Determination of thermodynamic parameters of cadmium(II) association to glutathione using the fluorescent reagent FluoZin-1

A method was developed to determine the conditional association constants of cadmium(II) [Cd(II)] complexes based on the reagent FluoZin-1, which forms a fluorescent complex with Cd(II). A solution containing Cd(II) and FluoZin-1 was titrated with glutathione while determining fluorescence intensity of FluoZin-1 to estimate levels of free Cd(II). The results were analyzed with a nonlinear least-squares method using the Solver algorithm of Microsoft Excel to yield conditional association constants for 1:1 and 1:2 Cd(II)-glutathione complexes. The values obtained were consistent with those reported previously using isothermal titration calorimetry.     

Double-headed protease inhibitors from black-eyed peas. IV. Half-site reactivity in the formation of complexes with trypsin and chymotrypsin.

Complex formation between two new double-headed protease inhibitors from black-eyed peas, trypsin-chymotrypsin inhibitor (BEPCI) and a trypsin inhibitor (BEPTI), and trypsin and chymotrypsin was investigated in the concentration range from 10-8 to 10-4 M by titration experiments and gel filtration chromatography. Dissociation equilibrium constants measured for complexes detected in the titration experiments range from as large as 10-8 M for trypsin bound nonspecifically to the chymotrypsin site of BEPCI to as small as 10-18 M2 for the interaction of BEPCI with chymotrypsin. The identity and stoichiometry of complexes detected during titration experiments were confirmed by gel filtration of mixtures of native and fluorescently labeled proteases and inhibitors. Half-site reactivity is observed in the formation of complexes between BEPCI or BEPTI and trypsin and chymotrypsin at all experimentally practical concentrations. The double-headed complex contains 1 molecule each of trypsin, chymotrypsin, and BEPCI dimer. The bimolecular rate constants of complex formation between trypsin or chymotrypsin and isolated BEPCI oligomers range from 1.8 X 10(5) M-1 S-1 for chymotrypsin and BEPCI monomer to 4.4 X 10(7) M-1 S-1 for trypsin and the rapidly equilibrating BEPCI dimer. The estimated rate constants for the dissociation of half-site-liganded dimer complexes and liganded monomer complexes range from 7.5 X 10-3 S-1 for the trypsin-liganded BEPCI monomer complex to 1.6 X 10-6 S-1 for the chymotrypsin-liganded BEPCI dimer complex.     

Interactions of bismuth complexes with metallothionein(II).

Bismuth complexes are widely used as anti-ulcer drugs and can significantly reduce the side effects of platinum anti-cancer drugs. Bismuth is known to induce the synthesis of metallothionein (MT) in the kidney, but there are few chemical studies on the interactions of bismuth complexes with metallothionein. Here we show that Bi(3+) binds strongly to metallothionein with a stoichiometry bismuth:MT = 7:1 (Bi(7)MT) and can readily displace Zn(2+) and Cd(2+). Bismuth is still bound to the protein even in strongly acidic solutions (pH 1). Reactions of bismuth citrate with MT are faster than those of [Bi(EDTA)](-), and both exhibit biphasic kinetics. (1)H NMR data show that Zn(2+) is displaced faster than Cd(2+), and that both Zn(2+) and Cd(2+) in the beta-domain (three metal cluster) of MT are displaced by Bi(3+) much faster than from the alpha-domain (four metal cluster). The extended x-ray absorption fine structure spectrum of Bi(7)MT is very similar to that for the glutathione and N-acetyl-L-cysteine complexes [Bi(GS)(3)] and [Bi(NAC)(3)] with an inner coordination sphere of three sulfur atoms and average Bi-S distances of 2.55 A. Some sites appear to contain additional short Bi-O bonds of 2.2 A and longer Bi-S bonds of 3.1 A. The Bi(3+) sites in Bi(7)MT are therefore highly distorted in comparison with those of Zn(2+) and Cd(2+).     

Cadmium tri-tert-butoxysilanethiolates: Structural and spectroscopic models of metal sites in proteins

Syntheses and crystal structures of two molecular, heteroleptic cadmium complexes with CdS₂NO₂ and CdS₂N₂ kernels are described. Bis(tri-tert-butoxysilanethiolate)(1-methylimidazole)cadmium(II) and bis(tri-tert-butoxysilanethiolate)bis(1-methylimidazole)cadmium(II) coexist at equilibrium in chloroform solutions with varying concentrations of bis[bis(tri-tert-butoxysilanethiolate)cadmium(II)] and 1-methylimidazole. The equilibrium is characterized by solution ¹¹³Cd NMR spectra. Solid state CP MAS ¹³C, ²⁹Si, ¹¹³Cd NMR data for the complexes are also reported, analyzed and compared with the results obtained for cadmium-substituted proteins. The similarities and differences between the structures of cadmium complexes and their zinc analogues are discussed.     

Studies on the kinetic stabilities of the Gd(3+) complexes formed with the N-mono(methylamide), N'-mono(methylamide) and N,N"-bis(methylamide) derivatives of diethylenetriamine-N,N,N',N",N"-pentaacetic acid

High kinetic stability is an important requirement for the Gd(3+) complexes used as contrast enhancement agents in magnetic resonance imaging. The kinetic stabilities of the Gd(3+) complexes formed with DTPA-N-mono(methylamide) (L(3)), DTPA-N'-mono(methylamide) (L(2)) and DTPA-bis(methylamide) (L(1)) are characterized by the rates of the exchange reactions with Eu(3+) and the endogenous Cu(2+) and Zn(2+). The exchange reactions occur via the proton-assisted dissociation of the complexes and direct attack of the exchanging metal ions on the complex. On the basis of the line-shape analysis of the 1H NMR spectra of the LaL(2), obtained in the pH range 2.5-3.5, we assume that for the proton-assisted dissociation of the complexes the formation of an intermediate containing a free iminodiacetate group must be followed with the rupture of the metal-central nitrogen bond. At about pH > or = 5, the reactions between GdL(2) or GdL(3) and Cu(2+) or Zn(2+) proceed predominantly by direct reaction of the reactants, through the formation of dinuclear intermediates. The contribution of the proton-assisted dissociation is highly important for GdL(1), but its reaction with Zn(2+) is significantly slower than the reactions of GdL(2) and GdL(3). The overall rates of dissociation of GdL(1), GdL(2), GdL(3) and Gd(DTPA)(2-) through H(+) (pH 7.4), Cu(2+) (1 x 10(-6) M) and Zn(2+) (1 x 10(-5) M)-assisted reactions are surprisingly very similar. Replacement of one or two carboxylates with amide groups results in significantly decreased stability constants, but has practically no effect on the kinetic stability of the Gd(3+) complexes, indicating the lower reactivity of the amide groups with Cu(2+) and Zn(2+).     

Mechanism of zinc coordination by point-mutated structures of the distal CCHC binding motif of the HIV-1 NCp7 protein

The kinetics of Zn(2+) binding by two point-mutated forms of the HIV-1 NCp7 C-terminal zinc finger, each containing tridentate binding motif HCC [Ser49(35-50)NCp7] or CCC [Ala44(35-50)NCp7], has been studied by stopped-flow spectrofluorimetry. Both the formation and dissociation rate constants of the complexes between Zn(2+) and the two model peptides depend on pH. The results are interpreted on the basis of a multistep reaction model involving three Zn(2+) binding paths due to three deprotonated states of the coordinating motif, acting as monodentate, bidentate, and tridentate ligands. For Ser49(35-50)NCp7 around neutral pH, binding preferentially occurs via the deprotonated Cys36 in the bidentate state also involving His44. The binding rate constants for the monodentate and bidentate states are 1 x 10(6) and 3.9 x 10(7) M(-)(1) s(-)(1), respectively. For Ala44(35-50)NCp7, intermolecular Zn(2+) binding predominantly occurs via the deprotonated Cys36 in the monodentate state with a rate constant of 3.6 x 10(7) M(-)(1) s(-)(1). In both mutants, the final state of the Zn(2+) complex is reached by subsequent stepwise ligand deprotonation and intramolecular substitution of coordinated water molecules. The rate constants for the intermolecular binding paths of the bidentate and tridentate states of Ala44(35-50)NCp7 and of the tridentate state of Ser49(35-50)NCp7 are much smaller than expected according to electrostatic considerations. This is attributed to conformational constraints required to achieve proper metal coordination during folding. The dissociation of Zn(2+) from both peptides is again characterized by a multistep process and takes place fastest via the protonated Zn(2+)-bound bidentate and monodentate states, with rate constants of approximately 0.3 and approximately 10(3) s(-)(1), respectively, for Ser49(35-50)NCp7 and approximately 4 x 10(-)(3) and approximately 500 s(-)(1), respectively, for Ala44(35-50)NCp7.     

Effects of water chemistry variables on gill binding and acute toxicity of cadmium in rainbow trout (Oncorhynchus mykiss): A biotic ligand model (BLM) approach

This study investigated the short-term (3 h) cadmium binding characteristics of the gills, as well as the influence of various water chemistry variables [calcium, magnesium, sodium, pH, alkalinity and dissolved organic carbon (DOC)] on short-term gill accumulation and acute toxicity of cadmium in juvenile freshwater rainbow trout. The cadmium binding pattern revealed two types of cadmium binding sites in the gill: (i) saturable high affinity sites operating at a low range of waterborne cadmium concentration, and (ii) non-saturable low affinity sites operating at a higher range of cadmium concentration. Among the water chemistry variables tested, only calcium and DOC significantly reduced both gill accumulation and toxicity of cadmium. Interestingly, alkalinity (15-90 mg L(-1) as CaCO(3)) did not influence the gill cadmium accumulation but a significant increase in toxicity was recorded at a higher alkalinity level (90 mg L(-1)). Affinity constants (log K) for binding of competing cations (Cd(2+) and Ca(2+)) to the biotic ligand and for binding of Cd(2+) to DOC were derived separately from the 3 h gill binding tests and the 96 h toxicity tests. In general, the values agreed well, indicating that both tests targeted the same population of high affinity binding sites, which are likely Ca(2+) uptake sites on the gills. These parameters were then incorporated into a geochemical speciation model (MINEQL+) to develop a biotic ligand model for predicting acute toxicity of cadmium in trout. The model predictions exhibited a good fit with the measured toxicity data except for high alkalinity and pH.     

The formation constants of ionomycin with divalent cations in 80% methanol/water.

The protonation constants and complex formation constants of ionomycin have been determined in 80% methanol/water (w/w) at 25.0 degrees C and mu = 0.050 (tetraethylammonium perchlorate). Potentiometric and spectrometric titration techniques give the following values for the mixed-mode protonation constants of ionomycin: log KH1 = 11.94 +/- 0.02 and log KH2 = 6.80 +/- 0.03. Comparison of these values with those for model compounds indicates that KH1 and KH2 refer to equilibria involving the beta-diketone and carboxylic acid moieties, respectively. Titrations of ionomycin with metal ion at fixed values of pH produced changes in the UV-visual absorbance spectra which were analyzed to give conditional complex formation constants, KMI'. The pH dependence of the values of KMI' indicated that 1:1 divalent metal ion-ionomycin (MI) complexes and protonated MHI+ complexes were formed in the pH range studied. The values of log KMI ranged from 5.30 +/- 0.11 for Sr2+ to 10.25 +/- 0.03 for Ni2+. The selectivity pattern and relative affinities (in parentheses) for the formation of the species MI are as follows: Ni2+ (2000) greater than Zn2+ (600) greater than CO2+ (440) greater than Mn2+ (47) greater than Mg2+ (1.00) greater than Ca2+ (0.21) greater than Sr2+ (0.022). Logarithmic values of KMHI, for the reaction MI + H+ in equilibrium MHI+, ranged from 5.9 (Ni2+) to 8.4 (Sr2+). Calculations using the values of the equilibrium constants determined indicate that an appreciable fraction of the complexed ionophore exists as the protonated complex, MHI+, in the pH range of 6.5-8.5.     

Ternary nickel(II) complexes as hydrolytic DNA-cleavage agents

A series of small model complexes made from Ni(II) and the ligands ethylenediamine (en), histamine (hist), and histidylleucine (HisLeu) were prepared and studied as potential hydrolytic DNA-cleavage agents. The stability constants and species-distribution curves for these complexes were determined as a function of pH. The 1 : 1 : 1 ternary complexes [Ni(II)(en)(HisLeu)] (1) and [Ni(II)(hist)(HisLeu)] (2) were the only major species present at the physiologically relevant pH of 6-7, as further corroborated by ESI-MS analysis. The complex geometries of 1 and 2 were analyzed by UV/VIS experiments and molecular dynamics (MD) simulations. Both ternary complexes were found to intercalate with DNA, as shown by UV/VIS, thermal-denaturation, and fluorescence-titration studies with ethidium bromide (EB). The intrinsic binding constants (K(b)) for the bound complexes 1DNA and 2DNA were determined as 150 and 290, resp. Gel-electrophoresis experiments revealed that 1 and 2 cleave supercoiled (type-I) to nicked-circular (type-II) DNA at physiological pH, with rate constants of 0.64 and 0.75 h(-1), resp. A tentative mechanism for this hydrolytic cleavage is proposed.     

Interactions between different selenium compounds and zinc, cadmium and mercury

In this paper, we present a polarographic study of systems containing different inorganic and organic selenium compounds (sodium selenite, sodium selenate, seleno-methionine and seleno-urea) and metal ions (Zn2+, Cd2+ Hg2+) of the 12th group of elements in the periodic table. While zinc is a trace element known to be essential for plants and animals, cadmium and mercury are exogenous elements and are harmful pollutants that accumulate during aging; selenium is also recognized as an important micronutrient and is sometimes added to the diet. Experiments investigating the interactions were carried out using polarographic techniques in unbuffered systems. The three metal cations originated complexes with different strength and solubility in the presence of selenite anions; in the presence of selenate, polarography was not able to detect formation of complexes with these metal ions, at least under the experimental conditions used: a decrease of Hg2+ ion concentration was observed. Seleno-methionine did not react with Cd2+; in the presence of Zn2+, a soluble complex with a co-ordination number 1 was formed, while, again, the concentration of Hg2+ decreased in the presence of increasing concentrations of the selenium derivative. Seleno-urea did not react with Zn2+, but formed a complex with Cd2+ with limited solubility. Finally, this ligand could not be studied with Hg2+ because of the overlapping of the reduction potentials of both the ligand and the metal cation. Overall equilibrium constants for complex formation (Kf) and the solubility product (Ksp) for poorly soluble species are also reported.     

DNA interactions of new mixed-ligand complexes of cobalt(III) and nickel(II) that incorporate modified phenanthroline ligands

Four new mixed-ligand complexes, namely [Co(phen)(2)(qdppz)](3+), [Ni(phen)(2)(qdppz)](2+), [Co(phen)(2)(dicnq)](3+) and [Ni(phen)(2)(dicnq)](2+) (phen=1,10-phenanthroline, qdppz=naptho[2,3-a]dipyrido[3,2-H:2',3'-f]phenazine-5,18-dione and dicnq=dicyanodipyrido quinoxaline), were synthesized and characterized by FAB-MS, UV/Vis, IR, 1H NMR, cyclic voltammetry and magnetic susceptibility methods. Absorption and viscometric titration as well as thermal denaturation studies revealed that each of these octahedral complexes is an avid binder of calf-thymus DNA. The apparent binding constants for the dicnq- and qdppz-bearing complexes are in the order of 10(4) and >10(6) M(-1), respectively. Based on the data obtained, an intercalative mode of DNA binding is suggested for these complexes. While both the investigated cobalt(III) complexes and also [Ni(phen)(2)(qdppz)](2+) affected the photocleavage of DNA (supercoiled pBR 322) upon irradiation by 360 nm light, the corresponding dicnq complex of nickel(II) was found to be ineffective under a similar set of experimental conditions. The physico-chemical properties as well as salient features involved in the DNA interactions of the cobalt(III) and nickel(II) complexes investigated here were compared with each other and also with the corresponding properties of the previously reported ruthenium(II) analogues.     

Potentiometric measurement of stability constants of complexes between fulvic acid carboxylate and Fe3+

The stability constants of complexes formed between iron (III) and fulvic acid extracted from organic manures and wastes such as urban domestic sewage sludge, farmyard manure, poultry manure and sulfitation pressmud were investigated by the potentiometric titration method in an ionic medium of 0.1 M KNO₃ at 25±1 °C. A modification of the Katchalsky's model was employed for the estimation of stability constants. The displacement of the titration curves due to presence of Fe³⁺ in FA solutions formed the basis of calculations. The weak acidic property of fulvic acids due to carboxyl groups resulted in buffering over a wide range of pH; fulvic acids were completely neutralized in the pH range of 7.00–8.85. Apparent dissociation constants (pK_APP) of weakly acidic carboxyl groups were a direct function of degree of dissociation (α_L) in the mid-range of titration curves but were non-linear at high and low α_L values. The stability constants for formation of Fe–FA complexes (log β_Fe) calculated from the titration data were in the range of 5.64–7.55, depending upon α_L and electrostatic properties of fulvic acids. The relatively high stability constants of Fe–FA complexes in comparison to those with other competing cations suggest that the Fe–FA complexes are relatively stable in a soil environment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Uranium(VI) complexes with phospholipid model compounds--a laser spectroscopic study

We present the complex formation of the uranyl ion (UO(2)(2+)) in the aqueous system with phosphocholine, O-phosphoethanolamine and O-phosphoserine. These phosphonates (R-O-PO(3)(2-)) represent the hydrophilic head groups of phospholipids. The complexation was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH=2-6. An increase of the fluorescence intensity, connected with a strong red-shift of about 8 nm compared to the free uranyl ion, indicates a complex formation between UO(2)(2+) and the phosphonates already at pH=2. Even at pH=6 these complexes prevail over the uranyl hydroxide and carbonate species, which are generated naturally at this pH. At pH=4 and higher a 1:2 complex between uranyl and O-phosphoserine was found. Complexes with a metal-to-ligand ratio of 1:1 were observed for all other ligands. Fluorescence lifetimes, emission maxima and complex stability constants at T=22+/-1 degrees C are reported. The TRLFS spectra of uranyl complexes with two phosphatidic acids (1,2-dimyristoyl-sn-glycero-3-phosphate and 1,2-dipalmitoyl-sn-glycero-3-phosphate), which represent the apolaric site of phospholipids, show in each case two different species.     

Glutathione and N-acetylcysteinylglycine: protonation and Zn2+ complexation

The speciation study of the Zn(2+)/glutathione (GSH, H(3)G) and Zn(2+)/N-acetylcysteinylglycine (NAcCG, H(2)L) was performed in aqueous solution by means of potentiometry and ESI mass spectrometry. The ligand N-acetylcysteinylglycine was synthesized by protection/activation strategies. (1)H NMR data for the Zn(2+)/NAcCG system at different pH were also collected, to gain insight in the coordination modes for the ligand. The information collected for the NAcCG model ligand were used to propose the structure in solution for the Zn(2+)/GSH complexes. Dinuclear complexes of GSH with Zn(2+), which have never been proposed previously in the literature, were identified in solution and a model of their structure was proposed. Moreover, the Zn(2+) promoted deprotonation of the cysteinyl peptidic NH with formation of five membered (S,N(Cys)(-)) chelating rings was evidenced. The speciation study of the ternary Zn(2+)/GSH/NAcCG system was also performed, showing that the Zn(2+) does not bind preferentially to GSH in presence of NAcCG. The (1)H NMR protonation studies of both GSH and NAcCG were also performed, and a novel proton dissociation microconstant calculation procedure has been proposed and applied to GSH equilibria.     

Effects of N-alkyl and ammonium groups on the hydrolytic cleavage of DNA with a Cu(II)TACH (1,3,5-triaminocyclohexane) complex. Speciation, kinetic, and DNA-binding studies for reaction mechanism

cis,cis-1,3,5-Triaminocyclohexane (c-TACH), its N-alkyl-derivatives (alkyl = methyl, ethyl), and trans,cis-1,3,5-triaminocyclohexane (t-TACH) were prepared, and speciation and DNA cleaving property of Cu(II) complexes of these ligands were investigated. All of the complexes efficiently promote the hydrolytic cleavage of supercoiled plasmid DNA under physiological conditions without further additives. The DNA cleavage rate (V(obs)) trend at pH values between 8 and 9 is N-Me(3) = N-Et(1) < t-TACH < c-TACH < N-Et(2) < N-Et(3). At pH 7, the trend is c-TACH < N-Et(3) = N-Et(2) < N-Et(1) < N-Me(3) < t-TACH. The cleavage rate constants at 35 degrees C, for the c-TACH complex are 3 x 10(-1) h(-1) at pH 8.1 and 2 x 10(-1) h(-1) at pH 7.0 ([DNA] = 7 microM, [Cu(II)-complex] = 105 microM). The hydrolytically active species at pH > 8 is CuL(H(2)O)(OH)(+) in which L coordinates to Cu(II) as a tridentate ligand for all complexes except for t-TACH. The hydrolytically active species at pH 7 is CuLH(H(2)O)(3)(3+) or CuLH(H(2)O)(4)(3+) in which LH coordinates as bidentate ligand. DNA-binding constants of c-TACH and t-TACH complexes are presented and the effects of N-alkyl and ammonium groups are discussed in light of the proposed reaction mechanism.     

Escherichia coli thioredoxin inhibition by cadmium: two mutually exclusive binding sites involving Cys32 and Asp26.

Observations of thioredoxin inhibition by cadmium and of a positive role for thioredoxin in protection from Cd(2+) led us to investigate the thioredoxin-cadmium interaction properties. We used calorimetric and spectroscopic methods at different pH values to explore the relative contribution of putative binding residues (Cys32, Cys35, Trp28, Trp31 and Asp26) within or near the active site. At pH 8 or 7.5 two binding sites were identified by isothermal titration calorimetry with affinity constants of 10 x 10(6) m(-1) and 1 x 10(6) m(-1). For both sites, a proton was released upon Cd(2+) binding. One mole of Cd(2+) per mole of reduced thioredoxin was measured by mass spectrometry at these pH values, demonstrating that the two binding sites were partially occupied and mutually exclusive. Cd(2+) binding at either site totally inhibited the thiol-disulfide transferase activity of Trx. The absence of Cd(2+) interaction detected for oxidized or alkylated Trx and the inhibition of the enzymatic activity of thioredoxin by Cd(2+) supported the role of Cys32 at the first site. The fluorescence profile of Cd(2+)-bound thioredoxin differed, however, from that of oxidized thioredoxin, indicating that Cd(2+) was not coordinated with Cys32 and Cys35. From FTIR spectroscopy, we inferred that the second site might involve Asp26, a buried residue that deprotonates at a rather high and unusual pK(a) for a carboxylate (7.5/9.2). The pK(a) of the two residues Cys32 and Asp26 have been shown to be interdependent [Chivers, T. P. (1997) Biochemistry36, 14985-14991]. A mechanism is proposed in which Cd(2+) binding at the solvent-accessible thiolate group of Cys32 induces a decrease of the pK(a) of Asp26 and its deprotonation. Conversely, interaction between the carboxylate group of Asp26 and Cd(2+) at a second binding site induces Cys32 deprotonation and thioredoxin inhibition, so that Cd(2+) inhibits thioredoxin activity not only by binding at the Cys32 but also by interacting with Asp26.     

Metal-binding properties of phytochelatin-related peptides

Phytochelatins (PCs, (gamma Glu-Cys)(n)-Gly, n=2-11) are produced by higher plants, algae and some fungi in order to detoxify Cd(2+) by sequestration to form Cd-PCs complexes. In order to investigate what chemical structures of PCs are responsible for their metal-binding ability, various cysteine-rich peptides ((X-Cys)(7)-Gly, X=Glu, Asp, Lys, Gly, Ser and Gln) were chemically synthesized. Water-solubility, metal-binding property, and detoxification effect toward Cd(2+) were analyzed and compared with those of (gamma EC)(7)G. (SC)(7)G and (QC)(7)G were insoluble at pH below 10, and (GC)(7)G was not soluble at any pH between 1 and 12, indicating that charged side chains were at least required for the molecules to be solubilized in aqueous solution. By spectroscopic analyses using DTNB method and UV method, we found that (EC)(7)G and (DC)(7)G had almost equivalent abilities of Cd(2+)-binding as PC ((gamma EC)(7)G), indicating that the distance between each thiol group was not a major factor for the binding to Cd(2+). (beta DC)(7)G and (KC)(7)G interacted to Cd(2+) with fourth coordination as in the case of other soluble PC-related peptides. However, compared to (gamma EC)(7)G, (beta DC)(7)G displayed a slightly weaker binding to Cd(2+), and (KC)(7)G showed a drastic decrease in binding ability. The affinities of PC-related peptides toward Cd(2+) were evaluated as below; (gamma EC)(7)G=(EC)(7)G=(DC)(7)G>(beta DC)(7)G>(KC)(7)G=weak binding. The results of Cd(2+)-detoxification assays were consistent with the affinity between Cd(2+) and the peptides. We concluded that the structure consisting of thiol and carboxyl groups were essential for the formation of a tight Cd-peptides complex such as Cd-PCs.     

Binding of zinc and cadmium to human serum albumin

1. The interaction of zinc and cadmium ion with human serum albumin (HSA) is evaluated and compared by potentiometric titration method and computer simulation of complex equilibria. 2. Zinc binds to histidine and free amino groups, cadmium in addition to basic functional groups of the protein. 3. Whereas zinc binds stronger in 1:1 complexes, chelate binding favours cadmium ions. 4. Within biological pH-conditions, high amounts Zn(II) and even more of Cd(II) will be bound to HSA.