Addition of DNA to Cr(VI) and cytochrome b5 containing proteoliposomes leads to generation of DNA strand breaks and Cr(III) complexes

Chromium (Cr) is a cytotoxic metal that can be associated with a variety of types of DNA damage, including Cr-DNA adducts and strand breaks. Prior studies with purified human cytochrome b(5) and NADPH:P450 reductase in reconstituted proteoliposomes (PLs) demonstrated rapid reduction of Cr(VI) (hexavalent chromium, as CrO(4)(2-), and the generation of Cr(V), superoxide (O(2)(*-)), and hydroxyl radical (HO(*)). Studies reported here examined the potential for the species produced by this system to interact with DNA. Strand breaks of purified plasmid DNA increased over time aerobically, but were not observed in the absence of O(2). Cr(V) is formed under both conditions, so the breaks are not mediated directly by Cr(V). The aerobic strand breaks were significantly prevented by catalase and EtOH, but not by the metal chelator diethylenetriaminepentaacetic acid (DTPA), suggesting that they are largely due to HO(*) from Cr-mediated redox cycling. EPR was used to assess the formation of Cr-DNA complexes. Following a 10-min incubation of PLs, CrO(4)(2-), and plasmid DNA, intense EPR signals at g=5.7 and g=5.0 were observed. These signals are attributed to specific Cr(III) complexes with large zero field splitting (ZFS). Without DNA, the signals in the g=5 region were weak. The large ZFS signals were not seen, when Cr(III)Cl(3) was incubated with DNA, suggesting that the Cr(III)-DNA interactions are different when generated by the PLs. After 24 h, a broad signal at g=2 is attributed to Cr(III) complexes with a small ZFS. This g=2 signal was observed without DNA, but it was different from that seen with plasmid. It is concluded that EPR can detect specific Cr(III) complexes that depend on the presence of plasmid DNA and the manner in which the Cr(III) is formed.     

An oxygen dependence in chromium mutagenesis

Evidence is provided that mutagenicity in Salmonella by a chromium(VI) salt and a chromium(III) compound has a differential dependence on the presence of oxygen. The mutagenic chromium(III) compound, cis-dichlorobis(2,2'-bipyridyl)chromium(III), reverted Salmonella strains, TA102 and TA2638, only under aerobic conditions. Potassium dichromate (chromium VI) required the presence of oxygen to revert the Salmonella strain TA102 but induced a moderate reversion frequency in TA2638 under anaerobic conditions. The data also support a role for oxygen radicals in chromium-mediated mutagenesis and suggests at least two pathways by which chromium compounds can induce mutations.     

In vitro studies on the DNA impairments induced by Cr(III) complexes with cellular reductants

The influence of Cr(III) complexes with ascorbic acid, cysteine and glutathione on DNA has been studied spectrophotometrically and chromatographically. The toxic and genotoxic activities of these complexes were also investigated. It was found that these complexes bind to DNA weaker than hexaaqua Cr(III) complexes. It could be explained through the greater strength of the bi- and tridentate ligands coordinated to chromium in comparison to water molecules. The formation of DNA-DNA intermolecular bonds and DNA interstrand cross-linking has been also observed. These complexes were found to be non-toxic and non-genotoxic in the bacterial test.     

Effects of nitrilotriacetic acid on the induction of gene mutations and sister-chromatid exchanges by insoluble chromium compounds

The influence of nitrilotriacetic acid trisodium salt (NTA) on the mutagenic and clastogenic activity of several water-insoluble or poorly soluble chromium compounds was determined by means of the Salmonella/microsome assay (plate test on TA100 strain) and the sister-chromatid exchange (SCE) test in mammalian cell cultures (CHO line). NTA in itself did not induce gene mutations nor did it increase the frequency of SCE. Cr(VI) compounds (Pb, Ba, Zn, Sr and Ca chromates) and an industrial Cr(VI) pigment, chromium orange (containing PbCrO4 PbO), were inactive or scarcely active mutagens in the Salmonella/microsome test when dissolved in water, but they were increasingly mutagenic when solubilized by 0.5 N NaOH or NTA (10 or 100 mg/ml). Also, the mutagenic activity of Cr(VI), contaminating an industrial Cr(III) pigment (chromite), was slightly enhanced by NTA. Mutagenicity of chromates was correlated with the amounts of Cr(VI) solubilized by NTA or alkali, as determined by the colorimetric reaction with diphenylcarbazide and atomic absorption spectrophotometry, and was decreased by incubation with microsomes, due to reduction of Cr(VI) to the genetically inactive Cr(III) form. In the SCE assay, the insoluble or poorly soluble Ba, Zn, Sr and Ca chromates and the insoluble Cr(VI) pigments zinc yellow (containing ZnCrO4 Zn(OH2], chromium yellow and molybdenum orange (both containing PbCrO4) were directly clastogenic due to cellular endocytosis taking place in prolonged treatments, and NTA significantly increased their chromosome-damaging activity.     

Unusual reactivity in a commercial chromium supplement compared to baseline DNA cleavage with synthetic chromium complexes

Commercially available chromium supplements were tested for their DNA cleavage ability compared with synthetic chromium(III) complexes, including chromium(III) tris-picolinate [Cr(pic)3], basic chromium acetate [Cr3O(OAc)6]+, model complexes, and recently patented Cr-complexes for use in supplements or therapy. Four different supplements (P1-P4) were tested for their DNA cleaving activity in the presence and the absence of H2O2, dithiothreitol (DTT) or ascorbate. One supplement, P1, showed nicking of DNA in the absence of oxidant or reductant at 120 microM metal concentration. Different lot numbers of P1 were also tested for DNA cleavage activity with similar results. Commercial supplements containing Cr(pic)3 nicked DNA at 120 microM metal concentrations in the presence of 5 mM ascorbate or with excess hydrogen peroxide, analogous to reactions with synthetic Cr(pic)3 reported elsewhere. Another chromium (non-Cr(pic)3) supplement, P2, behaves in a comparable manner to simple Cr(III) salts in the DNA nicking assay. Chromium(III) malonate [Cr(mal)2] and chromium(III) acetate [Cr(OAc)] can nick DNA in the presence of ascorbate or hydrogen peroxide, respectively, only at higher metal concentrations. The Cr(III) complexes of histidine, succinate or N-acetyl-L-glutamate do not nick DNA to a significant degree.     

Pyridine-2,6-bis(thiocarboxylic acid) Produced by <Emphasis Type="BoldItalic">Pseudomonas stutzeri</Emphasis> KC Reduces Chromium(VI) and Precipitates Mercury,Cadmium, Lead and Arsenic

Interactions of the Pseudomonas stutzeri KC siderophore pyridine-2,6-bis(thiocarboxylic acid) (pdtc) with chromium(VI), mercury(II), cadmium(II), lead(II), and arsenic(III) are described. Pdtc was found to reduce Cr(VI) to Cr(III) in both bacterial cultures and in abiotic reactions with chemically synthesized pdtc. Cr(III) subsequently formed complexes with pdtc and pdtc hydrolysis products, and their presence was confirmed using electrospray ionization-mass spectrometry (ESI-MS). Cr(III):pdtc complexes were found to slowly release Cr(III) as chromium sulfide and possibly Cr(III) oxides. Pdtc also formed poorly soluble complexes with Hg, Cd, Pb, and As(III). Hydrolysis of those complexes led to the formation of their respective metal sulfides as confirmed by energy dispersive X-ray spectroscopy (EDS) elemental analysis. The pdtc-producing strain P. stutzeri KC showed higher tolerance to most of these metals as compared to a pdtc-negative mutant. A novel role of pdtc is postulated as its involvement in providing an extracellular pool of thiols that are used for redox processes in detoxification of the bacterial extracellular environment. These redox processes can be mediated by transition metal:pdtc complexes.     

Developing azo and formazan dyes based on environmental considerations: Salmonella mutagenicity

In previous papers, the synthesis and chemical properties of iron-complexed azo and formazan dyes were reported. It was shown that in certain cases iron could be substituted for the traditionally used metals such as chromium and cobalt, without having an adverse effect on dye stability. While these results suggested that the iron analogs were potential replacements for the commercially used chromium and cobalt prototypes, characterization of potentially adverse environmental effects of the new dyes was deemed an essential step in their further development. The present paper provides results from using the Salmonella/mammalian microsome assay to determine the mutagenicity of some important commercial metal complexed dyes, their unmetallized forms, and the corresponding iron-complexed analogs. The study compared the mutagenic properties of six unmetallized azo dyes, six commercial cobalt- or chromium-complexed azo dyes, six iron-complexed azo dyes, six unmetallized formazan dyes, and six iron-complexed formazan dyes. The results of this study suggest that the mutagenicity of the unmetallized dye precursors plays a role in determining the mutagenicity of the iron-complexes. For the monoazo dye containing a nitro group, metal complex formation using iron or chromium decreased or removed mutagenicity in TA100; however, little reduction in mutagenicity was noted in TA98. For the formazan dye containing a nitro group, metal-complex formation using iron increased mutagenicity. Results varied for metal-complexes of azo and formazan dyes without nitro groups, but in general, the metal-complexed dyes based on mutagenic ligands were also mutagenic, while those dyes based on nonmutagenic ligands were nonmutagenic.     

Incision of trivalent chromium [Cr(III)]-induced DNA damage by Bacillus caldotenax UvrABC endonuclease

Some hexavalent chromium [Cr(VI)]-containing compounds are lung carcinogens. Once within cells, Cr(VI) is reduced to trivalent chromium [Cr(III)] which displays an affinity for both DNA bases and the phosphate backbone. A diverse array of genetic lesions is produced by Cr including Cr-DNA monoadducts, DNA interstrand crosslinks (ICLs), DNA-Cr-protein crosslinks (DPCs), abasic sites, DNA strand breaks and oxidized bases. Despite the large amount of information available on the genotoxicity of Cr, little is known regarding the molecular mechanisms involved in the removal of these lesions from damaged DNA. Recent work indicates that nucleotide excision repair (NER) is involved in the processing of Cr-DNA adducts in human and rodent cells. In order to better understand this process at the molecular level and begin to identify the Cr-DNA adducts processed by NER, the incision of CrCl(3) [Cr(III)]-damaged plasmid DNA was studied using a thermal-resistant UvrABC NER endonuclease from Bacillus caldotenax (Bca). Treatment of plasmid DNA with Cr(III) (as CrCl(3)) increased DNA binding as a function of dose. For example, at a Cr(III) concentration of 1 microM we observed approximately 2 Cr(III)-DNA adducts per plasmid. At this same concentration of Cr(III) we found that approximately 17% of the plasmid DNA contained ICLs ( approximately 0.2 ICLs/plasmid). When plasmid DNA treated with Cr(III) (1 microM) was incubated with Bca UvrABC we observed approximately 0.8 incisions/plasmid. The formation of endonuclease IV-sensitive abasic lesions or Fpg-sensitive oxidized DNA bases was not detected suggesting that the incision of Cr(III)-damaged plasmid DNA by UvrABC was not related to the generation of oxidized DNA damage. Taken together, our data suggest that a sub-fraction of Cr(III)-DNA adducts is recognized and processed by the prokaryotic NER machinery and that ICLs are not necessarily the sole lesions generated by Cr(III) that are substrates for NER.     

Toxicity and mutagenicity of 2,4,-6-trinitrotoluene and its microbial metabolites.

TNT (2,4,6-trinitrotoluene) of explosive grade is highly toxic to marine forms that included fresh water unicellular green algae (Selenastrum capricornutum), tidepool copepods (Tigriopus californicus), and oyster larvae (Crassostrea gigas), and mutagenic to Salmonella typhimurium. On the basis of mutagenic assays carried out with a set of histidine-requiring strains of the bacterium, TNT was detected as a frameshift mutagen that significantly accelerates the reversion rate of a frameshift tester, TA-98. In contrast, the major microbial metabolites of TNT appeared to be nontoxic and nonmutagenic.     

Effect of organic Cr(III) complexes on chromium speciation

Abstract

Chromium speciation in the presence of organic chromium(III) complexes was investigated using solid-phase extraction. The adsorptions of Cr(VI) and Cr(III) on alumina and pumice powder were studied. Maximum sorption of Cr(VI) was obtained by alumina (90.22%), while Cr(III) was highly adsorbed onto pumice powder (86.65%). This result shows that pumice may be a new and promising adsorbent for Cr(III). The experimental equilibrium data for Cr(VI) adsorption onto alumina and Cr(III) sorption onto pumice were analysed using Langmuir and Freundlich isotherms. The separation and adsorption of Cr(VI), Cr(III) and five organic chromium(III) complexes onto pumice and alumina at different pH values were evaluated. Ethylenediaminetetraacetate (EDTA), oxalate, citrate, glycine, alanine and 8-hydroxyqinoline were used as ligands. Sorption of alanine and ethylenediaminetetraacetate complexes was higher onto alumina than pumice at pH>3. The enhancement of adsorption of chromium(III) complexes onto pumice was achieved by surface modification of pumice using a surfactant, namely hexadecyltrimethylammoniumbromür (HDTMA). The presence of surfactant enhanced the adsorption of Cr(III) citrate, oxalate, glycine and 8-hydroxyquinoline complexes onto pumice. However, the adsorption of EDTA and alanine complexes decreased, with ratio of 13.40% and 4.00% respectively. Here we demonstrate that chromium speciation methods depending on adsorption onto various adsorbents including alumina may lead erroneous results. Analytical measurements were performed by flame AAS, data were obtained by standard addition method.     

Cr(III)-mediated crosslinks of glutathione or amino acids to the DNA phosphate backbone are mutagenic in human cells.

Carcinogenic Cr(VI) compounds were previously found to induce amino acid/glutathione-Cr(III)-DNA crosslinks with the site of adduction on the phosphate backbone. Utilizing the pSP189 shuttle vector plasmid we found that these ternary DNA adducts were mutagenic in human fibroblasts. The Cr(III)-glutathione adduct was the most potent in this assay, followed by Cr(III)-His and Cr(III)-Cys adducts. Binary Cr(III)-DNA complexes were only weakly mutagenic, inducing a significant response only at a 10 times higher number of adducts compared with Cr(III)-glutathione. Single base substitutions at the G:C base pairs were the predominant type of mutations for all Cr(III) adducts. Cr(III), Cr(III)-Cys and Cr(III)-His adducts induced G:C-->A:T transitions and G:C-->T:A transversions with almost equal frequency, whereas the Cr(III)-glutathione mutational spectrum was dominated by G:C-->T:A transversions. Adduct-induced mutations were targeted toward G:C base pairs with either A or G in the 3' position to the mutated G, while spontaneous mutations occurred mostly at G:C base pairs with a 3' A. No correlation was found between the sites of DNA adduction and positions of base substitution, as adducts were formed randomly on DNA with no base specificity. The observed mutagenicity of Cr(III)-induced phosphotriesters demonstrates the importance of a Cr(III)-dependent pathway in Cr(VI) carcinogenicity.     

Genotoxicity and mutagenicity of chromium(VI)/ascorbate-generated DNA adducts in human and bacterial cells

Reduction of carcinogenic Cr(VI) by vitamin C generates ascorbate-Cr(III)-DNA cross-links, binary Cr(III)-DNA adducts, and can potentially cause oxidative DNA damage by intermediate reaction products. Here, we examined the mutational spectrum and the importance of different forms of DNA damage in genotoxicity and mutagenicity of Cr(VI) activated by physiological concentrations of ascorbate. Reduction of Cr(VI) led to a dose-dependent formation of both mutagenic and replication-blocking DNA lesions as detected by propagation of the pSP189 plasmids in human fibroblasts. Disruption of Cr-DNA binding abolished mutagenic responses and normalized the yield of replicated plasmids, indicating that Cr-DNA adducts were responsible for both mutagenicity and genotoxicity of Cr(VI). The absence of DNA breaks and abasic sites confirmed the lack of a significant production of hydroxyl radicals and Cr(V)-peroxo complexes in Cr(VI)-ascorbate reactions. Ascorbate-Cr(III)-DNA cross-links were much more mutagenic than smaller Cr(III)-DNA adducts and accounted for more than 90% of Cr(VI) mutagenicity. Ternary adducts were also several times more potent in the inhibition of replication than binary complexes. The Cr(VI)-induced mutational spectrum consisted of an approximately equal number of deletions and G/C-targeted point mutations (51% G/C --> T/A and 30% G/C --> A/T). In Escherichia coli cells, Cr(VI)-induced DNA adducts were only highly genotoxic but not mutagenic under either normal or SOS-induced conditions. Lower toxicity and high mutagenicity of ascorbate-Cr(III)-DNA adducts in human cells may result from the recruitment of an error-prone bypass DNA polymerase(s) to the stalled replication forks. Our results suggest that phosphotriester-type DNA adducts could play a more important role in human than bacterial mutagenesis.     

Mutagenicity of a series of hexacoordinate cobalt(III) compounds

15 cobalt(III) compounds have been tested for DNA-damaging capabilities using an E. coli differential repair assay and for mutagenicity in strains of Salmonella typhimurium. 4 of these compounds were active in both systems. Although the general ligand requirements for genetic activity of cobalt(III) appear to closely parallel those of chromium(III) and rhodium(III), the genetic activity of cobalt compounds seems particularly dependent upon the structure of the ligands coordinated about the metal ion. By a simple methyl substitution on the organic ligands, a compound completely devoid of activity, e.g. trans-[Co(pyr)4Cl2]Cl, could be made slightly mutagenic in Salmonella typhimurium strains e.g. trans-[Co(3-pic)4Cl2]Cl. Substitution at the 4-position rather than the 3-position on the same pyridine ring, e.g. trans-[Co(4-pic)4Cl2]Cl, results in a 50-fold enhancement of activity in both repair and mutagenesis systems. The difference in genetic activity is attributed to the influence of the ligands on the relative lability of the metal complex.     

Non-oxidative mechanisms are responsible for the induction of mutagenesis by reduction of Cr(VI) with cysteine: role of ternary DNA adducts in Cr(III)-dependent mutagenesis

Intracellular reduction of carcinogenic Cr(VI) generates Cr-DNA adducts formed through the coordination of Cr(III) to DNA phosphates (phosphotriester-type adduct). Here, we examined the role of Cr(III)-DNA adducts in mutagenesis induced by metabolism of Cr(VI) with cysteine. Reduction of Cr(VI) caused a strong oxidation of 2', 7'-dichlorofluoroscin (DCFH) and extensive Cr-DNA binding but no DNA breakage. Cr-DNA adducts induced unwinding of supercoiled plasmids and structural distortions in the DNA helix as detected by decreased ethidium bromide binding. Propagation of Cr-treated pSP189 plasmids in human fibroblasts led to a dose-dependent formation of the supF mutants and inhibition of replication. Blocking of Cr(III)-DNA binding by occupation of DNA phosphates with Mg(2+) or by sequestration of Cr(III) by inorganic phosphate or EDTA eliminated mutagenic responses and restored a normal yield of replicated plasmids. Dissociation of Cr(III) from DNA by a phosphate-based reversal procedure returned mutation frequency to background levels. The mutagenic responses at the different phases of the reduction reaction were unrelated to the amount of reduced Cr(VI) but reflected the number and the spectrum of Cr(III)-DNA adducts that were formed. Ternary cysteine-Cr(III)-DNA adducts were approximately 4-5 times more mutagenic than binary Cr(III)-DNA adducts. Although intermediate reaction products (CrV/IV, thiyl radicals) were capable of oxidizing DCFH, they were insufficiently reactive to damage DNA. Single-base substitutions at G/C pairs were the predominant type of Cr-induced mutations. The majority of mutations occurred at the sites where G had adjacent purine in the 3' or 5' position. Overall, our results present the first evidence that Cr(III)-DNA adducts play the dominant role in the mutagenicity caused by the metabolism of Cr(VI) by a biological reducing agent.     

Probing of DNA structure with osmium tetroxide. Effect of ligands

Fourteen OsO4 complexes with different ligands were tested as probes of DNA structure. Of these complexes, only OsO4-2,2'-bipyridine (Os-bipy), OsO4-bathophenanthrolinedisulfonic acid (Os-bpds) and OsO4-N,N,N',N'-tetramethylenediamine (Os-TMEN) site-specifically modified the ColE1 cruciform in a supercoiled plasmid pColIR215 at millimolar concentrations. Os-bipy, Os-bpds and Os-TMEN also displayed site-specific modification of the B-Z junctions in the supercoiled plasmid pRW751 containing (dC-dG)n inserts.     

Mutagenicity, comutagenicity, and antimutagenicity of erythrosine (FD and C red 3), a food dye, in the Ames/Salmonella assay

Erythrosine (diNa, tetraiodofluorescein) was nonmutagenic to the Ames/Salmonella typhimurium strains TA97a, TA98, TA100, TA102, and TA104, to a concentration of 2 mg/plate. No mutative intermediates were detected on metabolism by rat caecal cell-free extracts or rat liver S9 mixture; or on incubation with the comutagens, harman and norharman (+/- S9). Instead, an unexpected dose-dependent suppression in spontaneous reversion frequencies was observed (maximum approximately equal to 35% decrease). Erythrosine was antimutagenic to benzo[a]pyrene, but it did not decrease the mutagenicity of the other adduct-forming mutagen, 4-nitroquinoline N-oxide. The food dye was strongly antimutagenic to the bifunctional alkylating agent, mitomycin C, though it did not exhibit a similar effect on the mutagenicity of the corresponding monofunctional agent, methyl methanesulphonate. It partially depressed the mutagenic potentials of sodium azide. The antimutagenic effect of erythrosine on an intercalating agent, ethidium bromide, was discernible only at the highest dose (2 mg/plate). These results have been interpreted in terms of a genointeractive role of erythrosine. Erythrosine produced differential toxic effects in repair-deficient (TA97a, TA98, TA100) and repair-proficient (TA102, TA104) Salmonella tester strains; survival of the repair-deficient strains was found to be decreased. Photoinduced potentiation of erythrosine toxicity was observed, although light irradiation in the presence of erythrosine did not modify the reversion frequencies of the tester strains. The evidence strongly suggests that erythrosine, which exhibits nonmutagenicity in the Ames/Salmonella test, can interact with DNA repair enzymes and/or with DNA.     

Biphenyl and fluorinated derivatives: liver enzyme-mediated mutagenicity detected in Salmonella typhimurium and Chinese hamster V79 cells.

Hepatocarcinogenic polychlorinated and polybrominated biphenyls usually show negative results in in vitro mutagenicity assays. Problems in their testing result from their low water solubility and their slow rate of metabolism. We therefore investigated better soluble model compounds, namely biphenyl and its 3 possible monofluorinated derivatives. In the direct test, these compounds proved to be nonmutagenic in Salmonella typhimurium TA98 and TA100 (reversion to histidine prototrophy) and in Chinese hamster V79 cells (acquisition of resistance to 6-thioguanine). However, when the exposure was carried out in the presence of NADPH-fortified postmitochondrial fraction of liver homogenate from Aroclor 1254-treated rats, all 4 compounds showed mutagenic activity in V79 cells. 3-Fluorobiphenyl produced strong mutagenic effects in S. typhimurium TA100 as well, whereas the other biphenyls were inactive. In strain TA98, 3- and 4-fluorobiphenyl showed mutagenic activity. This mutagenicity was enhanced in the presence of 1,1,1-trichloropropene 2,3-oxide, an inhibitor of microsomal epoxide hydrolase, thus suggesting that epoxides may be active metabolites.     

Generation of PM2 DNA breaks in the course of reduction of chromium(VI) by glutathione

The carcinogen chromate is efficiently taken up and reduced to chromium(III) compounds by various biological systems. To test the possible DNA damage induced in the course of chromium(VI) reduction, we used a combination of chromate with the reductant glutathione (GSH) as well as a green complex of chromium(V), which is formed in the reaction of chromate with GSH. The combination of chromate and glutathione was found to cause single-strand breaks in supercoiled circular DNA of the bacteriophage PM2. The green chromium(V) complex Na4(GSH)4Cr(V).8H2O, prepared from chromate and glutathione, also cleaved supercoiled PM2 DNA. No DNA-degrading effects were observed with either chromate or the final product of the reaction with GSH, a purple anionic chromium(III) GSH complex. The nature of the buffering agents revealed a strong influence on the extent of DNA strand breaks produced by chromate and GSH. A variation of the GSH concentration in the reaction with chromate and PM2 DNA, performed in sodium phosphate-buffered solutions showed an initial increase in the number of strand breaks at GSH concentrations up to 1 mM followed by a decline at higher GSH concentrations. Since neither chromate, when administered individually, nor the final product of chromium(VI) reduction, the purple chromium(III) GSH complex, produced any detectable DNA cleavage, the critical steps leading to DNA strand breaks occur in the course of the conversion of chromium(VI) to chromium(III) by GSH, the most abundant intracellular low molecular thiol. Moreover, the demonstration that DNA cleavage is induced in the presence of the chromium(V) complex identifies chromium(V) as the oxidation state of the metal, which is involved in the steps leading to DNA-damaging effects of chromate.     

A study of the interaction of Cr(III) complexes and their selective binding with B-DNA: a molecular modeling approach

Molecular modeling and energy minimisation calculations have been used to investigate the interaction of chromium(III) complexes in different ligand environments with various sequences of B-DNA. The complexes are [Cr(salen)(H(2)O)(2)](+); salen denotes 1, 2 bis-salicylideneaminoethane, [Cr(salprn)(H(2)O)(2)](+); salprn denotes 1, 3 bis- salicylideneaminopropane, [Cr(phen)(3)](3+); phen denotes 1, 10 phenanthroline and [Cr(en)(3)](3+); en denotes ethylenediamine. All the chromium(III) complexes are interacted with the minor groove and major groove of d(AT)(12), d(CGCGAATTCGCG)(2) and d(GC)(12) sequences of DNA. The binding energy and hydrogen bond parameters of DNA-Cr complex adduct in both the groove have been determined using molecular mechanics approach. The binding energy and formation of hydrogen bonds between chromium(III) complex and DNA has shown that all complexes of chromium(III) prefer minor groove interaction as the favourable binding mode.     

Hydrolysis of DNA by a Dipeptides Containing Histidine

Three ligands which contain histidine and conjugated by a flexible linker, have been characterized and evaluated as DNA cleavage agents. The cleavage activity of metal complexes were evaluated by monitoring the conversion of supercoiled plasmid DNA (pUC19) (Form I) to nicked circular DNA (Form II) by agarose gel electrophoresis. The results showed that the cleavage activity of Cu(II) complexes was enhanced compared with histidine. Specially, at a high reaction concentration (0.2 mM), Cu(II) complexes can cleave the plasmid DNA with some selectivity.