysteines under biological conditions are identified in the context of deleterious implications for gene expression.

Isothermal titration calorimetry measurements of Ni(II) and Cu(II) binding to His, GlyGlyHis, HisGlyHis, and bovine serum albumin: a critical evaluation

The binding of Ni(II) and Cu(II) to histidine, to the tripeptides GlyGlyHis and HisGlyHis, and to the protein bovine serum albumin has been studied by isothermal titration calorimetry (ITC) to determine the experimental conditions and data analysis necessary to reproduce literature values for the binding constants and thermodynamic parameters. From analysis of the ITC data, we find that there are two major considerations for the use of this method to accurately quantify metal ion interaction with biological macromolecules. First, to determine true pH-independent binding constants, ITC data must be corrected for metal ion competition with protons by accounting for the experimental pH and pKa values of the metal-binding residues. Second, metal interaction with the buffer (stability and enthalpy of formation of metal-buffer complex(es)) must be included in the analysis of the ITC data to determine the binding constants and the change in enthalpy. While it may be possible to use a buffer that forms only weak, and therefore negligible, complexes with the metal, a buffer that has a strong and well-characterized interaction has the benefit of suppressing metal ion hydrolysis and precipitation, and of allowing the quantification of high-affinity metal-binding sites on biological macromolecules. This study has also quantified the contribution of the N-terminal imidazole of HisGlyHis to the stability of the Cu(II) and Ni(II) complexes of this protein sequence and has provided new insight about Cu(II) binding to albumin.