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Synthesis and properties of the monomethyl ester of meso-dimercaptosuccinic acid and its chelates of lead (II), cadmium(II), and mercury(II).

The monomethyl ester of meso-dimercaptosuccinic acid (MoMeDMSA) and its chelates with lead(II), cadmium(II), and mercury(II) have been synthesized. The mercury(II) chelate of MoMeDMSA is formed by the coordination of the two sulfur atoms in MoMeDMSA, whereas the lead(II) and cadmium(II) chelates are formed by the coordination of one sulfur and one oxygen atom. The solubilities of the chelates are pH dependent; the mercury(II) chelate dissolves when the uncoordinated carboxylic acid dissociates, but the lead(II) and cadmium(II) chelates are solubilized only after the uncoordinated mercapto group is dissociated. The cadmium(II) chelate is dimeric and the lead(II) and mercury(II) chelates are monomeric at the concentrations and conditions used in this study. The acid dissociation constants of the chelating agent and the uncoordinated groups in its metal chelates have been determined in 50% v/v methanol-water. These acid-base properties in addition to the polarity of the chelating agent contribute to the effectiveness in the in vivo mobilization of intracellular in vivo deposits of cadmium. The biliary excretion of cadmium in rats increased by a factor of 173 upon administration of the relatively toxic, nonpolar dimethyl ester of DMSA whereas the administration of the less toxic, more polar monomethyl ester increased the biliary excretion of cadmium by a factor of 63. On the other hand, meso-DMSA which is highly polar and less toxic is known to be without effect on biliary excretion of cadmium. The monomethyl DMSA, therefore, appears to have properties that are intermediate between those of DMSA and its dimethyl ester, as far as both chelating properties and biliary excretion of cadmium are concerned.

Animals↗

The dimethyl ester of meso-2,3-dimercaptosuccinic acid and its interactions with Cd2+ and rabbit liver metallothionein I.

The ion association complex formed with the cadmium chelate of the dimethyl ester of meso-2,3-dimercaptosuccinic acid and the tetramethylammonium cation, [(Me)4N]2[Cd(Di-MeDMSA)2], has been synthesized. The structures of the ion association complex and of the ligand (meso-DiMeDMSA) were determined by single-crystal X-ray diffraction. The two methyl ester groups and the two sulfhydryl groups in meso-DiMeDMSA were in a staggered conformation. The anion [Cd(DiMeDMSA)2]2- is essentially a distorted tetrahedron, with a mononuclear CdS4 kernel. The structure of the [Cd(DiMeDMSA)2]2- anion in solution was found to be similar to its structure in the solid state by multinuclear (1H, 13C, 113Cd) magnetic resonance spectroscopy. In the presence of rabbit liver metallothionein I (MT), meso-DiMeDMSA was found to coordinate Zn2+ and Cd2+ ions. 1H NMR spectroscopy proved to be an effective analytical technique for monitoring the competition between meso-DiMeDMSA and the protein for these metal ions. This was accomplished by probing the tertiary structure of MT with 1H NMR spectroscopy after incubation with meso-DiMeDMSA. In addition, the 1H NMR spectra of rabbit liver metallothionein I demonstrated that MT is more susceptible to oxidation at physiological pH after the metal ions have been removed. It appears that intra- as well as intermolecular disulfide cross-linkages are formed upon oxidation. When rabbit liver metallothionein I was incubated with meso-DiMeDMSA in a 20:1 (meso-DiMeDMSA:MT) mole ratio, 32% of the cadmium and 87% of the zinc bound to MT were removed.

Animals↗

Synthesis, structure, and stability of the zinc complex of the dimethyl ester of meso-2,3-dimercaptosuccinic acid.

The dimethyl ester of meso-2,3-dimercaptosuccinic acid (meso-DiMeDMSA) reacts with Zn2+ ions to form protonated and polynuclear complexes. The species [Zn2L3H]- and [Zn2L3]2- are formed at pH values between 3 and 6.5 and have overall formation constants of 10(37.53) and 10(32.52), respectively, whereas the mononuclear complex [ZnL2]2- is formed at physiological pH and has a formation constant of 10(18.06). The mononuclear complex was synthesized and isolated as an ion association complex with the tetramethylammonium ion, [(Me)4N]2[Zn(DiMeDMSA)2], and its structure in solution was determined by low-temperature 1H and 13C NMR spectroscopy. In this structure the Zn2+ ion is tetrahedrally coordinated to four thiolate groups from two meso-DiMeDMSA molecules.

Hydrogen-Ion Concentration↗

Determination and metabolism of dithiol chelating agents: the zinc chelate of the dimethyl ester of meso-2,3-dimercaptosuccinic acid increase biliary excretion of cadmium and platinum.

meso-2,3-Dimercaptosuccinic acid is an orally active chelating agent useful for the treatment of lead intoxication. Since it is believed to be extracellular in its distribution, analogs have been synthesized in a search for one that will enter the cell and successfully complete for firmly bound intracellular toxic heavy metals such as cadmium and platinum. The biological properties of the zinc chelate of DiMeDMSA, ([Zn(DiMeDMSA)2]2- with tetramethylammonium or sodium as the counterion, have been investigated in the rat. In short-term (hourly) experiments, [Zn(DiMeDMSA)2]2- increased the biliary excretion of cadmium and platinum. In experiments of longer duration (days), severe renal toxicity was noted even in the absence of any exogenously administered cadmium chloride or cis-dichlorodiammineplatinum(II). The zinc content of the kidneys of rats receiving Na2[Zn(DiMeDMSA)2]2- was found to be about 10-fold greater than rats receiving saline or meso-dimethyldimercaptosuccinic acid. Although it appears that the source of the zinc is the injected [Zn(DiMeDMSA)2]2-, at this time it is unknown as to whether the toxicity is due to the Zn chelate molecule, per se, or Zn derived from the molecule after its degradative biotransformation.

Animals↗

Transfer of arsenite from glutathione to dithiols: a model of interaction.

The interactions of arsenate and arsenite with meso-2,3-dimercaptosuccinic acid (DMSA) have been characterized using carbon-13 nuclear magnetic resonance. These studies show that DMSA reduces arsenate to arsenite and complexes arsenite. Monitoring the carbon-13 signals of complexed DMSA and liberated glutathione shows that DMSA readily extracts arsenite from a (glutathione)3-arsenite complex, proving the affinity of arsenite for dithiols is greater than that for monothiols. Competition between DMSA (vicinal thiols) and dithioerythritol (1,4-dimercapto-2,3-butanediol) for binding of arsenite indicates that the binding affinity is inversely related to the distance between the two thiol groups. On the basis of these findings, a model for the interaction of arsenic with mono- and dithiol-containing molecules is proposed.

Arsenates↗

Synthesis, structure, and properties of rac-2,3-dimercaptosuccinic acid, a potentially useful chelating agent for toxic metals.

rac-2,3-Dimercaptosuccinic acid (rac-DMSA) may be a more effective antidote for the treatment of heavy metal poisoning than meso-2,3-dimercaptosuccinic acid (meso-DMSA), which is used at present. A comparative study of these two chelating agents has been undertaken in order to investigate this possibility. The structures of rac-DMSA and the precursor in its synthesis, rac-2,3-bis(acetylthio)succinic acid, have been determined by single-crystal X-ray analysis and compared with the structures of the corresponding meso compounds. The concentration distributions of the various ionic and molecular species of rac-DMSA that are present in aqueous solutions in the pH range 1.8-12.8 have been calculated from the four acid dissociation constants that have been determined potentiometrically at an ionic strength of 0.10. The conformations of these species in aqueous solutions have been determined by 1H NMR spectroscopy as a function of pH. The rac-DMSA exists in the crystal lattice as a double-stranded structure in which each molecule is in a gauche configuration. The meso-DMSA, however, exists in a single strand in which each molecule has adopted an anti configuration and the individual molecules are joined together at each end by hydrogen bonds. These differences in the conformations of rac- and meso-DMSA may account for the significant difference in the solubilities of the two compounds in aqueous solutions. In acidic solutions at elevated temperatures of about 95 degrees C, rac-DMSA is slowly but irreversibly converted to meso-DMSA. This conversion does not occur at ambient temperatures.

Crystallography, X-Ray↗

Design of in vivo cadmium-mobilizing agents: synthesis and properties of monobenzyl meso-2,3-dimercaptosuccinate.

A novel method for the synthesis of monoesters of meso-2,3-dimercaptosuccinic acid (DMSA) is presented which utilizes the reaction of the vicinal sulfhydryl-protected anhydride with the corresponding alcohol in the presence of a base. The product is then treated successively with mercuric chloride to remove the protecting group and form the mercuric complex, and hydrogen sulfide to regenerate thiol groups by removal of mercury as HgS. This strategy was exploited to synthesize monobenzyl meso-2,3-dimercaptosuccinate (MBzDMS), C6H5CH2O(O)CCH(SH)CH(SH)C(O)OH, and demonstrates a feasible synthesis of monoesters difficult to obtain by direct esterification, via the use of the reactive anhydride. The resultant compound was found to be an effective cadminum-mobilizing agent when used with cadmium-exposed rats or mice and when administered by any one of several routes (ip, iv, po). This monobenzyl ester (MBzDMS) of DMSA was found to be somewhat less effective than the corresponding monoisoamyl ester (Mi-ADMS) in mobilizing cadmium from such cadmium deposits. The ability of MBzDMS to mobilize cadmium into the urine is significantly decreased by the coadministration of p-aminobenzoic acid, in support of the hypothesis that MBzDMS enters renal cells via an anion transport system. An analysis of the structural features of vicinal dithiols examined as antagonists for chronic cadmium intoxication allows a hypothesis to be formulated indicating essential features required for the design of effective new cadmium antagonists of this type.

Administration, Oral↗

Efficacy of combined chelation in lead intoxication.

The combination of a bone lead mobilizer, viz., CaNa2EDTA, and a soft tissue depleter such as meso-2,3-dimercaptosuccinic acid (DMSA) or the sodium salt of 2,3-dimercapto-1-propane-sulfonic acid (DMPS) was investigated for its efficacy to enhance the excretion of lead, to reduce its body burden, and to reverse certain lead-induced biochemical alterations by either of them alone in rats. The administration of CaNa2EDTA or DMSA was more effective than that of DMPS, and the combination of CaNa2EDTA and DMSA was more efficient than that of CaNa2EDTA and DMPS or the individual chelators in enhancing urinary/fecal excretion of lead, in reducing hepatic, renal, and femur concentration of lead, and in restoring lead-induced inhibition of blood delta-aminolevulinic acid dehydratase and increase in blood zinc protoporphyrin and urinary delta-aminolevulinic acid levels. The brain lead was depleted by DMSA only, but the combined chelation failed to improve upon the ability of DMSA. However, chelation therapy increased diuresis and tissue depletion of zinc.

Animals↗

Complexation of arsenic species in rabbit erythrocytes.

The binding of arsenite, As(III), and arsenate, As(V), by molecules in the intracellular compartment of rabbit erythrocytes has been studied by 1H- and 31P-NMR spectroscopy, uptake of 73As, and ultrafiltration experiments. For intact erythrocytes to which 0.1-0.4 mM arsenite was added, direct evidence was obtained for entry of 76% within 1/2 h and subsequent binding of As(III) by intracellular glutathione and induced changes in the hemoglobin structure (NMR), likely due to binding of As(III). These results were compared with the effect of addition of As(V) on intact erythrocytes and revealed that a smaller amount of As(V) (approximately 25%) enters the cells; the main fraction of As(V) enters the phosphate pathway, depletes ATP, and increases Pi. In contrast, As(III) did not affect the ATP level. Both 1H- and 31P-NMR data indicated striking differences between As(III) and As(V) behavior when incubated with rabbit erythrocytes. These differences were confirmed by 73As uptake and binding experiments. meso-2,3-Dimercaptosuccinic acid (DMSA), a dithiol ligand, released glutathione from its arsenite complexes in erythrocytes.

Adenosine Triphosphate↗

A comparative study of meso- and rac-2,3-dimercaptosuccinic acids and their zinc complexes in aqueous solution.

The conformations of meso-2,3-dimercaptosuccinic acid (meso-DMSA) in aqueous solution have been postulated from proton NMR titrations. The complexes formed with zinc(II) and the ligands rac-DMSA and meso-DMSA have been postulated from potentiometric titrations of solutions containing varying ratios of zinc:ligand. The complex formation behavior of rac-DMSA with zinc(II) is dramatically different from that of meso-DMSA. These differences are reflected in the complex formation constants of the zinc(II) species and their distributions in solution as a function of zinc:ligand ratios and as a function of pH. On the basis of these results we have predicted that rac-DMSA is more effective than meso-DMSA in mobilizing lead in vivo and that the 1:2 zinc complex of rac-DMSA can be used effectively for the treatment of heavy metal poisoning because endogenous zinc will not be depleted by this chelation treatment.

Potentiometry↗

Conformations of zinc chelates of meso- and rac-2,3-dimercaptosuccinic acid in aqueous solution.

rac-2,3-Dimercaptosuccinic acid (DMSA) was found to be superior to meso-2,3-dimercaptosuccinc acid in mobilizing in vivo heavy metals such as Cd, Hg, and Pb. The disadvantage of using rac-DMSA alone as a clinical antidote for heavy metal poisoning is that it causes a greater loss of endogenous zinc than its meso isomer. The difference between the two diastereoisomers of DMSA in the excretion of endogenous zinc has been rationalized on the basis of the differences in the conformations of their zinc complexes. The zinc complexes of rac-DMSA in aqueous solution are more stable than the corresponding complexes of its meso isomer because the rac-DMSA ligands always adopt staggered anti conformations, in which the electrostatic repulsion between two bulky carboxylate groups is minimized; in contrast, unlike in the crystal lattice, meso-DMSA ligands always adopt staggered gauche conformations in their zinc complexes. The conformations of various monomeric and dimeric zinc complexes with rac- and meso-2,3-dimercaptosuccinic acid in aqueous solution were determined by IR spectroscopy and proton NMR spectroscopy as a function of zinc:ligand ratio, by proton NMR spectroscopy as a function of pD, and by variable-temperature 13C NMR spectroscopy. rac-DMSA in ZnL2(6-) coordinates with zinc ion via two thiolate groups and one carboxylate group, whereas in Zn2L2(4-) each ligand complexes two zinc ions by using one carboxylate group and one thiolate group in the beta-position to bind to the same zinc ion.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetic Resonance Spectroscopy↗

Stereoisomeric selectivity of 2,3-dimercaptosuccinic acids in chelation therapy for lead poisoning.

The formation constants of lead chelates of the stereoisomers of 2,3-dimercaptosuccinic acid (DMSA) were determined from potentiometric titrations in the presence of the competing ligand, EDTA. The lead chelates formed at pH 7.4 with the stereoisomers of DMSA are the monomeric complexes PbL and HPbL. Formation of PbL and HPbL at pH 7.4 is independent of total concentrations of lead and DMSA present, and so is the concentration ratio of PbL:HPbL. Lead is completely chelated at pH 7.4 when the total concentration of ligand is equal to or greater than the total concentration of lead present. Lead tends to bind to a greater extent with rac- than with meso-DMSA, and the relative extent increases with an increase in the concentration ratio of ligand to lead and finally reaches a constant value of 45. The binding sites in the chelates, PbL, of the stereoisomers of DMSA are the two thiolate groups and one carboxylate group. rac-DMSA also forms a dimeric complex Pb2L2 in which both carboxylate groups of the ligands participate in binding with lead ions. The formation constants of the lead chelates of rac-DMSA were invariably found to be larger than those of the corresponding of meso-DMSA chelates, because in all the lead chelates of the stereoisomers of DMSA formed in solution, rac-DMSA existed in staggered anti conformations, whereas meso-DMSA preferred a staggered gauche conformation with respect to carboxylate groups in the ligands. The potential of using ZnL2 of rac-DMSA as a therapeutical lead chelator was assessed by considering its lead-mobilizing ability and its ability to deplete endogenous zinc; on this basis it is predicted that ZnL2 of rac-DMSA is a better chelator than meso-DMSA for the treatment of lead poisoning.

Chelating Agents↗

Mercury binding to the chelation therapy agents DMSA and DMPS and the rational design of custom chelators for mercury.

Clinical chelation therapy of mercury poisoning generally uses one or both of two drugs--meso-dimercaptosuccinic acid (DMSA) and dimercaptopropanesulfonic acid (DMPS), commercially sold as Chemet and Dimaval, respectively. We have used a combination of mercury L(III)-edge X-ray absorption spectroscopy and density functional theory calculations to investigate the chemistry of interaction of mercuric ions with each of these chelation therapy drugs. We show that neither DMSA nor DMPS forms a true chelate complex with mercuric ions and that these drugs should be considered suboptimal for their clinical task of binding mercuric ions. We discuss the design criteria for a mercuric specific chelator molecule or "custom chelator", which might form the basis for an improved clinical treatment.

Chelating Agents↗

Comparison of rac- and meso-2,3-dimercaptosuccinic acids for chelation of mercury and cadmium using chemical speciation models.

The formation constants of various mercury and cadmium chelates of the stereoisomers of 2,3-dimercaptosuccinic acid (DMSA) have been determined from potentiometric titrations in the presence of the competing ligand EDTA. The mercury chelates formed at pH 7.4 are the monomeric HgL of the DMSA diastereoisomers and HHgL2 of rac-DMSA. Mercury is completely complexes at pH greater than 3.0 in solutions containing more than 1 equiv of either rac- or meso-DMSA. At high concentrations (10 microM and above) mercury tends to bind to a greater extent to rac- than to meso-DMSA. At pH 7.4, the predominant cadmium meso-DMSA chelate species in solution is CdL, and HCdL is present at a much smaller concentration. With rac-DMSA, however, the predominant cadmium chelate species is HCdL at a low concentration of the ligand, and at a high concentration of the ligand the species CdL2 predominates. Cadmium is completely chelated at pH 7.4 in solutions containing more than 1 equiv of either rac- or meso-DMSA. At pH around 5.5, which corresponds to the pH of the kidney, however, a significant amount of free cadmium is present in solutions containing 1 equiv or less of either DMSA stereoisomer. From the results of an analysis of speciation models, probable kidney damage, that may result from free cadmium ion release in the kidney during chelation therapy, is inferred when meso-DMSA is used for mobilizing cadmium. In contrast, the release of free cadmium ion is negligible in the pH range in the kidney when rac-DMSA is used. On the basis of the speciation models, rac-DMSA is found to be far superior to meso-DMSA in the treatment of acute cadmium poisoning.

Cadmium↗

Meso-2,3-dimercaptosuccinic acid mono-N-alkylamides: syntheses and biological activity as novel in vivo cadmium mobilizing agents.

Three meso-2,3-dimercaptosuccinic acid mono-N-alkylamides (meso-RNHCOCH(SH)CH(SH)-COOH, where R = CHMe2, Mi-PDMA; CH2CHMe2, Mi-BDMA; and CH2CH2CHMe2, Mi-ADMA), were prepared via a synthetic route using the sulfhydryl-protected anhydride. 2,2-Dimethyl-1,3-dithiolane-4,5-cis-dicarboxylic acid anhydride was opened up with 1 mol of corresponding amine to give the SH-protected monoamide. Subsequent deblocking of the vicinal dithiol functionality was accomplished by conversion of the dithiolane into the mercury complex followed by reaction with H2S to give the target molecule. The potential utility of these compounds in chronic cadmium intoxication was examined by evaluation of their cadmium mobilizing efficacy in vivo in cadmium-loaded female albino rats using sodium N-benzyl-D-glucamine-N-carbodithioate (BGDTC) as the standard drug. Compared to BGDTC, the new compounds were, except at the highest dosage studied, equally or more effective in decreasing retention of hepatic cadmium, while mostly less effective in decreasing renal cadmium. The greatest reductions were obtained with Mi-BDMS at 4 x 1.5 mmol/kg, where liver and kidney cadmium levels were reduced to 12% and 59% of control levels, while at the same dosage BGDTC induced a reduction to 50% and 13% of control levels. The order of the efficacy of the monoamides as hepatic cadmium mobilizing agents was found to be Mi-PDMA > Mi-BDMA > Mi-ADMA. However, the isopropyl analog, though very effective at reducing hepatic cadmium at a low dosage, was found to be more toxic than the isobutyl and isoamyl monoamides. While the new compounds were shown to be effective cadmium mobilizing agents, the specific compounds examined did not possess optimized structures in terms of the balance between effectiveness and toxicity.

Amides↗

Comparative inhibition of yeast glutathione reductase by arsenicals and arsenothiols.

Tri(gamma-glutamylcysteinylglycinyl)trithioarsenite (AsIII(GS)3) is formed in cells and is a more potent mixed-type inhibitor of the reduction of glutathione disulfide (GSSG) by yeast glutathione (GSH) reductase than either arsenite (AsIII) or GSH. The present work examines the effects of valence and complexation of arsenicals with GSH or L-cysteine (Cys) upon potency as competitive inhibitors of the reduction of GSH disulfide (GSSG) by yeast GSH reductase. Trivalent arsenicals were more potent inhibitors than their pentavalent analogs, and methylated trivalent arsenicals were more potent inhibitors than was inorganic trivalent As. Complexation of either inorganic trivalent As or methylarsonous diiodide (CH3As(III)I2) with Cys or GSH produced inhibitors of GSH reductase that were severalfold more potent than the parent arsenicals. In contrast, dimethylarsinous iodide ((CH3)2As(III)I) was a more potent inhibitor than its complexes with either GSH or Cys. Complexes of CH3AsIII with GSH (CH3-AsIII(GS)2) or with Cys (CH3AsIII(Cys)2) were the most potent inhibitors, with Ki's of 0.009 and 0.018 mM, respectively. Inhibition of GSH reductase by arsenicals or arsenothiols was prevented by addition of meso-2,3-dimercaptosuccinic acid (DMSA) to a mixture of enzyme, GSSG, and inhibitor before addition of NADPH. DMSA added to the reaction mixture after NADPH reversed inhibition by (CH3)2As(III)I but had little effect on inhibition by CH3As(III)I2, Ch3AsIII(GS)2, CH3AsIII(Cys)2, or AsIII(GS)3. Partial redox inactivation of the enzyme with NADPH increased the inhibitory potency of CH3As(III)I2 and (CH3)2As(III)I and changed the mode of inhibition for CH3As(III)I2 from competitive to noncompetitive. The greater potency of methylated trivalent arsenicals and arsenothiols than of inorganic trivalent As suggests that biomethylation of As could yield species that inhibit reduction of GSSG and alter the redox status of cells.

Arsenicals↗

The presence of lead decreases the availability of meso-2, 3-dimercaptosuccinic acid for analysis in the monobromobimane assay.

meso-2,3-Dimercaptosuccinic acid is a suitable chelating agent for routine pharmacotherapy of lead poisoning in children. Administration of meso-2,3-dimercaptosuccinic acid presumably permits complexation of lead in vivo, allowing excretion through urine or feces. Quantification of the lead is achieved independently from the analysis of meso-2,3-dimercaptosuccinic acid and metabolites from the monobromobimane assay. To date, no direct chemical characterization of the Pb species excreted in urine has been successful. Pharmacokinetic correlation of lead excretion with excretion of meso-2,3-dimercaptosuccinic acid and metabolites has been utilized as an indirect method to draw conclusions regarding the identity of the active chelating agent. In this study, we hypothesized that the Pb-coordinated thiols are not reactive with respect to monobromobimane, and thus, the active chelator contained in the lead complex escapes detection. We performed variations of the assay and found that (1) the fluorescence detector response for the meso-2,3-dimercaptosuccinic acid-monobromobimane adduct was clearly attenuated as a function of added Pb, (2) when meso-2, 3-dimercaptosuccinic acid and monobromobimane were mixed prior to the addition of lead, the lead had no effect on detector response, (3) the addition of dithiothreitol does not affect the ability of Pb to react with meso-2,3-dimercaptosuccinic acid and verifies that oxidation of meso-DMSA had not occurred, and (4) the addition of ethylenediaminetetraacetic acid to the assay reverses the result found in point 1, presumably through trans chelation of the Pb-DMSA complex. Indirect quantification of the Pb-DMSA complexes found in urine might be accomplished through modification of the standard monobromobimane assay for analysis of meso-2,3-dimercaptosuccinic acid.

Bridged Bicyclo Compounds↗