Metal complexes as enzyme inhibitors

Kinetics of inhibition of rabbit intestine alkaline phosphatase by heteroligand peroxo complexes of vanadium(V) and tungsten(VI)

The present work was undertaken to examine and compare some biologically important properties of peroxo compounds of V(V) and W(VI) containing biogenic species as ancillary ligand. New anionic peroxovanadate(V) complex of the type Na[VO(O(2))(2)(triglycine)].3H(2)O (pV1) and a molecular peroxotungstate(VI) [WO(O(2))(2)(triglycine)].3H(2)O (pW1) were synthesized and characterized for the purpose and their stability in solution was ascertained. Studies on kinetics of inhibition of alkaline phosphatase activity by the newly synthesized compounds and series of dipeptide and amino acid containing peroxo complexes of vanadium and tungsten synthesized previously by us viz., Na[VO(O(2))(2)(gly-gly)(H(2)O)].H(2)O (gly-gly = glycyl-glycine), Na[VO(O(2))(2)(asn)].H(2)O (asn = asparagine), Na[VO(O(2))(2)(gln)].H(2)O (gln = glutamine), and [WO(O(2))(2)(gly-gly)(H(2)O)].3H(2)O, revealed that each of these species is a potent mixed-type inhibitor of the enzyme. Significant difference was noted between the peroxovanadium (pV) and peroxotungsten (pW) compounds in terms of their oxidant activity with reduced glutathione.

Theoretical investigations on the hydrolysis pathway of tin verdoheme complexes: elucidation of tin's ring opening inhibition role

In order to obtain a better molecular understanding of inhibitory role of tin metal in the verdoheme ring opening process, hydrolysis of three possibly six, five, and four coordinate verdoheme complexes of tin(IV) and (II) have been studied using DFT method. The results of calculations indicate that, in excellent accord with experimental reports, hydrolysis of different possibly coordinated tin(IV) and (II) verdohemes does not lead to the opening of the macrocycle. Contrary to iron and zinc verdohemes, in five and four coordinate verdoheme complexes of tin(IV) and (II), formation of open ring helical complexes of tin are unfavorable both thermodynamically and kinetically. In these pathways, coordination of hydroxide nucleophile to tin metal due to the highly charged, exclusive oxophilicity nature of the Sn center, and high affinity of Sn to increase coordination state are proposed responsible as inhibiting roles of tin via the ring opening. While, in saturated six coordinate tin(IV) and (II) verdoheme complexes the ring opening of tin verdohemes is possible thermodynamically, but it is not predicted to occur from a kinetics point of view. In the six coordinate pathway, tin plays no coordination role and direct addition of hydroxide nucleophile to the positive oxo-carbon centers and formation of closed ring hydroxy compounds is proposed for preventing the verdoheme ring opening. These key points and findings have been corroborated by the results obtained from atomic charge analysis, geometrical parameters, and molecular orbital calculations. In addition, the results of inhibiting ring opening reaction of tin verdoheme complexes could support the great interest of tin porphyrin analogues as pharmacologic means of chemoprevention of neonatal jaundice by the competitive inhibitory action of tin porphyrins on heme oxygenase.

Organometallic compounds in oncology: implications of novel organotins as antitumor agents

Since the introduction of cisplatin in cancer therapy, metal complexes and organometallic compounds have been gaining growing importance in oncology. The impressive clinical effectiveness of cisplatin is limited by significant side effects and the emergence of drug resistance. Thus, novel classic and unconventional Pt(II) and Pt(IV) complexes have been introduced in therapy or are presently in advanced clinical trials. Moreover, innovative non-platinum metal-based antitumor agents, whose activity does not rely on direct DNA damage and may involve proteins and enzymes, have been developed. Gold and tin derivatives are enjoying an increasing interest and appear very promising as potential drug candidates.

Targeting proteins with metal complexes

Unique properties of metal complexes, such as structural diversity, adjustable ligand exchange kinetics, fine-tuned redox activities, and distinct spectroscopic signatures, make them exciting scaffolds not only for binding to nucleic acids but increasingly also to proteins as non-traditional targets. This feature article discusses recent trends in this field. These include the use of chemically inert metal complexes as structural scaffolds for the design of enzyme inhibitors, new strategies for inducing selective coordination chemistry at the protein binding site, recent advances in the development of catalytic enzyme inhibitors, and the design of metal complexes that can inject electrons or holes into redox enzymes. A common theme in many of the discussed examples is that binding selectivity is at least in part achieved through weak interactions between the ligand sphere and the protein binding site. These examples hint to an exciting future in which "organic-like" molecular recognition principles are combined with properties that are unique to metals and thus promise to yield compounds with novel and unprecedented properties.

Spectral characterization, cyclic voltammetry, morphology, biological activities and DNA cleaving studies of amino acid Schiff base metal(II) complexes

Metal complexes are synthesized with Schiff bases derived from o-phthalaldehyde (opa) and amino acids viz., glycine (gly) l-alanine (ala), l-phenylalanine (pal). Metal ions coordinate in a tetradentate or hexadentate manner with these N(2)O(2) donor ligands, which are characterized by elemental analysis, molar conductance, magnetic moments, IR, electronic, (1)H NMR and EPR spectral studies. The elemental analysis suggests the stoichiometry to be 1:1 (metal:ligand). Based on EPR studies, spin-Hamiltonian and bonding parameters have been calculated. The g-values calculated for copper complexes at 300K and in frozen DMSO (77K) indicate the presence of the unpaired electron in the dx2-y2 orbital. The evaluated metal-ligand bonding parameters showed strong in-plane sigma- and pi-bonding. X-ray diffraction (XRD) and scanning electron micrography (SEM) analysis provide the crystalline nature and the morphology of the metal complexes. The cyclic voltammograms of the Cu(II)/Mn(II)/VO(II) complexes investigated in DMSO solution exhibit metal centered electroactivity in the potential range -1.5 to +1.5V. The electrochemical data obtained for Cu(II) complexes explains the change of structural arrangement of the ligand around Cu(II) ions. The biological activity of the complexes has been tested on eight bacteria and three fungi. Cu(II) and Ni(II) complexes show an increased activity in comparison to the controls. The metal complexes of opapal Schiff base were evaluated for their DNA cleaving activities with calf-thymus DNA (CT DNA) under aerobic conditions. Cu(II) and VO(II) complexes show more pronounced activity in presence of the oxidant.

Complexation of Al(III) with reduced glutathione in acidic aqueous solutions

The complexation of reduced glutathione (GSH) in its free and Al(III)-bound species in acidic aqueous solutions was characterized by means of multi-analytical techniques: pH-potentiometry, multinuclear ((1)H, (13)C and (27)Al) and two-dimensional nuclear Overhauser enhancement NMR spectroscopy ((1)H, (1)H-NOESY), electrospray mass spectroscopy (ESI-MS), and ab initio electronic structure calculations. The following results were found. In the 25 degrees C 0.1M KCl and 37 degrees C 0.15M NaCl ionic medium systems, Al(3+) coordinates with the important biomolecule GSH through carboxylate groups to form various mononuclear 1:1 (AlHL, AlH(2)L and AlH(-1)L), 1:2 (AlL(2)) complexes, and dinuclear (Al(2)H(5)L(2)) species, where H(4)L(+) denotes totally protonated GSH. Besides the monodentate complexes through carboxylate groups, the amino groups and the peptide bond imino and carbonyl groups may also be involved in binding with Al(3+) in the bidentate and tridentate complexes. The present data reinforce that the glycine carboxylate group of GSH has a higher microscopic complex formation constant than gamma-glutamyl carboxylate. Compared with simple amino acids, the tripeptide GSH displays a greater affinity for the Al(3+) ion and thus may interfere with aluminum's biological role more significantly.

Re and (99m)Tc organometallic complexes containing pendant l-arginine derivatives as potential probes of inducible nitric oxide synthase

Aiming to design radioactive compounds based on the core "(99m)Tc(CO)(3)" for probing inducible nitric oxide synthase (iNOS) levels in vivo, we have synthesized conjugates containing a pyrazolyl-diamine chelating unit and pendant l-arginine analogues (substrates and inhibitors of NOS). Reaction of the conjugates with fac-[M(CO)(3)](+) (M = Re, (99m)Tc) gave bioorganometallic complexes of the type fac-[M(CO)(3)(k(3)-L)] in good yield. After in vitro testing using the oxyhemoglobin NO capture assay, we concluded that the affinity of the inhibitor-containing conjugates to iNOS seems to be less affected upon metallation with rhenium than the substrate-containing conjugates. The complexes bearing guanidino substituted analogues of l-arginine still present considerable inhibitory action (N(omega)-monomethyl-l-arginine, K(i) = 36 microM; N(omega)-nitro-l-arginine, K(i) = 84 microM), being the first examples of organometallic complexes able to inhibit the iNOS. These results seem to indicate that (99m)Tc(CO)(3)-labeled L-argininine analogues, namely NOS inhibitors, may hold potential for monitoring increased levels of iNOS in vivo.

Synthesis and antimalarial activity of novel chiral and achiral benzenesulfonamides bearing 1, 3, 4-oxadiazole moieties

A series of new benzenesulfonamides, most of which are chiral, incorporating 1, 3, 4-oxadiazole and amino acid moieties have been synthesized. Some of these compounds were screened for antimalarial activity and also evaluated for their ability to inhibit hem polymerization. The electrophoretic analysis indicated that one compound was effective in inhibiting the degradation of hemoglobin. The synthesized compounds were tested in mice infected with Plasmodium berghei. These derivatives have the potential for the development of novel antimalarial lead compounds.

Copper-adenine complex, a compound, with multi-biochemical targets and potential anti-cancer effect

A series of adenine-copper complexes (1-6) with various ligands (Cl(-), SCN(-), BF(4)(-) and acac [acetylacetonate ion]) have been synthesized and characterized by elemental analysis, infrared spectroscopy and thermal analysis. Among the six complexes only complex (1), Cu(2)(adenine)(4)Cl(4).2EtOH (abbreviated as Cu-Ad), demonstrated some toxic effect on different cell lines. In vitro investigations of the biological effect of Cu-Ad complex have shown that it: (1) binds genomic DNA; (2) decreases significantly, the viability of cells in culture in a concentration (15-125 microM)-dependant manner; an estimated IC(50) of: 45 microM with HepG2; 73 microM with C2C12; 103 microM with NIH3T3; and 108 microM with MCF7. Cu-Ad had no effect on A549 cells; (3) inhibits Taq polymerase-catalyzed reaction; (4) inhibits the binding of the transcription factor GATA-5 to labeled DNA probes; (5) inhibits mitochondrial NADH-UQ-reductase with an estimated IC(50) of 2.8 nmol, but had no effect on succinate dehydrogenase activity; (6) increases reactive oxygen species (60%) at 45 microM Cu-Ad; and (7) decreases ATP (80%) at 50 microM Cu-Ad. The new compound Cu(2)(adenine)(4)Cl(4).2EtOH (Cu-Ad), belongs to a class of copper-adenylate complexes that target many biochemical sites and with potential anti-cancer activity.

New approaches for medicinal applications of bioinorganic chemistry

Inorganic and bioinorganic chemistry have made important contributions to medical science and human health in the past half century. Today, metal-containing imaging agents and therapeutics constitute a multi-billion dollar industry. Recent discoveries in bioinorganic chemistry of potential biomedical importance include the use of metal ions as synthetic scaffolds for the preparation of small molecule therapeutics, which opens up a new route to molecular structure and diversity, as well as the examination of metal-organic frameworks as biological imaging and drug delivery agents. These areas represent some of the most recent and still relatively unexplored themes in inorganic and bioinorganic chemistry that might be exciting and fruitful topics of study for the community interested in 'metals in medicine'.

Interaction of some Pd(II) complexes with Na+ / K+-ATPase: inhibition, kinetics, prevention and recovery

The aim of this work was to investigate the influence of [PdCl4]2-, [PdCl(dien)]+ and [PdCl(Me4dien)]+ complexes on Na+ / K+-ATPase activity. The dose-dependent inhibition curves were obtained in all cases. IC50 values determined by Hill analysis were 2.25 x 10(-5) M, 1.21 x 10(-4) M and 2.36 x 10(-4) M, respectively. Na+ / K+-ATPase exhibited typical Michelis-Menten kinetics in the presence of Pd(II) complexes. Kinetic parameters (Vmax, Km) derived using Eadie-Hofstee transformation indicated a noncompetitive type of Na+ / K+-ATPase inhibition. The inhibitor constants (Ki) were determined from Dixon plots. The order of complex affinity for binding with Na+ / K+-ATPase, deducted from Ki values, was [PdCl4]2- > [PdCl(dien)]+ > [PdCl(Me4dien)]+. The results indicated that the potency of Pd(II) complexes to inhibit Na+/ K +-ATPase activity depended strongly on ligands of the related compound. Furthermore, the ability of SH-donor ligands, L-cysteine and glutathione, to prevent and recover the Pd(II) complexes-induced inhibition of Na+ / K+-ATPase was examined. The addition of 1 mM L-cysteine or glutathione to the reaction mixture before exposure to Pd(II) complexes prevented the inhibition by increasing the IC50 values by one order of magnitude. Moreover, the inhibited enzymatic activity was recovered by addition of SH-donor ligands in a concentration-dependent manner.

Organometallic compounds with biological activity: a very selective and highly potent cellular inhibitor for glycogen synthase kinase 3

A chiral second-generation organoruthenium half-sandwich compound is disclosed that shows a remarkable selectivity and cellular potency for the inhibition of glycogen synthase kinase 3 (GSK-3). The selectivity was evaluated against a panel of 57 protein kinases, in which no other kinase was inhibited to the same extent, with a selectivity window of at least tenfold to more than 1000-fold at 100 microM ATP. Furthermore, a comparison with organic GSK-3 inhibitors demonstrated the superior cellular activity of this ruthenium compound: wnt signaling was fully induced at concentrations down to 30 nM. For comparison, the well-established organic GSK-3 inhibitors 6-bromoindirubin-3'-oxime (BIO) and kenpaullone activate the wnt pathway at concentrations that are higher by around 30-fold and 100-fold, respectively. The treatment of zebrafish embryos with the organometallic inhibitor resulted in a phenotype that is typical for the inhibition of GSK-3. No phenotypic change was observed with the mirror-imaged ruthenium complex. The latter does not, in fact, show any of the pharmacological properties for the inhibition of GSK-3. Overall, these results demonstrate the potential usefulness of organometallic compounds as molecular probes in cultured cells and whole organisms.

Noble metals strip peptides from class II MHC proteins

Class II major histocompatibility complex (MHC) proteins are essential for normal immune system function but also drive many autoimmune responses. They bind peptide antigens in endosomes and present them on the cell surface for recognition by CD4(+) T cells. A small molecule could potentially block an autoimmune response by disrupting MHC-peptide interactions, but this has proven difficult because peptides bind tightly and dissociate slowly from MHC proteins. Using a high-throughput screening assay we discovered a class of noble metal complexes that strip peptides from human class II MHC proteins by an allosteric mechanism. Biochemical experiments indicate the metal-bound MHC protein adopts a 'peptide-empty' conformation that resembles the transition state of peptide loading. Furthermore, these metal inhibitors block the ability of antigen-presenting cells to activate T cells. This previously unknown allosteric mechanism may help resolve how gold(I) drugs affect the progress of rheumatoid arthritis and may provide a basis for developing a new class of anti-autoimmune drugs.

New oxo-bridged peroxotungsten complexes containing biogenic co-ligand as potent inhibitors of alkaline phosphatase activity

Novel dinuclear peroxo complexes of tungsten with coordinated cystine of the type A(2)[W(2)O(3)(O(2))(4)(cystine)].4H(2)O, A = Na (1) or K (2) have been synthesized from the reaction of A(2)WO(4,)cysteine and 30% H(2)O(2)at pH 2.5. The synthesized compounds were characterized by elemental analysis, spectral and physico-chemical methods. The two W(VI) centres with side-on bound peroxo groups of the dinuclear complex species are bridged by an oxo group and a cystine ligand, formed from the oxidation of cysteine. Cystine occurring as zwitterion binds the metal centers of the complex ion through O(carboxylate) atoms leading to hepta co-ordination around each W(VI). The compounds exhibit high stability toward decomposition in solution of acidic as well as physiological pH and serve as weak substrates to catalase, undergoing degradation in presence of the enzyme at a rate much slower relative to H(2)O(2). The compounds efficiently oxidized GSH to GSSG, a reaction in which only two of the peroxide groups of the complex species were found to participate. The compounds induce strong inhibitory effect on alkaline phosphatase activity with a potency higher than that of the free cystine, tungstate, or peroxotungstate.

Metal complexes in medicinal chemistry: new vistas and challenges in drug design

An overview is presented of selected metal-based pharmaceuticals, either diagnostic or therapeutic, with emphasis on specific attributes and in vivo interactions of these compounds relevant to their use in medicinal applications. Both the advantages and the challenges of this approach are outlined, with possibilities for future developments accentuated.

Silicon and plant disease resistance against pathogenic fungi

Silicon (Si) is a bioactive element associated with beneficial effects on mechanical and physiological properties of plants. Silicon alleviates abiotic and biotic stresses, and increases the resistance of plants to pathogenic fungi. Several studies have suggested that Si activates plant defense mechanisms, yet the exact nature of the interaction between the element and biochemical pathways leading to resistance remains unclear. Silicon possesses unique biochemical properties that may explain its bioactivity as a regulator of plant defense mechanisms. It can act as a modulator influencing the timing and extent of plant defense responses in a manner reminiscent of the role of secondary messengers in induced systemic resistance; it can also bind to hydroxyl groups of proteins strategically involved in signal transduction; or it can interfere with cationic co-factors of enzymes influencing pathogenesis-related events. Silicon may therefore interact with several key components of plant stress signaling systems leading to induced resistance.

Anticancer metal compounds in NCI's tumor-screening database: putative mode of action

Clustering analysis of tumor cell cytotoxicity profiles for the National Cancer Institute (NCI)'s open compound repository has been used to catalog over 1100 metal or metalloid containing compounds with potential anticancer activity. The molecular features and corresponding reactivity of these compounds have been analyzed in terms of properties of their metals, their associated organic components (ligands) and their capacity to inhibit tumor cell growth. Cytotoxic responses are influenced by both the identity of the metal and the properties of its coordination ligand, with clear associations between structural similarities and cytotoxicity. Assignments of mechanisms of action (MOAs) for these compounds could be segregated into four broad response classes according to preference for binding to biological sulfhydryl groups, chelation, generation of reactive oxygen species (ROS), and production of lipophilic ions. Correlations between specific cytotoxic responses and differential gene expression profiles within the NCI's tumor cell panel serve as a validation for candidate biological targets and putative MOA classes. In addition, specific sensitivity toward subsets of metal containing agents has been found for certain tumor cell panels. Taken together, our results expand the knowledge base available for evaluating, designing and developing new metal-based anticancer drugs that may provide the basis for target-specific therapeutics.

Metal complexes with Schiff-base ligands - pyridoxal and semicarbazide-based derivatives

The most important results of extensive studies (syntheses, spectral, magnetic, voltammetric and structural characteristics and biological activity) of metal complexes with pyridoxal semi-, thiosemi- and isothiosemicarbazones are reviewed.

Inhibition of the Escherichia coli RecA protein: zinc(II), copper(II) and mercury(II) trap RecA as inactive aggregates

In bacteria, the RecA protein plays important roles in a number of DNA recombination and repair processes, including homologous recombination, SOS induction and recombinational DNA repair. We have explored the idea that the Escherichia coli RecA protein's functions could be controlled by small molecules. We investigated the 2:1 complex of zinc(II) with 1,4-dithio-l-threitol (l-DTT) that inhibits the E. coli rho transcription terminator, which is a hexameric ATP motor protein and is structurally homologous to RecA. We found that both the complex and ZnCl(2) inhibit the single-stranded DNA-dependent ATPase activity of RecA at sub-millimolar concentrations. Investigation of a variety of metal dications (0.4 mM final concentration) determined that zinc(II), copper(II) and mercury(II) all induce the precipitation of RecA, while the dichloride salts of calcium, manganese, barium, cobalt, and nickel do not. The inhibition of RecA activity by Zn(II), Cu(II) and Hg(II) results from the metal-dependent initiation of RecA aggregation. These observations may have implications for the design of biophysical experiments requiring solid-phase RecA protein, for a more complete understanding of metal toxicities, and for the design of metal-chelate inhibitors of prokaryotic DNA repair.

Ruthenium complexes as protein kinase inhibitors

[reaction: see text] Replacing complex natural products with simple metal complexes could lead to a new class of metallopharmaceuticals in which the metal center plays mainly a structural role. A strategy is introduced for the creation of ruthenium complex-based protein kinase inhibitors 1 (X = CO or CH(2)), morphed out of the class of indolocarbazole inhibitors with the alkaloid staurosporine as its most prominent member.

Spectroscopic properties, catalytic activities and mechanism studies of [(TpPh)Co(X)(CH3OH)m]·nCH3OH: bicarbonate dehydration in the presence of inhibitors

Two inhibitor-containing 'half-sandwich' cobalt(II) complexes [(TpPh)Co(X)(CH3OH)m] x nCH3OH ((TpPh) = hydrotris (3-phenylpyrazolyl)borate; 1: X- = N3-, m = 1, n = 2; 2: X- = NCS-, m = 0, n = 0) have been synthesized and used as the catalysts in the bicarbonate dehydration reaction. The structures of 1 and 2 were determined by X-ray diffraction analysis, which shows that N3- and NCS- coordinate to the Co(II) ions of 1 and 2, respectively, with the Co-N bond lengths of 1.992(6) A and 1.901(3) A. The coordination geometries of the Co(II) complexes in solution are five-coordinated trigonal bipyramid as revealed by the spectroscopic measurements. The dehydration kinetic measurements of HCO3- are performed by the stopped-flow techniques at pH < 7.9. The apparent dehydration rate constant k(obs) varies linearly with Co(II) complex and H+ concentrations, respectively, and the catalytic activity of 2 is lower than that of 1. The aqua Co(II) complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3-. The k(obs) values increase with increasing reaction temperature, and the large negative entropy of activation also indicates the associative activation mode. The inhibition ability of NCS- is stronger than that of N3-, which can be rationalized by the decreases in the Co-N(N3-/NCS-) bond lengths and effective atomic charges of the Co(II) ions based on the X-ray crystallographic data and theoretical calculations in this work.