Metabolic activation of the mutagen azide in biological systems

Enhancing genetic variability in Trigonella species through sodium azide induction: morpho-physiological and chromosomal amelioration

Plant breeding, aimed at enhancing desired traits, depends on genetic diversity. Mutation breeding is a powerful method of rapidly expanding genetic diversity, facilitating crop improvement, and ensuring food security. In a recent study, researchers evaluated the genetic variability of Trigonella species using different doses of sodium azide (SA) (0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) through morphological, physiological, and cytogenetic studies. Morphological variations were observed in cotyledonary leaves, vegetative leaves, and overall plant growth and habit. Several quantitative parameters, such as plant height, fertile branches per plant, pods per plant (or clusters), seeds per pod, and seed yield, increased when treated with 0.2% and 0.4% SA compared to the control. Furthermore, the total chlorophyll content and carotenoids increased in the sample treated with 0.2% SA over the control but decreased with higher concentrations. Scanning electron microscopy revealed that stomatal aperture and seed dimensions increased at lower concentrations of sodium azide treatment. The study found a positive correlation between the different parameters studied in the Trigonella species, as indicated by high r-values. Based on their findings, it was concluded that the genotype of fenugreek can be improved by using 0.2% and 0.4% concentrations of sodium azide. However, the evaluation of observed variants in successive generations is a critical and necessary process to validate their potential as keystones for crop genetic improvements.

Chemical Composition, In Silico and In Vitro Antimutagenic Activities of Ethanolic and Aqueous Extracts of Tigernut (Cyperus esculentus)

Tigernut, also known as Cyperus esculentus, is said to be high in nutritional and medicinal value. The purpose of this study was to determine the C. esculentus’s antimutagenic activity. The ethanolic and aqueous extracts of the nut were analyzed for chemical constituents, antioxidants, ultraviolet-visible, and gas chromatography-mass spectrometry using standard procedures. The extracts contained a total of 17 major compounds that were docked against human RecQ-like protein 5 (RECQL5) helicase protein. The antimutagenic property of the ethanolic extract in vitro was assessed using the Allium cepa chromosome assay. Onion bulbs were pre-treated with 200 mg/kg of ethanolic extract of C. esculentus for 24 h and then, grown in NaN₃ (250 μg/L) for 24 h; onion bulbs were also first exposed to NaN₃ (250 μg/L) for 24 h before treatment with 100 mg/kg and 200 mg/kg of the ethanolic extract respectively. Standard methods were used to determine the mitotic index and chromosomal aberrations. Results revealed that C. esculentus ethanolic extract contained flavonoids (22.47 mg/g), tannins (0.08 mg/g), alkaloids (19.71 mg/g), glycosides, phenol, and tannin and showed high scavenging activity against 2,2-diphenyl-1-picrylhydrazyland H₂O₂. Docking with RECQL5 showed good binding energies (∆G>−7) of five compounds in C. esculentus ethanolic extract. The A. cepa assay results revealed a significant (P<0.05) reduction in chromosomal aberrations and a higher mitotic index in groups treated with the C. esculentus ethanolic extract. The antimutagenic activity of C. esculentus ethanolic extract was attributed to its high levels of phytosterols and phenolic compounds.

Potent mutagenicity of an azide, 3-azido-1,2-propanediol, in human TK6 cells

Sodium azide is a strong mutagen that has been successfully employed in mutation breeding of crop plants. In biological systems, it is metabolically converted to the proximate mutagen azidoalanine, which requires further bioactivation to a putative ultimate mutagen that remains elusive. The nature of the DNA modifications induced by azides leading to mutations is also unknown. Other mutagenic organic azido compounds seem to share the same bioactivation pathway to the ultimate mutagenic species as they induce point mutations dependent on the same DNA repair pathways. We investigated mutations induced by the representative mutagen 3-azido-1,2-propanediol (azidoglycerol, AZG) in the human TK6 cell line. Until now, azides have been considered to be non-mutagens and non-carcinogens in mammals, including humans, as judged only by the conventional clastogenicity chromosomal aberration types of bioassays. Here, we show the potent mutagenicity of AZG in cultured human cells, comparable to alkylating agents such as methyl methanesulfonate at concentrations with similar lethality. The potent ability of an organic azide to induce base substitutions in a mammalian system raises an alert with respect to human exposure to organic and inorganic azido compounds.

Discovery of rare mutations in extensively pooled DNA samples using multiple target enrichment

Chemical mutagenesis is routinely used to create large numbers of rare mutations in plant and animal populations, which can be subsequently subjected to selection for beneficial traits and phenotypes that enable the characterization of gene functions. Several next-generation sequencing (NGS)-based target enrichment methods have been developed for the detection of mutations in target DNA regions. However, most of these methods aim to sequence a large number of target regions from a small number of individuals. Here, we demonstrate an effective and affordable strategy for the discovery of rare mutations in a large sodium azide-induced mutant rice population (F2 ). The integration of multiplex, semi-nested PCR combined with NGS library construction allowed for the amplification of multiple target DNA fragments for sequencing. The 8 × 8 × 8 tridimensional DNA sample pooling strategy enabled us to obtain DNA sequences of 512 individuals while only sequencing 24 samples. A stepwise filtering procedure was then elaborated to eliminate most of the false positives expected to arise through sequencing error, and the application of a simple Student's t-test against position-prone error allowed for the discovery of 16 mutations from 36 enriched targeted DNA fragments of 1024 mutagenized rice plants, all without any false calls.

Exclusive induction of G:C to A:T transitions by 3-azido-1,2-propanediol in yeast

Sodium azide is a strong mutagen which has been successfully employed in mutation breeding of crop plants. In biological systems, it is metabolized to azidoalanine, but further bioactivation to a putative ultimate mutagen as well as the nature of the induced DNA modifications leading to mutations remain elusive. In this study, mutations induced in the CAN1 gene of yeast Saccharomyces cerevisiae by the representative mutagen 3-azido-1,2-propanediol (azidoglycerol, AZG) have been sequenced. Analysis of the forward mutation spectrum to canavanine resistance revealed that AZG induced nearly exclusively G:C to A:T transitions. AZG also induced reversions to tryptophan prototrophy by base-pair substitutions in a dose-dependent manner. This unusual mutational specificity may be shared by other organic azido compounds.

Antimutagenic and antioxidant activities of some bioflavours from wine

Monoterpenes limonene and its metabolic derivatives, α-terpineol and 1,8-cineol, commonly found as aroma wine components, were studied for their antimutagenicity by the bacterial reverse mutation assay on different strains. Substances were also tested for their antioxidant activity, i.e. radical scavenger, chelation, reduction, and lipid peroxidation inhibition. Limonene and its metabolites, α-terpineol and 1,8-cineol, resulted able to inhibit the chemically-induced mutagenesis, although with a different specificity. The antimutagenicity of limonene has been generally retained by its metabolites and sometimes increased. In particular, α-terpineol exhibited the strongest inhibition, moreover it showed to be a remarkable ferrous ions chelating agent. Limonene and 1,8-cineol were devoid of antioxidant activity. Present results are a starting point in evaluating the potential of α-terpineol as a chemopreventive agent and suggest potential functional dietary benefits of wine.

Genotoxic, antigenotoxic and antioxidant properties of methanol extracts obtained from Peltigera horizontalis and Peltigera praetextata

Now-a-days, there is a big need to reduce genotoxic effects of mutagenic and carcinogenic agents in environment, which are increased by the technological development. Lichens produce a wide variety of unique metabolites due to being in various extreme areas and being symbiotic organisms of fungi and algae. Therefore, this study was planned to search new sources having antimutagenic activity by researching two different lichen species and to determine whether their usage is safe. With this respect, the mutagenic and antimutagenic properties of methanol extracts of the lichens were determined by the bacterial reverse mutation and sister chromatid exchange assays. Furthermore, the malondialdehyde level, superoxide dismutase, glutathione and glutathione peroxidase activities against aflatoxin B1 were determined for understanding the ways in which the lichens showed their genotoxic properties.

Generation of rice mutants by chemical mutagenesis

Chemical mutagenesis of rice has been used extensively to generate useful genetic variation for the purpose of breeding improved varieties. More recently, advances in high-throughput genotyping platforms have enabled the efficient detection of point mutations generated by chemical agents. This in turn has renewed interest in using traditional chemical mutagenesis to generate mutant populations for gene discovery and functional characterization. Targeting of Induced Local Lesions in Genomes (TILLING) is a powerful reverse genetics method which combines chemical mutagenesis with the high-throughput discovery of point mutations. Numerous chemical mutagens have been shown to be effective in generating point mutations and small deletions in rice. This chapter describes the use of a combination of sodium azide (NaN(3)) and N-nitroso-N-methylurea to generate populations that are suitable for TILLING as well as forward genetics and mutation breeding.

Determination of genotoxic and antigenotoxic properties of essential oil from Ferula orientalis L. using Ames/Salmonella and E. coli WP2 bacterial test systems

The essential oils having many application fields such as medicine, flavoring, cosmetics are natural products obtained from aromatic plants. As the natural products of Ferula species have a wide range of use in folk medicine, this study was planned to evaluate the mutagenic and antimutagenic activities of essential oils of leaves and flowers of Ferula orientalis grown in Erzurum, through the bacterial reverse mutation assay. Furthermore, the chemical compositions of essential oils isolated by the hyrodistillation method were analysed by gas chromatography (GC) and gas chromatography-mass spectroscopy (GC-MS), as their biological activities were connected to their contents. According to our results, any tested essential oil at any used concentration on Salmonella typhimurium TA1535 and TA1537 strains and in Escherichia coli WP2 uvrA strain showed no mutagenic activity. However, the tested materials at different concentrations showed antimutagenic activities against the used mutagens. The inhibition rates ranged against sodium azide (NaN3) on S. typhimurium TA1535 from 29% to 36%, against 9-aminoacridine (9-AA) on S. typhimurium TA1537 from 40% to 68% and against N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on E. coli WP2 uvrA from 23% to 52%, respectively. Also, it is revealed by GC and GC/MS analysis of the essential oils isolated from the leaves and flowers, respectively. The major compounds in these oils were determined as α-cadinol, δ-cadinene and germacrene D-4-ol. The results of this study indicate that as the essential oils of F. orientalis have many constituents, they show no mutagenic activity but significant antimutagenic activity, and these materials can be safely used in medicinal applications after further investigations.

Protective properties of five newly synthesized cyclic compounds against sodium azide and N-methyl-N'-nitro-N-nitrosoguanidine genotoxicity

The current study aims to determine the antimutagenic potential of five newly synthesized cyclic compounds against the genotoxic agents sodium azide (NaN₃) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The mutant bacterial tester strains were NaN₃-sensitive Salmonella typhimurium TA1535 and MNNG-sensitive Escherichia coli WP2uvrA. According to the results, all the test compounds showed significant antimutagenic activity. The inhibition rates ranged from 26.05% (Compound 4-1 µg/plate) to 68.54% (Compound 5-0.01 µg/plate) for NaN₃ and from 32.44% (Compound 3-1 µg/plate) to 60.77% (Compound 5-1 µg/plate) for MNNG genotoxicity. Moreover, the mutagenic potential of the test compounds was investigated using the same strains. The results showed that all the test compounds do not have mutagenic potential on the bacterial strains at the tested concentrations. Thus, the findings of the present study give valuable information about chemical prevention from NaN₃ and MNNG genotoxicity.

Protective effects of methanol extracts from Cladonia rangiformis and Umbilicaria vellea against known mutagens sodium azide and 9-aminoacridine

Lichens and their various extracts have been occasionally used in the treatment of many diseases. Cladonia rangiformis and Umbilicaria vellea are two important species of these lichens and they have several biological activities. In the present study, methanol extracts of these lichens, which are grown in the Eastern Anatolia Region of Turkey, were isolated, and their mutagenic and antimutagenic properties were investigated by using AMES-Salmonella and Zea mays Root Tip Mitotic Index mutagenicity and antimutagenicity assay systems. Known mutagens sodium azide (NaN3) and 9-Aminoacridine (9-AA) were used to determine antimutagenic properties of methanol extracts. The results showed that all methanol extracts, investigated in the present study, can be considered genotoxically safe because they do not have mutagenic activity at the tested concentrations. Besides, all of them have antimutagenic activity against 9-AA known as a model intercalator agent in the AMES-Salmonella test system. The inhibition rates obtained from the antimutagenicity assays ranged from 37.07% (C. rangiformis—5 µg/plate) to 54.39% (C. rangiformis—5 µg/plate). Furthermore, all the methanol extracts have significant antimutagenic activity against NaN3 mutagenicity in Z. mays Root Tip Mitotic Index assay system. These activities are valuable towards an extension of the employ of these drugs as new phytotherapeutic or preservative ingredients.

Mutagenic and antimutagenic effects of hexane extract of some Astragalus species grown in the eastern Anatolia region of Turkey

Medical plants and their various extracts have been occasionally used in the treatment of many diseases. Astragalus is one of those medical plants and it has several biological activities. In the present study, the hexane extracts of six Astragalus species, which are grown in the eastern Anatolia region of Turkey, were isolated, and their mutagenic and antimutagenic properties were investigated by using Salmonella typhimurium TA1535, TA1537 and Escherichia coli WP2uvrA tester strains at 0.05, 0.5 and 5 microg/plate concentrations. Known mutagens sodium azide (NaN(3)), 9-Aminoacridine (9-AA) and N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) were used to determine antimutagenic properties of hexane extracts. The results showed that all hexane extracts, investigated in the present study, can be considered genotoxically safe because they do not have mutagenic activity at the tested concentrations. But, a great many of them have antimutagenic activity against 9-Aminoacridine known as a model intercalator agent. The inhibition rates obtained from the antimutagenicity assays ranged from 27.51% (A. macrocephalus--0.05 microg/plate) to 54.39% (A. galegiformis--5 microg/plate). These activities are valuable toward an extension of the employ of these drugs as new phytotherapeutic or preservative ingredients.

TILLMore, a resource for the discovery of chemically induced mutants in barley

A sodium azide-mutagenized population of barley (cv. 'Morex') was developed and utilized to identify mutants at target genes using the 'targeting induced local lesions in genomes' (TILLING) procedure. Screening for mutations at four agronomically important genes (HvCO1, Rpg1, eIF4E and NR) identified a total of 22 new mutant alleles, equivalent to the extrapolated rate of one mutation every 374 kb. All mutations except one were G/C to A/T transitions and several (approximately 68%) implied a change in protein amino acid sequence and therefore a possible effect on phenotype. The high rate of mutation detected through TILLING is in keeping with the high frequency (32.7%) of variant phenotypes observed amongst the M(3) families. Our results indicate the feasibility of using this resource for both reverse and forward genetics approaches to investigate gene function in barley and related crops.

Antimutagenic potential of curcumin on chromosomal aberrations in Allium cepa

Turmeric has long been used as a spice and food colouring agent in Asia. In the present investigation, the antimutagenic potential of curcumin was evaluated in Allium cepa root meristem cells. So far there is no report on the biological properties of curcumin in plant test systems. The root tip cells were treated with sodium azide at 200 and 300 microg/ml for 3 h and curcumin was given at 5, 10 and 20 microg/ml for 16 h, prior to sodium azide treatment. The tips were squashed after colchicine treatment and the cells were analyzed for chromosome aberration and mitotic index. Curcumin induces chromosomal aberration in Allium cepa root tip cells in an insignificant manner, when compared with untreated control. Sodium azide alone induces chromosomal aberrations significantly with increasing concentrations. The total number of aberrations was significantly reduced in root tip cells pretreated with curcumin. The study reveals that curcumin has antimutagenic potential against sodium azide induced chromosomal aberrations in Allium cepa root meristem cells. In addition, it showed mild cytotoxicity by reducing the percentage of mitotic index in all curcumin treated groups, but the mechanism of action remains unknown. The antimutagenic potential of curcumin is effective at 5 microg/ml in Allium cepa root meristem cells.

Effects of chaotropic anions on the distribution of conformational substates of amicyanin, wild type and Cys3Ala/Cys26Ala azurin mutant

The effect of azide and thiocyanate on the structure and dynamics of wild type and disulfide bond depleted azurin and of amicyanin has been investigated by electron paramagnetic resonance (EPR) spectroscopy at low temperature. The analysis of the EPR spectra, which can be described in terms of Gaussian distributions of the components of the axial symmetric <--> g and <--> A tensors of the spin-Hamiltonian, has shown that the two small exogenous ligands, known as chaotropic agents, are effective in reducing the structural heterogeneity of the proteins. Such a reduction, quantified by the standard deviations sigma(g axially) and sigma(A axially) and obtained by simulation of the experimental EPR spectra, depends on azide and thiocyanate concentration in solution. In particular, the comparison of the sigma(g axially) and sigma(A axially) values found for the protein samples investigated points out that the lower the protein to anion molar ratios (1:50; 1:100) are, the more marked the reduction in structural heterogeneity is. The thiocyanate effect is stronger than the azide one. Furthermore, the reduction in structural heterogeneity is more marked in the azurins than in amicyanin and the Cys3Ala/Cys26Ala azurin mutant is less flexible compared to the wild-type protein. The effect observed upon N(-)(3) and SCN(-) addition in solution is very similar to that observed when glycerol is added to the solution, suggesting that such perturbing agents behave like cryoprotectors, affecting the protein-solvent interactions in such a way as to suppress the large amplitude motions.

Azide-resistant mutants of Azorhizobium caulinodans with enhanced symbiotic effectiveness

Azide-resistant mutants of Azorhizobium caulinodans strains Sb3, S78, SrR13 and SrS8 were isolated and screened for nitrate reductase activity. Selected nitrate reductase negative mutants were inoculated on Sesbania bispinosa and S. rostrata under sterile conditions in chillum jars to study their symbiotic behavior. Azide-resistant mutants exhibited either similar or higher symbiotic effectiveness than the parent strain after 30 d of plant growth. Nodule mass, nitrogenase activity and uptake hydrogenase activity of the mutants varied depending on the host as well as on the plant growth stage. In comparison to wild-type parent strains, four azide-resistant mutants, Sb3Az18, S78Az21, SrR13Az17 and SrS8Az6 showed significant increase in nodulation and nitrogen fixation as well as shoot dry mass of the inoculated plants.

Mutagenicity of sodium azide and its metabolite azidoalanine in Drosophila melanogaster

The mutagenic and toxic activities of sodium azide (NaN(3) ) and its organic metabolite L-azidoalanine [N(3)-CH(2)-CH(NH)(2)-COOH] were examined in the different stages of spermatogenesis in Drosophila melanogaster. Both azide and azidoalanine were toxic to the injected males, but azidoalanine was significantly less toxic than sodium azide. Following the injection with 0.2 microl of these compounds in the hemocoel of young adult wild-type males, the minimum concentrations of these compounds with complete toxic effects (zero survival) were 40 mM sodium azide and 160 mM azidoalanine. Sex-linked recessive lethals were scored by the Muller-5 method in three successive broods, representing sperms (brood A), spermatids (brood B), and a compiled group of meiotic and premeiotic germ cell stages (brood C). The results provide strong experimental evidence that azidoalanine is significantly (p<0.01) mutagenic to all stages of spermatogenesis in Drosophila melanogaster. Sodium azide, however, was not significantly (p>0.05) mutagenic and did not increase the rate of sex-linked recessive lethals over those produced by the control group injected with 0.45% NaCl. These results indicate the requirement of metabolic activation of azide in Drosophila as a prerequisite for its mutagenic effects.

Mutation induction and mutation spectra of S. typhimurium TA100 after exposure to isohistidines

L-Isohistidine and D,L-isohistidine, but not D-isohistidine, caused an increase of the number of mutant colonies in S. typhimurium strain TA100. Spontaneous and also sodium azide or 2-aminoanthracene induced mutant numbers were enhanced by L-isohistidine and by an isomeric mixture of D,L- and L-isohistidine. These effects could not be attributed to a growth-enhancing property. The colony probe hybridization procedure was used to investigate the effects of the histidines on the spontaneous and azide-induced spectra of the hisG46 allele in strain TA100. D,L-Isohistidine, but not the D-isomer, caused and increase of transitions (CCC-->CTC) and transversions (CCC-->CAC) in the spontaneous spectrum. Sodium azide alone induced a strong increase of CCC-->CTC transitions; combination with the D,L-isohistidine led to a further enhancement of this type of base substitutions, whereas with the L-isomer, no such effect was observed. This supports the hypothesis that the activity of D,L-isohistidine is probably not due to DNA-damaging properties, but rather to indirect mechanisms, such as enhancement of the infidelity of DNA replication and/or interference with DNA-repair or proofreading functions.

Studies on the synthesis of the Fe-S cluster of dihydroxy-acid dehydratase in escherichia coli crude extract. Isolation of O-acetylserine sulfhydrylases A and B and beta-cystathionase based on their ability to mobilize sulfur from cysteine and to participate in Fe-S cluster synthesis

The apoprotein of Escherichia coli dihydroxy-acid dehydratase, which contains a catalytically essential [4Fe-4S] cluster in its active form, has been used as a substrate to investigate Fe-S cluster synthesis. The inactive apoprotein could be reactivated in vitro by factors present in the crude extract of E. coli and to a much smaller extent in the presence of Fe3+, S2-, and dithiothreitol. This reactivation occurs as a result of Fe-S cluster synthesis. It is anticipated that the Fe-S cluster synthesis observed in crude extracts in vitro may involve some of the components that participate in Fe-S cluster synthesis in vivo. The origin of the sulfur used to form Fe-S clusters was investigated. Four enzymatic activities in the crude extract of E. coli were found that can provide sulfur for Fe-S cluster synthesis in vitro by mobilizing the sulfur from cysteine. The purification of the proteins responsible for three of these activities is reported in this paper. The three proteins have been identified as O-acetylserine sulfhydrylase A, O-acetylserine sulfhydrylase B, and beta-cystathionase. The rate and extent of sulfide mobilization from cysteine in the reaction catalyzed by O-acetylserine sulfhydrylases A and B depend on the presence of nucleophiles that can add to the aminoacrylate formed on the enzyme following the removal of sulfide from cysteine. A new amino acid is formed when the nucleophiles add to the aminoacrylate. Sulfur mobilization by beta-cystathionase does not require a nucleophile, and the reaction is a minor variation on the cleavage of beta-cystathionine, with pyruvate, ammonia, and sulfide being the products. Once sulfur is mobilized by these enzymes, its efficient use in Fe-S cluster synthesis seems to be affected by the presence of yet unidentified factors present in crude extract. In crude extract and partially purified preparations from E. coli where these factors are present, the rapidity with which Fe-S clusters are formed and the efficiency with which sulfur is used imply an orderly controlled formation of Fe-S clusters that is generally typified by enzymatic reactions.

Comparative mutagenicity of chemicals selected for test in the International Program on Chemical Safety's collaborative study on plant systems for the detection of environmental mutagens

A review has been made for the four compounds (maleic hydrazide, methyl nitrosourea, sodium azide, azidoglycerol) tested in the International Program on Chemical Safety's collaborative study on plant systems. Maleic hydrazide (MH) is a weak cytotoxic/mutagenic chemical in mammalian tissues and is classified as a class 4 chemical. In contrast, with few exceptions such as Arabidopsis, MH is a potent mutagen/clastogen in plant systems. The difference in its response between plant and animal tissue is likely due to differences in the way MH is metabolized. MH appears to be noncarcinogenic and has been given a negative NCI/NTP carcinogen rating. Methyl nitrosourea (MNU) is a toxic, mutagenic, radiomimetic, carcinogenic, and teratogenic chemical. It has been shown to be a mutagen in bacteria, fungi, Drosophila, higher plants, and animal cells both in vitro and in vivo. MNU is a clastogen in both animal and human cell cultures, plant root tips and cell cultures inducing both chromosome and chromatid aberrations as well as sister-chromatid exchanges. Carcinogenicity has been confirmed in numerous studies and involves the nervous system, intestine, kidney, stomach, bladder and uterus, in the rat, mouse, and hamster. MNU produces stage-specific teratogenic effects and also interferes with embryonic development. The experimental evidence that strongly indicates the mutagenic effects of MNU underlines the possible hazard of this compound to human beings. The experimental evidence for the stringent handling of this compound is clear. Sodium azide (NaN3) is cytotoxic in several animal and plant systems and functions by inhibiting protein synthesis and replicative DNA synthesis at low dosages. It is mutagenic in bacteria, higher plants and human cells and has been used as a positive control in some systems. In general, tests for clastogenicity have been negative or weakly positive. No evidence of carcinogenicity has been reported in a 2-year study seeking carcinogenic activity in male and female rats. Its advantages in comparison to other efficient mutagens are claimed to be a high production of gene mutations accompanied by a low frequency of chromosomal rearrangements and safer handling because of its nonclastogenic and noncarcinogenic action on humans. Azidoglycerol (AG) is a very potent mutagen in bacteria, yeast and higher plants including Arabidopsis and Tradescantia; however, it only slightly enhances the frequencies of recessive lethals in Drosophila. AG is at best a weak clastogen and is without effect in inducing chromosomal aberrations and SCEs in human peripheral lymphocytes in vitro. In microbial and plant systems, AG is considerably more potent than sodium azide in the maximal frequencies of mutation induced. In particular, in Saccharomyces cerevisae, AG is 3000-fold more mutagenic than sodium azide. Its carcinogenic and teratogenic properties are unknown.

Effects of deuterium labeling on azido amino acid mutagenicity in Salmonella typhimurium

The mutagenic effects of azide (N3-) anion in bacterial test systems require the formation of the novel mutagenic metabolite, 3-azido-L-alanine (AZAL). Although the mechanism of AZAL-induced mutagenicity is unknown, subsequent bioactivation of this metabolite appears likely. Earlier studies have shown that other azide-containing amino acids are mutagenic as well. In fact, the mutagenic potency of the synthetic AZAL homologue, L-2-amino-4-azidobutanoic acid (HomoAZAL), was several times that of AZAL. To gain insight into the biochemical processing and mutagenicity of azido amino acids in Salmonella typhimurium, several specifically deuterium-labeled azido amino acids have been prepared and tested for mutagenic potency. In addition, the effect of (aminooxy)acetic acid (AOA) (a potent inhibitor of pyridoxal-dependent processes) on AZAL-induced mutagenesis was examined. The results showed that 2-deuterium substitution of AZAL resulted in a slight increase in mutagenic potency, while AOA treatment resulted in no change in AZAL potency. Taken together these findings did not support the involvement of pyridoxal-dependent processes in AZAL bioactivation. In contrast, deuterium substitution adjacent to the azide group in HomoAZAL and 5-azido-L-norvaline (N3-Norval) resulted in a large decrease in mutagenic potency when compared to the non-deuterium labeled compounds. These observations are consistent with a bioactivation mechanism involving rate-limiting C-H bond cleavage in the formation of the ultimate mutagen. Moreover, this effect of deuterium labeling points to processing of the azide-containing side chain as a key feature in the mutagenic activation mechanism.

Mutation spectrum of a binary mixture of mutagens (methapyrilene and sodium azide) in strain TA1535 of Salmonella

Methapyrilene (MP) is a rat-liver carcinogen and cocarcinogen that exhibits a narrow spectrum of mutagenic activity in Salmonella typhimurium, inducing only a 2-fold increase in revertants only in the base-substitution strain TA1535; it also enhances the mutagenic activity of sodium azide (NaN3) in the same strain. To examine the effects of MP at the molecular level, we used the colony probe hybridization procedure developed by Cebula and Koch (Mutation Res., 229 (1990) 79-87) to identify the base substitutions in approximately 800 background, MP-, NaN3-, and MP + NaN3-induced revertants of the hisG46 allele of strain TA1535. The predominant mutation in all 4 mutation spectra was a CCC-->CTC transition. The results suggest a mechanism by which MP enhances the infidelity of the DNA replication complex or inhibits a DNA repair or proofreading function, resulting in the production of more of the same error that occurs normally and that is also induced by NaN3. Such a mechanism might be the basis for the carcinogenic and cocarcinogenic activities of MP. To our knowledge, this is the first report of the molecular analysis of mutants produced by exposure of cells to a binary mixture of mutagens.

Mutagenicity of 3-azido-1,2-propanediol and 9-(3-azido-2-hydroxypropyl)-adenine in repair deficient strains of Escherichia coli

The mutagenicity of two non-aromatic organic azido compounds, 3-azido-1,2-propanediol (AG) and 9-(3-azido-2-hydroxypropyl)-adenine (AHPA), was studied in E. coli repair deficient strains uvrA6, uvrA6 + umuC36, uvrA6+ umuC122::Tn5, polA1, tagA1+ alkA1, ada and dam3. The mutagenicity of both agents was markedly enhanced by defects of UvrABC excinuclease (uvrA6) and was independent of umuC function of the SOS error-prone pathway. Neither azido compound promoted umuDC operon expression. The mutagenicity of AG in tag A1, alkA1 and ada mutants does not differ from that found in the wild-type strain. The expression of both ada and alkA genes was not elevated by AG. Experiments on polA1 and dam3 mutants suggest that DNA polymerase I as well as the mutHLS mismatch repair pathway does not contribute to the removal of putative DNA lesions induced by AG.

Sodium azide mutagenesis: preferential generation of A.T-->G.C transitions in the barley Ant18 gene

The molecular basis for the absence of anthocyanins and proanthocyanidins in four independent sodium azide-induced ant18 mutants of barley was examined by sequencing the gene encoding dihydroflavonol 4-reductase in these mutants. Sodium azide generated 21 base substitutions, which corresponds to 0.17% of the 12,704 nucleotides sequenced. Of the substitutions, 86% were nucleotide transitions, and 14% were transversions. A.T-->G.C base pair transitions were about 3 times more frequent than G.C-->A.T transitions. No deletions or mutation hot spots were found. The absence of dihydroflavonol 4-reductase activity in ant18-159, ant18-162, and ant18-164 plants is caused by missense mutations in the respective genes. By using microprojectile bombardment, a plasmid harboring the wild-type Ant18 gene was introduced into ant18-161 mutant cells and resulted in the development of anthocyanin pigmentation, which demonstrates that the mutation is corrected by expression of the introduced gene. On the other hand, a plasmid derivative with the two ant18-161-specific base transitions at the 5' splice site of intron 3 prevented complementation. It is concluded that the absence of detectable mRNA for dihydroflavonol 4-reductase in ant18-161 cells is due to the mutations in the pre-mRNA splice donor site.

The lack of L-azidoalanine interaction with DNA. In vitro studies

A Bacillus subtilis transformation system was used to investigate the possible direct effect of L-azidoalanine on DNA in vitro. A B. subtilis (trp-) deletion and repair deficient (uvr-) strain was constructed and used as a recipient for treated DNA. The data obtained indicate that L-azidoalanine, at the described conditions, does not interact with DNA in vitro. Thus, L-azidoalanine failed to produce any DNA damage even in the absence of an excision repair mechanism.

Prophage induction by DNA topoisomerase II poisons and reactive-oxygen species: role of DNA breaks

Various compounds were evaluated for their ability to induce prophage lambda in the Escherichia coli WP2s(lambda) microscreen assay. The inability of a DNA gyrase subunit B inhibitor (novobiocin) to induce prophage indicated that inhibition of the gyrase's ATPase was insufficient to elicit the SOS response. In contrast, poisons of DNA gyrase subunit A (nalidixic acid and oxolinic acid) were the most potent inducers of prophage among the agents examined here. This suggested that inhibition of the ligation function of subunit A, which also has a DNA nicking activity, likely resulted in DNA breaks that were available (as single-stranded DNA) to act as strong SOS-inducing signals, leading to prophage induction. Agents that both intercalated and produced reactive-oxygen species (the mammalian DNA topoisomerase II poisons, adriamycin, ellipticine, and m-AMSA) were the next most potent inducers of prophage. Agents that produced reactive-oxygen species only (hydrogen peroxide and paraquat) were less potent than adriamycin and ellipticine but more potent than m-AMSA. Agents that intercalated but did not generate reactive-oxygen species (actinomycin D) or that did neither (teniposide) were unable to induce prophage, suggesting that intercalation alone may be insufficient to induce prophage. These results illustrate the variety of mechanisms (and the relative effectiveness of these mechanisms) by which agents can induce prophage. Nonetheless, these agents may induce prophage by producing essentially the same type of DNA damage, i.e., DNA strand breaks. The potent genotoxicity of the DNA gyrase subunit A poisons illustrates the genotoxic consequences of perturbing an important DNA-protein complex such as that formed by DNA and DNA topoisomerase.

Antimutagenicity of propionic acid bacteria

The antimutagenic effect of dialysed cell extracts of 4 strains of propionic acid bacteria was examined against the mutagenicity of sodium azide in the TA1535 tester strain of Salmonella typhimurium using the Ames test. It was noted that dialysates of 2 strains of Propionibacterium shermanii, P. pentosaceum and P. acnes, significantly reduced sodium azide-induced revertants. The dialysate of propionic acid cocci did not show an antimutagenic effect. The inhibitory activity was enhanced if the mutagen and extract were coincubated for 20 min prior to performing the mutagenicity assay. Antimutagenicity of dialysates from P. shermanii VKM-103 against MNNG and 9-aminoacridine was shown in S. typhimurium strains TA1535 and TA97. The antimutagenic activity was found in the protein fraction of the cell extract of P. shermanii. The proteins of the dialysate of P. shermanii were separated using a Toyopearl gel column into 3 main peaks according to their molecular weights. The antimutagenic activity towards sodium azide was found in the second and the third peaks. We suggest that dialysates of the cells of propionic acid bacteria contain several kinds of antimutagenic substances with different molecular weights.

Mutagenic activity of 6-azido deoxyhexoses and azido alcohols in Salmonella typhimurium and its inhibition by a structure-similar carbon source in the medium

6-Azido-6-deoxy (AZd) derivatives of D-glucose, D-mannose, D-altrose, D-allose, L-idose, D-galactose, D-galactonic acid and D-galactitol, 3-azido-1,2-propanediol (azidoglycerol), 3,1-diazido-2-propanol (diazidoglycerol) and (at much higher doses) 2-azidoethanol were mutagenic in Salmonella typhimurium strains TA100 and TA1535. The mutagenic response was similar to that induced by sodium azide, i.e., the azido compounds failed to induce mutations in strain TA98, and mutagenesis was independent of plasmid pKM101, and independent of external activation. The specific mutagenicity (his+ rev/mmole) of AZd-glucose and AZd-galactose was decreased with increasing concentrations of D-glucose or D-galactose in the minimal agar medium and enhanced 100-fold or more when 0.2% citrate rather than 0.2% glucose served as the carbon source in the medium. Similarly, the mutagenic efficiency of azidoglycerol was inhibited by glycerol but not by D-glucose or D-galactose; however, the mutagenicity of sodium azide was not influenced by any of these carbon sources in the medium. The inhibition of the mutagenic action of azido hexoses and azido alcohols by non-azido structural analogs is assumed to reside in competition in transmembrane transport or for the metabolic pathways.

Structure of the Hordeum vulgare gene encoding dihydroflavonol-4-reductase and molecular analysis of ant18 mutants blocked in flavonoid synthesis

A full-length cDNA clone encoding barley dihydroflavonol-4-reductase was isolated from a kernel-specific cDNA library by screening with the cDNA of the structural gene (A1) for this enzyme from maize. Subsequently, the gene corresponding to the barley dihydroflavonol-4-reductase cDNA was cloned and sequenced. The gene contains three introns at the same positions as in the Zea mays gene, corresponding to the positions of the first three of the five introns present in the genes of Petunia hybrida and Antirrhinum majus. In vitro transcription and translation of the Hordeum vulgare cDNA clone yielded a protein which converts dihydroquercetin into 2,3-trans-3,4-cis-leucocyanidin with NADPH as cofactor. The protein has a deduced amino acid sequence of 354 residues and a molecular weight of 38,400 daltons. Dihydroflavonol reductases of barley, maize, petunia and snapdragon are highly polymorphic in the NH2- and C-terminal parts of the polypeptide chain while a central region of 324 residues contains 51% identical amino acids. This identity increases to 81% when only the barley and maize enzymes are compared. Recessive mutants in the Ant18 gene tested so far lack dihydroflavonol-4-reductase activity and accumulate small amounts of dihydroquercetin but have retained activity for at least two other enzymes in the flavonoid pathway. In testa-pericarp tissue of mutants ant18-159, ant18-162 and ant18-164, wild-type levels of steady state mRNA for dihydroflavonol reductase have been measured, while mRNA for this enzyme is not transcribed in mutant ant18-161. These data are consistent with the proposal that the Ant18 locus carries the structural gene for dihydroflavonol-4-reductase of barley.

Medicinal azides. Part 8. The in vitro metabolism of p-substituted phenyl azides

1. A series of p-substituted aromatic azides was synthesized and their metabolism investigated in suspensions of mouse liver microsomes and mouse hepatocytes. Metabolite analysis was performed by h.p.l.c. 2. On incubation with microsomes under anaerobic conditions p-nitro, p-cyano- and p-chlorophenyl azide afforded metabolites which co-chromatographed with the respective aromatic amines. The rate at which p-nitrophenyl azide was metabolically reduced was approximately 20-fold that observed for p-cyano- and p-chlorophenyl azide. 3. Phenyl azide, p-methoxyphenyl azide and the aliphatic congener, phenethyl azide, did not furnish detectable amounts of metabolites on incubation with microsomes under anaerobic conditions. When phenyl azide and p-methoxyphenyl azide were incubated with hepatocytes or microsomes under aerobic conditions the resulting chromatograms furnished peaks which co-eluted with authentic p-hydroxyphenyl azide. 4. The microsomal reduction of p-nitrophenyl azide was dependent upon the presence of viable microsomes and NADPH, and on the absence of oxygen above the incubation medium.

Cloning of the E. coli O-acetylserine sulfhydrylase gene: ability of the clone to produce a mutagenic product from azide and O-acetylserine

The gene coding for O-acetylserine sulfhydrylase (OASS) from E. coli K12 was cloned into the vector pBR322 plasmid and expressed in a cysk mutant strain of E. coli that is deficient in O-acetylserine sulfhydrylase (OASS-). The clone containing the OASS gene was selected by using tetracycline-ammonium bismuth citrate medium. Retransformation of the hybrid plasmid into competent cysk mutant cells resulted in the recovery of a clone containing normal levels of O-acetylserine sulfhydrylase. Negative selection of retransformed cysk cells on 1,2,4-triazole plates resulted in the complete inhibition of growth indicating the presence of a functional OASS gene. The ability of the new clone to convert azide to its mutagenic metabolite was tested. Cultures of the clone cells containing significant levels of OASS activity were able to produce a mutagenic product from azide and O-acetylserine as tested on Salmonella typhimurium TA1530. This cloning method could be applied also to clone the same gene from eukaryotic sources.

Mutations conferring resistance to azide in Escherichia coli occur primarily in the secA gene

Mutant strains of Escherichia coli were screened for the ability to grow on L agar plates containing 3.4 or 4.6 mM sodium azide. Most mutants had mutations located in the leucine region, presumably at the azi locus. Two of these mutants were found to have a mutation in the secA gene, but expression of the resistance phenotype also required the presence of upstream gene X. While a plasmid carrying the X-secA mutant gene pair was able to confer azide resistance to a sensitive host, a similar plasmid harboring the wild-type secA allele rendered a resistant strain sensitive to azide, indicating codominance of the two alleles. That azide inhibits SecA is consistent with the fact that SecA has ATPase activity, an activity that is often prone to inhibition by azide.

Differential metabolism of sodium azide in maize callus and germinating embryos

Sodium azide is a potent mutagen of maize (Zea mays L.) kernels that may have potential as a point mutagen for inducing biochemical mutations in maize tissue cultures. Azide mutagenicity was evaluated in friable, embryogenic maize callus and a nonregenerable maize suspension culture by determining the number of resistant variant cell lines able to grow on media containing inhibitory concentrations of lysine plus threonine (LT). The number of LT-resistant variants selected from either culture type did not increase in response to azide treatment. In addition, there was no increase in somatic mutations in more than 100 plants regenerated from azide treated LT-resistant lines. The levels of mutagenic metabolite of azide (presumably azidoalanine), were determined by bioassay in the two azide-treated maize callus types and compared to levels of mutagenic metabolite in embryos isolated from azide-treated kernels. The two types of maize tissue cultures and isolated embryos contained similar levels of mutagenic metabolite 4 h after azide treatment indicating similar uptake and conversion of azide to mutagenic metabolite in the three tissues. Mutagenic metabolite in azide-treated embryos did not significantly decrease after 40 h. However, mutagenic metabolite levels in both azide-treated tissue cultures decreased to near background levels within 20 h providing evidence for rapid metabolism of the azide mutagenic metabolite. The lack of evidence for azide mutagenicity in maize callus and its known potent mutagenicity in kernels appears to be associated with specific differences in azide metabolism between callus tissues and kernel embryos.

Mutagenic activity of promutagens in plants: indirect evidence of their activation

This review summarizes data concerning mutagenic activity of promutagens in various plant in vivo assays. These data are compared with the present knowledge about the metabolism of xenobiotics and activation of promutagens in plants obtained by biochemical studies and by the separation of the activation process from the genetic endpoints assayed for the mutagenicity. The article documents a differential response of plant species in the endogenous transforming of various classes of promutagens into mutagens. Attention is devoted to the following types of promutagens: nitrosamines, polycyclic aromatic hydrocarbons and aromatic amines, aflatoxins, pyrrolizidine alkaloids, diallate, styrene, vinylchloride, ethanol, cycasin, nitrofurans, sodium azide, s-triazine herbicides, 1,2-dibromoethane and maleic hydrazide.