Metal complexes as enzyme inhibitors

Multi-NMR and fluorescence spectra study the effects of aluminum(III) on coenzyme NADH in aqueous solutions

The interactions of dihydronicotinamide adenine dinucleotide (NADH) with Al(III) in near neutral aqueous solutions were studied by means of multinuclear (31P, 27Al, 1H and 13C)-NMR and fluorescence spectra techniques. The results suggested that Al(III) interacts with NADH by occupying the binding sites of pyrophosphate oxygen atoms and locks the adenine moiety of coenzyme in an anti folded conformation Meanwhile, the weak attractive interactions ('association') may occur between Al(III) and the hydroxyl groups of ribose rings through the intramolecular hydrogen bonding. Furthermore, at biologically relevant pH and concentrations of Al(III) and NADH (pH 6.5, C(Al)=10(-6)-10(-5) M), Al(III) could increase the amount of folded forms of NADH, which will result in reducing the coenzyme NADH activity in hollow-dehydrogenases reaction systems. However, in the presence of possible competing organic acids such as citrate, oxalate and tartate, could detoxify these Al(III) toxic effect.

Metal-1,4-dithio-2,3-dihydroxybutane chelates: novel inhibitors of the Rho transcription termination factor

Rho is an enzyme that is essential for the growth and survival of Escherichia coli, and bicyclomycin (1) is its only known selective inhibitor. We show that metal (Cd(2+), Ni(2+), and Zn(2+)) complexes of 1,4-dithio-2,3-dihydroxybutanes (2) serve as effective and potent rho inhibitors with I(50) values that can exceed that of 1. Maximal inhibition for ZnCl(2) and L-dithiothreitol (2a) corresponded to Zn(2):L-DTT stoichiometry. The I(50) value for the 2:1 Zn-L-DTT solution was 20 microM, which made it 3 times more potent than 1 (I(50) = 60 microM). Kinetic studies showed that a Zn-L-DTT solution functioned as a noncompetitive inhibitor with respect to ATP in the rho poly(C)-dependent ATPase assay and as a competitive inhibitor with respect to ribo(C)(10) in the poly(dC).ribo(C)(10)-stimulated ATPase assay. These findings demonstrated that both 1 and a Zn-L-DTT solution disrupted rho-mediated ATP hydrolysis but that they inhibit using different mechanisms. Substitution of L-DTT with 1,2-ethanedithiol in ZnCl(2) solutions led to a comparable loss of rho poly(C)-dependent ATPase activity, indicating that other metal chelates can serve as efficient inhibitors. The site and pathway of rho inhibition by the putative metal-1,4-dithio-2,3-dihydroxybutane chelates are discussed in light of the current data.

New metal complexes as potential therapeutics

The many activities of metal ions in biology have stimulated the development of metal-based therapeutics. Cisplatin, as one of the leading metal-based drugs, is widely used in treatment of cancer, being especially effective against genitourinary tumors such as testicular. Significant side effects and drug resistance, however, have limited its clinical applications. Biological carriers conjugated to cisplatin analogs have improved specificity for tumor tissue, thereby reducing side effects and drug resistance. Platinum complexes with distinctively different DNA binding modes from that of cisplatin also exhibit promising pharmacological properties. Ruthenium and gold complexes with antitumor activity have also evolved. Other metal-based chemotherapeutic compounds have been investigated for potential medicinal applications, including superoxide dismutase mimics and metal-based NO donors/scavengers. These compounds have the potential to modulate the biological properties of superoxide anion and nitric oxide.

Zinc(II)-mediated inhibition of a ribonuclease by an N-hydroxyurea nucleotide

The inhibition of ribonuclease Bi by 3'-N-hydroxyurea-3'-deoxythymidine 5'-phosphate is enhanced by 30-fold in the presence of Zn(2+). Thus, an N-hydroxyurea nucleotide can recruit Zn(2+) to inhibit the enzymatic activity of a ribonuclease. This result engenders a general strategy for the inhibition of non-metalloenzymes by metal complexes.

Multimethod characterization of the interaction of aluminum ion with alpha-ketoglutaric acid in acidic aqueous solutions

It has recently been reported that aluminum plays a very important role in reducing the activity of Krebs-cycle enzymes and glutamate dehydrogenase in rat brain homogenate. Therefore, it is necessary to identify the aluminum binding ability with the pivotal substrate alpha-ketoglutarate in biological systems. The interactions of aluminum with alpha-ketoglutarate were studied with pH-potentiometry, cyclic voltammetry, UV-vis, 1H, 27Al-NMR and Raman spectra multi-analytical techniques in acidic aqueous solution to measure the stoichiometries and stability constants of the complexes and its keto-enol tautomerism. The alpha-ketoglutarate was found to bind Al in a bidentate manner at the carboxylate and carbonyl moieties. The mononuclear 1:1 (AlLH(-1), AIL+, AlHL2+) and 2:1 (AlL2-, AlL2H(-2)3-) species, and dinuclear 2:1 (Al2L4+) species were found in acidic aqueous solution. Meanwhile, Al can promote alpha-KG tautomerize to its enolic-structure compounds in solutions. These findings may help to further understand the influence of Al on GDH enzyme reactions in biological systems.

Metal-organic compounds: a new approach for drug discovery. N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide copper(II) complex as an inhibitor of human immunodeficiency virus 1 protease

The use of metal-organic complexes is a potentially fruitful approach for the development of novel enzyme inhibitors. They hold the attractive promise of forming stronger attachments with the target by combining the co-ordination ability of metals with the unique stereoelectronic properties of the ligand. We demonstrated that this approach can be successfully used to inhibit the protease of the human immunodeficiency virus (type 1). Several ligands bearing substituents designed to interact with the catalytic site of the enzyme when complexed to Cu(2+) were synthesised. The inhibition pattern of the resulting copper(II) complexes was analysed. We showed that the copper(II) complex of N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide (C1) interacts with the active site of the enzyme leading to competitive inhibition. On the other hand, N2-pyridine-amide ligands and oxazinane carboxamide ligand were found to be poor chelators of the cupric ion under the enzymatic assay conditions. In these cases, the observed inhibition was attributed to released cupric ions which react with cysteine residues on the surface of the protease. While unchelated metal cations are not likely to be useful agents, metal chelates such as C1 should be considered as promising lead compounds for the development of targeted drugs.

Nickel(II) 2,6-diacetylpyridine bis(isonicotinoylhydrazonate) and bis(benzoylhydrazonate) complexes: structure and antimycobacterial evaluation. Part XI

The reaction of 2,6-diacetylpyridine (dap) and isonicotinoyl- or benzoylhydrazide leads to bishydrazones H(2)dapin (1a) and H(2)dapb (1b), respectively. The condensation can either take place as a bimolecular kinetic process between the two reactants or as a monomolecular metal-templated synthesis in the presence of nickel(II) ions. In the latter case the reaction products are charged 2,6-diacetylpyridine bis(hydrazone) nickel(II) complexes, which can be easily deprotonated to neutral hydrazonates. Diffractometric analysis of one of these [Ni(dapb)](2) (8b) has shown a binuclear structure with two octahedral nickel(II) ions bridged by two helicoidal dap (bishydrazonates) in a spheroidal structure of C(2V) symmetry. The synthesized complexes 8 are promising as antimycobacterial agents against M. tuberculosis H37Rv. In particular, 8b displays significant activity (MIC=0.025 microg/mL) 10-fold higher than rifampin and equal to isoniazid, while its ligand is ineffective. Compound 8b is also capable of reducing HIV-induced cytopathogenic effect in human T(4 )lymphocytes.

Antifungal activity of Ag(I) and Zn(II) complexes of aminobenzolamide (5-sulfanilylamido-1,3,4-thiadiazole-2-sulfonamide) derivatives

Aminobenzolamide (5-sulfanilylamido-1,3,4-thiadiazole-2-sulfonamide) is a potent inhibitor of the zinc enzyme carbonic anhydrase (CA, EC 4.2.1.1), being at the same time structurally similar to the antimicrobial sulfonamides. Here we report that the reaction of aminobenzolamide with arylsulfonyl isocyanates affords a series of new arylsulfonylureido derivatives which were subsequently used as ligands (in the form of conjugate bases, as sulfonamide anions) for the preparation of metal complexes containing Ag(I) and Zn(II). All the new compounds proved to be very potent inhibitors of CA (isozymes I, II and IV). The newly synthesized complexes, unlike the free ligands, also act as effective antifungal agents against several Aspergillus and Candida spp., some of them showing activities comparable to ketoconazole, with minimum inhibitory concentrations in the range of 1.8-5 microg/mL. The mechanism of antifungal action of these complexes seem to be unconnected with inhibition of lanosterol-14-alpha-demethylase, since the levels of sterols assessed in the fungi cultures were equal in the absence or in the presence of the tested compounds. Probably the new complexes act as inhibitors of phosphomannose isomerase, a key enzyme in the biosynthesis of yeast cell walls.

The McbB component of microcin B17 synthetase is a zinc metalloprotein

The microcin B17 synthetase converts glycine, serine, and cysteine residues in a polypeptide precursor into oxazoles and thiazoles during the maturation of the Escherichia coli antibiotic Microcin B17. This multimeric enzyme is composed of three subunits (McbB, McbC, and McbD), and it employs both ATP and FMN as cofactors. The McbB subunit was purified as a fusion with the maltose-binding protein (MBP), and metal analysis revealed that this protein binds 0.91+/-0.17 zinc atoms. Upon incubation of MBP-McbB with excess zinc, the stoichiometry increased to two atoms of zinc bound, but metal binding to the second site resulted in a decrease in the heterocyclization activity when MBP-McbB was reconstituted with the other components of the synthetase. Apo-protein was prepared by using p-hydroxymercuriphenylsulfonic acid (PMPS), and loss of the metal caused a severe reduction in enzymatic activity. However, if dithiothreitol was added to the PMPS reactions within a few minutes, enzymatic activity was retained and MBP-McbB could be reconstituted with zinc. Spectroscopic analysis of the cobalt-containing protein and extended X-ray absorption fine structure analysis of the zinc-containing protein both provide evidence for a tetrathiolate coordination sphere. Site-directed mutants of MBP-McbB as well as the synthetase tagged with the calmodulin-binding peptide were constructed. Activity assays and metal analysis were used to determine which of the six cysteines in McbB are metal ligands. These results suggest that the zinc cofactor in McbB plays a structural role.

Apoptosis-inducing oxovanadium(IV) complexes of 1,10-phenanthroline against human ovarian cancer

In a systematic effort to identify a potent anticancer agent against human ovarian cancer, we synthesized 15 oxovanadium(IV) complexes, and examined their cytotoxic activity against human ovarian cancer cell lines PA-1, SKOV-3, ES-2 and OVCAR-3 using a MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyletetrazolium bromide]-based assay. The apoptosis-inducing ability of the oxovanadium compounds was evaluated by the two-color flow cytometric terminal deoxynucleotidyl transferase-based assay that labels 3'-hydroxyl ends of fragmented DNA (TUNEL) assay and confocal laser scanning microscopy. Notably, all eight oxovanadium complexes of 1,10 phenanthroline exhibited significant cytotoxicity and induced apoptosis within 24 h. The mono-chelated, VO(NO2-phen) and bis-chelated, VO(Me2-phen)2, VO(Cl-phen)2 and VO(NO2-phen)2 complexes were the most potent oxovanadium compounds, and killed target cancer cells at low micromolar concentrations. The marked differences in the cytotoxic activity of oxovanadium(IV) complexes containing different heterocyclic ancillary ligands suggest that the cytotoxic activity of these compounds is determined by the identity of the five-member bidentate ligands, as well as the nature of the substituents on the heterocyclic aromatic rings. Our results presented herein provide experimental evidence that oxovanadium compounds induce apoptosis in human ovarian cancer cells. The lead compounds, VO(Me2-phen)2 and VO(NO2-phen)2, may be useful in the treatment of ovarian cancer.

Antifungal activity of silver and zinc complexes of sulfadrug derivatives incorporating arylsulfonylureido moieties

Two well known antimicrobial sulfonamides, sulfadiazine and sulfamerazine were reacted with arylsulfonyl isocyanates, affording several new arylsulfonylureido derivatives. These compounds were subsequently used as ligands (in the form of conjugate bases, as sulfonamide anions) for the preparation of metal complexes containing silver and zinc. The newly synthesized complexes, unlike the free ligands, proved to act as effective antifungal agents against several Aspergillus and Candida spp., some of them showing activities comparable to ketoconazole, with minimum inhibitory concentrations in the range of 1.5-5 microg/ml. The mechanism of antifungal action of these complexes seems to be different from that of the azole antifungals acting as lanosterol-14-alpha-demethylase inhibitors. Levels of sterols assayed in the fungi cultures treated with these new antifungals were equal in the absence or in the presence of the tested compounds. This is in strong contrast with similar experiments in which ketoconazole has been used as antifungal, when drastically reduced ergosterol amounts could be detected. Thus, it is probable that the inhibition of phosphomannose isomerase, a key enzyme in the biosynthesis of yeast cell walls, imparts antifungal activity to the new metal complexes reported here.

Capillary electrophoretic analysis of alkaline phosphatase inhibition by theophylline

An analytical method for studying enzyme inhibition has been developed using capillary electrophoresis with laser-induced fluorescence detection. This technique is based on electrophoretic mixing of zones of enzyme and inhibitor in substrate-filled capillaries. Enzyme catalytic activity is measured by detecting the fluorescent reaction product as it migrates past the detector. Reversible enzyme inhibition is indicated by a transient decrease in product formation. The enzyme, alkaline phosphatase, has been studied using the fluorogenic substrate AttoPhos ([2,2'-bibenzothiazol]-6-hydroxy-benzthiazole phosphate). This assay has been used to quantify theophylline, a noncompetitive, reversible inhibitor of alkaline phosphatase. The detection limit for theophylline is estimated at 3 microM, and 8.6 amole of alkaline phosphatase are required for each assay. The calculated K(i) for theophylline is 90 microM for the capillary electrophoretic enzyme-inhibitor assays.