DNA strand breaking by the carbon-centered radical generated from 4-(hydroxymethyl) benzenediazonium salt, a carcinogen in mushroom Agaricus bisporus

Nodding syndrome: A key role for sources of nutrition?

Nodding Syndrome (NS) has occurred among severely food-stressed communities in northern Uganda and several other East African populations that, with their forced physical displacement, have resorted to nutritional support from available wild plants and fungi, some of which have neurotoxic potential. Among the latter is an agaric mushroom with an unknown content of hydrazine-generating agaritine, namely Agaricus bingensis, the unusually wide consumption of which may relate to the low serum levels of vitamin B6 in Ugandan NS subjects relative to controls. Hydrazine-related compounds induce patterns of DNA damage that promote neuropathological changes (tauopathy) reminiscent of those associated with established NS. While the cause of this childhood brain disease is unknown, we encourage increased attention to the role of malnutrition and B6 hypovitaminosis in the etiology of this devastating brain disease.

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Idiopathic epileptic encephalopathy with tauopathy (Nodding syndrome) impacts East African children

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Associated factors include nematode infection, food insecurity, and food use of wild plants and fungi

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Food use of hydrazinic fungi induces B6 hypovitaminosis, which may be marked in Nodding syndrome

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Vitamin B6 deficiency promotes tau phosphorylation in mouse models of human tauopathy

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Hydrazine generates carbon free radicals associated with DNA-damage and neurodegenerative disease

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Increased research attention to nutritional practices associated with Nodding syndrome is merited.

Direct Intracellular Delivery of Benzene Diazonium Ions As Observed by Increased Tyrosine Phosphorylation

A challenge within the field of bioconjugation is developing probes to uncover novel information on proteins and other biomolecules. Intracellular delivery of these probes offers the promise of giving relevant context to this information, and these probes can serve as hypothesis-generating tools within complex systems. Leveraging the utility of triazabutadiene chemistry, herein, we discuss the development of a probe that undergoes reduction-mediated deprotection to rapidly deliver a benzene diazonium ion (BDI) into cells. The intracellular BDI resulted in an increase in global tyrosine phosphorylation levels. Seeing phosphatase dysregulation as a potential source of this increase, a tyrosine phosphatase (PTP1B) was tested and shown to be both inhibited and covalently modified by the BDI. In addition to the expected azobenzene formation at tyrosine side chains, key reactive histidine residues were also modified.

Warning on False or True Morels and Button Mushrooms with Potential Toxicity Linked to Hydrazinic Toxins: An Update

Recently, consumption of the gyromitrin-containing neurotoxic mushroom Gyromitra sp. (false morel), as gourmet food was hypothesized to play a role in sporadic amyotrophic lateral sclerosis genesis. The present review analyses recent data on edibility and toxicity of false and true morels and Agaricus spp. Controversy about the toxic status of Gyromitra esculenta was due to variable toxin susceptibility within consumers. We suggest that Verpa bohemica, another false morel, is also inedible. We found a temporary neurological syndrome (NS) with cerebellar signs associated with high consumption of fresh or dried true morels Morchella sp. After ingestion of crude or poorly cooked fresh or dried morels, a gastrointestinal “haemolytic” syndrome was also observed. Agaritine, a water soluble hydrazinic toxin closely related to gyromitrin is present along with metabolites including diazonium ions and free radicals, in Agaricus spp. and A. bisporus, the button mushroom, and in mice after ingestion. It is a potential weak carcinogen in mice, but although no data are available for humans, a lifetime low cumulative extra cancer risk in humans can be estimated to be about 10⁻⁵. To conclude, a safety measure is to avoid consuming any true morels or button mushrooms when crude or poorly cooked, fresh or dried.

Benzotriazine Di-Oxide Prodrugs for Exploiting Hypoxia and Low Extracellular pH in Tumors

Extracellular acidification is an important feature of tumor microenvironments but has yet to be successfully exploited in cancer therapy. The reversal of the pH gradient across the plasma membrane in cells that regulate intracellular pH (pHi) has potential to drive the selective uptake of weak acids at low extracellular pH (pHe). Here, we investigate the dual targeting of low pHe and hypoxia, another key feature of tumor microenvironments. We prepared eight bioreductive prodrugs based on the benzotriazine di-oxide (BTO) nucleus by appending alkanoic or aminoalkanoic acid sidechains. The BTO acids showed modest selectivity for both low pHe (pH 6.5 versus 7.4, ratios 2 to 5-fold) and anoxia (ratios 2 to 8-fold) in SiHa and FaDu cell cultures. Related neutral BTOs were not selective for acidosis, but had greater cytotoxic potency and hypoxic selectivity than the BTO acids. Investigation of the uptake and metabolism of representative BTO acids confirmed enhanced uptake at low pHe, but lower intracellular concentrations than expected for passive diffusion. Further, the modulation of intracellular reductase activity and competition by the cell-excluded electron acceptor WST-1 suggests that the majority of metabolic reductions of BTO acids occur at the cell surface, compromising the engagement of the resulting free radicals with intracellular targets. Thus, the present study provides support for designing bioreductive prodrugs that exploit pH-dependent partitioning, suggesting, however, that that the approach should be applied to prodrugs with obligate intracellular activation.

Reasons for the yellowness of photocured samples by the benzophenone/1,3-benzodioxole photoinitiating system

Reasons for the yellowness in samples photocured by benzophenone/1,3-benzodioxole.

Characterisation of radicals formed by the triazine 1,4-dioxide hypoxia-activated prodrug, SN30000

The radical species underlying the activity of the bioreductive anticancer prodrug, SN30000, have been identified by electron paramagnetic resonance and pulse radiolysis techniques. Spin-trapping experiments indicate both an aryl-type radical and an oxidising radical, trapped as a carbon-centred radical, are formed from the protonated radical anion of SN30000. The carbon-centred radical, produced upon the one-electron oxidation of the 2-electron reduced metabolite of SN30000, oxidises 2-deoxyribose, a model for the site of damage on DNA which leads to double strand breaks. Calculations using density functional theory support the assignments made.

Characterization of radicals formed following enzymatic reduction of 3-substituted analogues of the hypoxia-selective cytotoxin 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine)

The mechanism by which the 1,2,4-benzotriazine 1,4-dioxide (BTO) class of bioreductive hypoxia-selective prodrugs (HSPs) form reactive radicals that kill cancer cells has been investigated by steady-state radiolysis, pulse radiolysis (PR), electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. Tirapazamine (TPZ, 3-amino BTO, 1) and a series of 3-substituted analogues, -H (2), -methyl (3), -ethyl (4), -methoxy (5), -ethoxymethoxy (6), and -phenyl (7), were reduced in aqueous solution under anaerobic steady-state radiolysis conditions, and their radicals were found to remove the substrates by short chain reactions of different lengths in the presence of formate ions. Multiple carbon-centered radical intermediates, produced upon anaerobic incubation of the compounds with cytochrome P(450) reductase enriched microsomes, were trapped by N-tert-butyl-alpha-phenylnitrone and observed using EPR. The highly oxidizing oxymethyl radical, from compound 5, was identified, and experimental spectra obtained for compounds 1, 2, 3, and 7 were well simulated after the inclusion of aryl radicals. The identification of a range of oxidizing radicals in the metabolism of the BTO compounds gives a new insight into the mechanism by which these HSPs can cause a wide variety of damage to biological targets such as DNA.

The metabolisms of agaritine, a mushroom hydrazine in mice

The mushroom hydrazine agaritine was measured in mouse plasma and urine using LC/MS/MS, which is highly specific. Agaritine concentration peaked 20 min after oral administration to mice (4.0 and 40 mg/kg). The concentration gradually decreased and returned to the basal level in 100 min. The maximum concentration, the time to the maximum concentration, and the half life were 0.37 microg/ml plasma, 0.33 h, and 0.71 h, respectively after administration of agaritine at 40 mg/kg body weight. One agaritine metabolite was found in the plasma and the urine from agaritine-administered mice. The structure of metabolites of agaritine by gamma-GT was next investigated using LC/MS. HMPH proved to be generated from agaritine. The oxidative stress marker 8-OHdG was detected in agaritine-administered mouse urine. After administration, the 8-OHdG level immediately tripled, and then decreased to the control level over 48 h. Its level then elevated again and remained high for 11 days. These results suggest that agaritine quickly metabolizes and disappears in the plasma, whereas DNA damage lasts for a long time after a single administration of agaritine to mice.

Radical production and DNA damage induced by carcinogenic 4-hydrazinobenzoic acid, an ingredient of mushroom Agaricus bisporus

4-Hydrazinobenzoic acid, an ingredient of mushroom Agaricus bisporus, is carcinogenic to rodents. To clarify the mechanism of carcinogenesis, we investigated DNA damage by 4-hydrazinobenzoic acid using (32)P-labeled DNA fragments obtained from the human p53 and p16 tumor suppressor genes. 4-Hydrazinobenzoic acid induced Cu(II)-dependent DNA damage especially piperidine-labile formation at thymine and cytosine residues. Typical hydroxyl radical scavengers showed no inhibitory effects on Cu(II)-mediated DNA damage by 4-hydrazinobenzoic acid. Bathocuproine and catalase inhibited the DNA damage, indicating the participation of Cu(I) and H(2)O(2) in the DNA damage. These findings suggest that H(2)O(2) generated by the autoxidation of 4-hydrazinobenzoic acid reacts with Cu(I) to form reactive oxygen species, capable of causing DNA damage. Interestingly, catalase did not completely inhibit DNA damage caused by a high concentration of 4-hydrazinobenzoic acid (over 50 microM) in the presence of Cu(II). 4-Hydrazinobenzoic acid induced piperidine-labile sites frequently at adenine and guanine residues in the presence of catalase. 4-Hydrazinobenzoic acid increased formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), a characteristic oxidative DNA lesion, in calf thymus DNA, whereas 4-hydrazinobenzoic acid did not increase the formation of 8-oxodG in the presence of catalase. ESR spin-trapping experiments showed that the phenyl radical was formed during the reaction of 4-hydrazinobenzoic acid in the presence of Cu(II) and catalase. Matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF/mass) spectrometry analysis showed that phenyl radical formed adduct with adenosine and guanosine. These results suggested that 4-hydrazinobenzoic acid induced DNA damage via not only H(2)O(2) production but also phenyl radical production. This study suggests that both oxidative DNA damage and DNA adduct formation play important roles in the expression of carcinogenesis of 4-hydrazinobenzoic acid.

Tumor-promoting activity of p-hydroxybenzenediazonium is accelerated in Mg-deficient rats

Tumor-promoting activity caused by the short-term administration of p-hydroxybenzenediazonium (PDQ) has been assayed in rats fed on a Mg-deficient diet as a reference model versus rats fed on a standard diet as controls. For 5 weeks groups of 20 rats, fed either on the Mg-deficient or standard diet, were treated simultaneously with PDQ. A group of 10 Mg-deficient rats remained untreated. Topical application of PDQ was followed by the appearance of macroscopic alterations in the skin, which were more evident in the Mg-deficient rats. No deaths occurred during the treatment. After 5 weeks' PDQ treatment the rats were killed and histological analyses were made. Tissues from the skin, liver, heart, kidney, lung and thymus were screened by conventional staining methods. Both the PDQ-treated Mg-deficient and PDQ-treated control rats presented tissue lesions, although to a different extent. The untreated Mg-deficient rats showed no such lesions. Mg-deficient rats treated with PDQ developed significant incipient fibrosarcomas (p<0.05) and extended hyperplasia (p<0.001), particularly in the skin, accompanied by a significant increase in the thickness of collagen (mean value: 445.4+/-47.2microm, p<0.05) compared to the control PDQ-treated group (mean values: 258.7+/-36.4microm). The overall results provide objective proof of tumor-promoting activity after 5 weeks' treatment with PDQ. Such a fast response is interpreted as being linked to the increased vulnerability of the membrane caused by Mg deficiency, which would more readily facilitate the toxic activity of p-hydroxybenzenediazonium ions.

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Molecular mechanisms of DNA damage induced by procarbazine in the presence of Cu(II)

Procarbazine [N-isopropyl-alpha-(2-methylhydrazino)-p-toluamide], a hydrazine derivative, which has been shown to have effective antineoplastic activity, induces cancer in some experimental animals and humans. To clarify a new mechanism for its carcinogenic effect, we examined DNA damage induced by procarbazine in the presence of metal ion, using 32P-5'-end-labeled DNA fragments obtained from the human p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. Procarbazine plus Cu(II) induced piperidine-labile and formamidopyrimidine-DNA glycosylase-sensitive lesions at the 5'-ACG-3' sequence, complementary to a hotspot of the p53 gene, and the 5'-TG-3' sequence. Catalase partially inhibited DNA damage, suggesting that not only H(2)O(2) but also other reactive species are involved. Procarbazine plus Cu(II) significantly increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine, which was completely inhibited by calatase. Electron spin resonance spin-trapping experiments revealed that methyl radicals were generated from procarbazine and Cu(II). On the basis of these findings, it is considered that procarbazine causes DNA damage through non-enzymatic formation of the Cu(I)-hydroperoxo complex and methyl radicals. In conclusion, in addition to alkylation, oxidative DNA damage may play important roles in not only antitumor effects but also mutagenesis and carcinogenesis induced by procarbazine.

Suppression of free radical-induced DNA strand breaks by linoleic acid and low density lipoprotein in vitro

The results of the present study have shown that unoxidized linoleic acid (LA) and low density lipoprotein (LDL) suppressed free radical-induced supercoiled plasmid DNA strand breaks. Unoxidized LA suppressed DNA strand breaks induced by free radicals generated from hydrogen peroxide/Fe(II) ion, 2'-azobis(2-amidinopropane)hydrochloride (AAPH), and 4-(hydroxymethyl)benzene diazonium salt. Thiobarbituric acid reactive substances (TBARS) of LA were increased on treatment with the radical generators. The intensities of the electron spin resonance (ESR) signals of the spin adducts of the radicals were reduced by unoxidized LA. Although LA hydroperoxide caused DNA strand breaks as has already been shown, its strand breaking activity was observed only at the higher concentrations. Unoxidized LDL inhibited ascorbic acid/Cu(II) ion-, ascorbic acid/Fe(II) ion-, peroxynitrite- and AAPH-induced DNA strand breaks. The TBARS of LDL were increased by treatment with the agents. LDL oxidized with Cu(II) ion did not cause DNA strand breaks. The results indicate that the potency of the free radicals to cause DNA strand breaks was attenuated by the fatty acid and the lipoprotein through lipid peroxidation.

Carcinogenic semicarbazide induces sequence-specific DNA damage through the generation of reactive oxygen species and the derived organic radicals

Semicarbazide, a hydrazine derivative, is carcinogenic to mice but shows no or little mutagenicity in the Salmonella-microsome test. To clarify whether or not the genotoxic mechanism contributes to the non-mutagenic carcinogenicity of semicarbazide, we investigated DNA damage induced by semicarbazide using 32P-5'-end-labeled DNA fragments obtained from the c-Ha-ras-1 protooncogene and the p53 tumor suppressor gene. Semicarbazide caused DNA damage frequently at the thymine and cytosine residues in the presence of Cu(II). Catalase and bathocuproine partially inhibited DNA damage, suggesting that hydrogen peroxide plus Cu(I) participates in DNA damage. When a high concentration of semicarbazide was used in the presence of catalase, DNA damage was induced, especially at G in 5'-AG and slightly at 5'-G in GG and GGG sequences. An electron paramagnetic resonance (EPR) spectroscopic study has confirmed that the reaction of semicarbazide with Cu(II) produces carbamoyl radicals (z.rad;CONH(2)), possibly generated via the nitrogen-centered radicals of semicarbazide. Azodicarbonamide also produced carbamoyl radicals and induced DNA damage frequently at 5'-G in GG and GGG sequences, suggesting that carbamoyl radicals participate in this sequence-specific DNA damage by semicarbazide. On the basis of our previous reports, we consider that the sequence-specific DNA damage at G in 5'-AG in the present study is due to the nitrogen-centered radicals. This study has shown that semicarbazide induces DNA damage in the presence of Cu(II) through the formation of hydrogen peroxide and Cu(I). In addition, semicarbazide-derived free radicals participate in DNA damage. DNA damage induced by these reactive species may be relevant to the carcinogenicity of semicarbazide.

Decomposition of N-nitrosamines, and concomitant release of nitric oxide by Fenton reagent under physiological conditions

N-Nitrosodimethylamine (NDMA) in phosphate buffer was rapidly decomposed by Fenton reagent composed of H2O2, and Fe(II) ion. Electron spin resonance (ESR) studies using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) showed that characteristic four line 1:2:2:1 ESR signals due to the DMPO-OH adduct formed on treatment of DMPO with Fenton reagent disappeared in the presence of NDMA, and N-nitrosodiethylamine (NDEA), suggesting the interaction of the N-nitrosamines with Fenton reagent. Treatment of the N-nitrosamines with Fenton reagent generated nitric oxide (NO) as estimated by ESR technique using cysteine-Fe(II), and N-methyl-D-glucaminedithiocarbamate (MGD)-Fe(II) complexes. Characteristic 3, and single line signals due to 2 cysteine-Fe(II)-NO, and 2 cysteine-Fe(II)-2 NO complexes, respectively, and three line signals due to MGD-Fe(II)-NO were observed. Considerable amount of NO were liberated as determined by NO2-, the final oxidation product of NO formed by reaction with dissolved oxygen in the aqueous medium. Spontaneous release of a small amount of NO from the N-nitrosamines was observed only on incubation in neutral buffers. Above results indicate that the N-nitrosamines were decomposed accompanying concomitant release of NO on contact with reactive oxygen species.

Activation of AP-1 through the MAP kinase pathway: a potential mechanism of the carcinogenic effect of arenediazonium ions

Arenediazonium ions such as those found in the common mushroom Agaricus bisporus have been convincingly demonstrated to be tumorigenic. The specific mechanism of their tumorigenicity remains unclear. It has been shown that arenediazonium ions can be metabolized to aryl radicals, and that reaction of these aryl radicals with DNA produces aryl adducts. These metabolic processes also produce the reactive oxygen species superoxide and hydroxyl radicals which have been implicated in AP-1 activation. To further investigate the mechanism of tumorigenesis by arenediazonium ions, we studied the effect of a representative arenediazonium ion on AP-1 activation and phosphorylation of the signal transduction proteins ERK1, ERK2, JNK, and p38 kinase, both in vitro and in vivo. We also identified the specific radicals produced by spin trapping and ESR analysis. Here, it was found that p-methylbenzenediazonium ion (2a) induced a 16-fold increase in the extent of AP-1 activation at micromolar concentrations, and that this increase coincided with phosphorylation of the signaling kinases ERK1 and -2 and p38 kinase, but not JNK, in JB6 mouse epithelial cells. In vivo studies using AP-1 luciferase reporter-bearing transgenic mice supported the increase in the extent of AP-1 activation in 2a-treated mice over controls, and showed that this effect was different in different tissue types. The antioxidant N-acetylcysteine (NAC), a general antioxidant, showed an inhibitory effect on 2a-mediated AP-1 induction, while aspirin, a hydroxyl radical scavenger, had no effect. Spin trapping studies showed that while NAC suppressed radical formation from 2a, aspirin did not alter radical production from 2a. It appears that 3a, a carbon-centered radical formed from 2a, is responsible for AP-1-induced activation, and therefore, radical species that are not oxygen-centered are also capable of inducing AP-1. These results represent a step toward understanding the mechanism of tumorigenicity of arenediazonium ions.

Dediazoniation of p-hydroxybenzenediazonium ion in a neutral aqueous medium

The dediazoniation of p-hydroxybenzenediazonium ion (PDQ) in a neutral aqueous medium has been studied under controlled experimental conditions to prevent photochemical and/or heterolytic side-reactions. Oxygen increased the dediazoniation rate of PDQ and caused the appearance of quinone and hydroquinone. An accumulation of quinone deriving from the reduction of PDQ by hydroquinone was also observed. In ESR analyses with different spin traps, the most stable signal was identified as belonging to the adduct of the p-hydroxyphenyl radical, even in the presence of dimethylsulfoxide or ethanol. A general scheme is proposed including three pathways for the homolytic dediazoniation of PDQ. Pathway 1 represents dediazoniation induced by a hydroxyl ion, a slow process at neutral pH and an even slower one with deaerated samples; a favored quinoid structure is put forward to explain these results. In pathway 2, the formation of a semiquinone radical via the reaction of an aryl radical with oxygen is put forward to justify the increase in the dediazoniation rate in the presence of oxygen. In pathway 3, hydroquinone, produced by semiquinone dismutation, may act as a reducing agent.

C8-Arylguanine and C8-aryladenine formation in calf thymus DNA from arenediazonium ions

Arylhydrazides, arylhydrazines, and N-alkyl-N-arylnitrosamines are metabolized to arenediazonium ions which yield C8-arylpurine adducts in calf thymus and cellular DNA. The mechanism of adduct formation has not been fully elucidated. C8-Arylguanine adducts likely form from direct aryl radical (Ar*) addition to the C8 position of guanine. However, the amounts of C8-aryladenine adducts measured here are inconsistent with direct radical attack at the C8 position of adenine. An intermediate product, an aryltriazene, is likely formed which then decomposes to the C8-aryladenine adduct. We have demonstrated that N1-aryl-N3-purinyltriazene adducts are formed from a variety of para-substituted arenediazonium ions with adenine. Decomposition of the N1-aryl-N3-purinyltriazene, at high pH and elevated temperatures, has been shown to give C8-aryladenine derivatives, and a free radical mechanism for this process has been proposed. Here we show that this process can occur under physiological conditions and that the C8-aryladenine adduct can be quantitated by HPLC. ESR studies, in which DMPO was used as a spin trap, have been used to demonstrate the intermediacy of aryl radicals during the decomposition of the N1-aryl-N3-purinyltriazenes and to demonstrate that this process also occurs in calf thymus (ct) DNA treated with arenediazonium ions. These results suggest the involvement of an aryl radical in the formation of the observed DNA adducts. Finally, we have found that the treatment of ct DNA with arenediazonium ions produces a significant amount of depurination. Both the formation of C8-arylguanine and C8-aryladenine adducts and the generation of apurinic sites may contribute to the genotoxicity of arylhydrazides, arylhydrazines, N-alkyl-N-arylnitrosamines, and arenediazonium ions.

Gamma-glutamyl transpeptidase-dependent mutagenicity and cytotoxicity of gamma-glutamyl derivatives: a model for biochemical targeting of chemotherapeutic agents

Many carcinomas in humans are rich in gamma-glutamyl transpeptidase (GGT), a plasma membrane enzyme that reacts with extracellular substrates. Thus, biochemical targeting of chemotherapeutic agents may be achieved by converting anticancer drugs into their gamma-glutamyl derivatives. Chemical conversion of phenylhydrazine (PH) and biochemical modification of daunomycin (DM) into their gamma-glutamyl derivatives gamma-glutamyl phenylhydrazine (GGPH) and gamma-glutamyl DM (GGDM) resulted in the abolishment of their mutagenicity and cytotoxicity, as judged by decreased viability and increased mutant yields in cultures of several Salmonella Ames strains. Commercial gamma-glutamyl-p-nitroanilide (GGPNA) was not toxic or mutagenic. Mutagenicity and/or cytotoxicity of these gamma-glutamyl derivatives were restored upon reaction with GGT, with concomitant release of PH, and p-nitroaniline (PNA). The GGT-dependent release of DM from GGDM was demonstrated by thin layer chromatography (TLC), spectral analysis, and specific mutagenicity. Mutagenicity and/or cytotoxicity of gamma-glutamyl derivatives increased in the presence of glycylglycine, a GGT activator, and decreased in the presence of serine-borate, a GGT inhibitor. GGDM retained considerable DNA binding capacity. Its inability to kill and mutagenize was due to altered transport properties. The results are compatible with the notion that gamma-glutamylation is a feasible method for biochemical targeting of drugs containing a primary amino group to GGT-rich tumors.

Synthesis and miscoding specificity of oligodeoxynucleotide containing 8-phenyl-2'-deoxyguanosine

Aryl radicals and arenediazonium ions are suspected to react with cellular DNA, resulting in C8-arylguanine adducts. 8-Phenyl-2'-deoxyguanosine (8-PhdG) was synthesized as a model adduct by reacting dG with benzenediazonium chloride and incorporated into oligodeoxynucleotides using phosphoramidite techniques. A site-specifically modified oligodeoxynucleotide containing a single 8-PhdG was then used as a template for primer extension reactions catalyzed by the intact (exo+) or 3'-->5' exonuclease-free (exo-) Klenow fragment of Escherichia coli DNA polymerase I and mammalian DNA polymerase alpha (pol alpha). Although primer extensions catalyzed by the Klenow fragments were retarded at the position of 8-PhdG, most of the primer extension passed the lesion to form the fully extended products. In contrast, primer extensions catalyzed by pol alpha were strongly blocked opposite the lesion. The fully extended products formed during DNA synthesis were analyzed to quantify the miscoding specificities of 8-PhdG. The exo- Klenow fragment incorporated primarily dCMP, the correct base, opposite 8-PhdG, along with small amounts of incorporation of dAMP. Two-base deletions were also observed. In contrast, the exo+ Klenow fragment incorporated dCMP opposite the lesion. When pol alpha was used, 8-PhdG promoted small amounts of misincorporation of dAMP and dGMP as well as one- and two-base deletions. The duplex containing 8-PhdG.dG was thermally and thermodynamically more stable than dG.dG. The duplex containing 8-PhdG.dA was thermodynamically more stable than dG.dA. We conclude that 8-PhdG is a weak miscoding lesion, capable of generating G-->T and G-->C transversions and deletions in cells.

Aryl radical formation during the metabolism of arylhydrazines by microsomes

Many arylhydrazines are genotoxins, although the mechanism of their genotoxicity is unknown. Previous studies have shown that arylhydrazines are metabolized to arenediazonium ions, which produce C8-arylguanine adducts in DNA suggesting the intermediacy of an aryl radical. Here we have looked for the formation of aryl radicals from arylhydrazines and microsomes by ESR spin trapping. Only hydroxyl radicals are trapped upon incubation of p-methylphenylhydrazine with rat liver microsomes and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). However, hydroxyl and aryl radicals were trapped upon incubation of p-(methoxymethyl)phenylhydrazine with rat liver microsomes. Evidence for hydroperoxyl radical formation was also obtained. In contrast, when either of these substrates was incubated with microsomes from C5O cells, aryl and hydroxyl radicals were trapped. The ESR signal intensity of the spin-trapped aryl radicals parallels the extent of C8-arylguanine formation in DNA, and therefore, the aryl radical is likely the intermediate responsible for C8-arylguanine adduct formation. Aryl radicals and C8-arylguanine adducts may be related to the genotoxicity of arylhydrazines and related compounds that are oxidatively metabolized to arenediazonium ions, the precursor to aryl radicals, including arylalkyl nitrosamines, arylazo compounds, and triazenes.

Bioactivation of mushroom hydrazines to mutagenic products by mammalian and fungal enzymes

Agaritine (N2-[L-(+)-glutamyl]-4-(hydroxymethylphenyl)hydrazine), the principal hydrazine found in the edible mushroom Agaricus bisporus, as well as the N'-acetyl derivative of 4-(hydroxymethyl)phenylhydrazine and 4-(hydroxymethyl)benzene diazonium ion, as the tetraborate salt, considered as the putative proximate and ultimate carcinogens of agaritine, were all synthesised chemically. The mutagenicity of these compounds and of 4-hydrazinobenzoic acid, a precursor of agaritine biosynthesis in mushroom, was investigated in the Ames test, using Salmonella typhimurium strain TA104, in the absence and in the presence of either mushroom tyrosinase or rat hepatic cytosol as activation systems. In the absence of an activation system the diazonium ion was clearly the most mutagenic of the four compounds studied. When tyrosinase was used as activation system, the mutagenicity of N'-acetyl-4-(hydroxymethyl)phenylhydrazine was enhanced; glutathione and superoxide dismutase markedly suppressed the mutagenic response. When the mutagenicity of the four compounds was evaluated in the presence of rat hepatic cytosol, an increase was seen only in the case of N'-acetyl-4-(hydroxymethyl)phenylhydrazine; this was shown to be due to deacetylation releasing the more mutagenic free hydrazine. Collectively, the above data are compatible with an activation of agaritine that involves an initial loss of the gamma-glutamyl group followed by microsomal oxidation of the free hydrazine to generate the diazonium ion. Also of interest is the observation that mushroom tyrosinase can convert N'-acetyl-4-(hydroxymethyl)phenylhydrazine to mutagenic product(s); whether these products contribute to the mutagenicity of mushroom extracts remains to be established.

8-Arylguanine adducts from arenediazonium ions and DNA

Arenediazonium ions (ArN2+) are genotoxic though the source of their genotoxicity is unknown. The present studies were undertaken to determine if reductive decomposition of ArN2+ to aryl radicals (Ar) in the presence of calf thymus DNA (ctDNA) or in cells results in the formation of DNA adducts. We found that when arenediazonium ions of the general structure p-X-ArN2+ (X = CH3, CH2OCH3, CH2OH) are allowed to react with ctDNA or incubated with cells under conditions that produce p-X-Ar, DNA adducts are formed with guanine. The structure of the adduct is the C8-substitution product derived from guanine and p-X-Ar. Formation of p-X-Ar was determined by ESR spin-trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The extent of C8-arylguanine adduction was measured by high performance liquid chromatography (HPLC) analysis of the DNA hydrosylate and comparison with authentic synthetic standards. The C8-arylguanine adducts observed to form may be important in regard to the genotoxicity of ArN2+, though other DNA adducts such as the N6-triazene of adenine or C8-aryladenine adducts can form. Finally, though the formation of C8-arylguanine adducts from arenediazonium ions has been proposed, this is the first report demonstrating their formation in DNA.

DNA strand cleavage by tumor-inhibiting antibiotic 6-diazo-5-oxo-L-norleucine

A tumor-inhibiting antibiotic, 6-diazo-5-oxo-L-norleucine (DON), caused DNA single-strand breaks. Thus, supercoiled plasmid DNA was transformed into an open circular relaxed form DNA by incubation with DON at pH 7.4. DNA strand cleavage by DON was not inhibited by superoxide dismutase, but inhibited by catalase. The inhibition by catalase may not be due to the destruction of hydrogen peroxide, but to the masking DON by the interaction with the heme moiety of the enzyme. DNA strand cleavage by DON was inhibited by azide, mannitol, ethanol, cysteine and 2-mercaptoethanol, suggesting the involvement of radical species in the cleavage. The cleavage, however, was not suppressed by removal of dissolved oxygen from the reaction mixture, indicating that no oxygen-derived radicals participated in the cleavage. Electron spin resonance spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) elucidated the generation of a carbon-centered radical from DON. Hence, the carbon-centered radical may participate in DNA strand cleavage by DON.