Modulation of SCE induction and cell proliferation by 2-mercaptoethanol in phytohemagglutinin-stimulated rat lymphocytes

Review: 2-mercaptoethanol alteration of in vitro immune functions of species other than murine

Descriptions that organosulfurs could alter biologically relevant cellular functions began some 40 years ago when cell mediated and humoral murine in vitro immune responses were reported to be dramatically enhanced by any of four xenobiotic, sulfhydryl compounds—2-mercaptoethanol (2-ME), dithiothreitol, glutathione, and l-cysteine; the most effective of the four was 2-ME. These findings triggered a plethora of reports defining 2-ME benefits for a multitude of immunological processes, primarily with murine models. This led to investigations on 2-ME alterations of (a) immune functions in other species, (b) activities of other cell-types, and (c) in situ diseases. In addition, the early findings may have been instrumental in the identification of the previously undefined anticarcinogenic chemicals in specific foods as organosulfurs. Outside the plant organosulfurs, there are no comprehensive reviews of these areas to help define mechanisms by which organosulfurs function as well as identify potential alternative uses. Therefore, the present review will focus on 2-ME alterations of in vitro immune functions in species other than murine; namely, fish, amphibian, reptile, avian, whales, dolphins, rat, hamster, rabbit, guinea pig, feline, canine, porcine, ovine, bovine, and human. Processes, some unique to a given species, were in general, enhanced and in some cases dependent upon the presence of 2-ME. The largest benefits occurred in media that were serum free, followed by those in autologous serum and then fetal bovine serum supplemented medium. Concentrations of 2-ME were generally in the low μM range, with exceptions of those for salamander (20 mM), turtles (70 mM) and dolphins (7 mM). The few studies designed to assess mechanisms found that changes induced by 2-ME were generally accompanied by alterations of reduced/oxidized glutathione cellular concentrations. The major benefit for most studies, however, was to increase the sensitivity of the culture environment, which permitted a specific process to be more easily dissected.

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2-ME altered in vitro immune functions of species other than murine.

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Benefits were found for species from fish to humans.

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Enhancement occurred in serum-free and in autologous or fetal bovine serum.

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Generally, optimal concentrations of 2-ME were in the low uM range.

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Concentration exceptions were salamander (20 mM), turtles (70 mM), and dolphins (7 mM).

Hprt lymphocyte mutant frequency in relation to DNA adduct formation in rats fed the hepatocarcinogen 2-acetylaminofluorene

The lymphocyte hypoxanthine-guanine phosphoribosyltransferase (Hprt) assay is frequently used as a biomarker for the exposure of both humans and laboratory animals to potentially carcinogenic agents. To obtain information concerning the sensitivity of the rat Hprt lymphocyte assay toward aromatic amine carcinogens, male F344 rats were fed 0.02% 2-acetylaminofluorene (2-AAF) for 1 month and then returned to control diet for 2 months. At 4, 27, 48, 62, and 90 days after the initiation of 2-AAF-feeding, the frequency of mutants in the Hprt gene was determined. In addition, DNA was isolated from liver nuclei, spleen lymphocytes, bone marrow, and thymus, and DNA adducts were analyzed by 32P-postlabeling. 2-AAF feeding resulted in a significant induction of 6-thioguanine-resistant T-lymphocytes and the mutant frequency continued to increase after the 2-AAF feeding was stopped. The same major DNA adduct, N-(deoxyguanosin-8-yl)-2-aminofluorene, was detected in liver, spleen lymphocytes, bone marrow, and thymus. DNA adduct levels were greatest in the tumor target tissue (liver) but occurred in all T-lymphocyte compartments, being highest in spleen lymphocytes. The DNA adduct levels were highest at the end of the 1-month 2-AAF feeding period and decreased rapidly in all tissues. The data indicate that the Hprt lymphocyte mutagenesis assay detects arylamine carcinogens, but with relatively low sensitivity.

Development and utilization of the rat lymphocyte hprt mutation assay

Much of the recent progress in the field of genetic toxicology has come from an increased understanding of the molecular and cellular biology of the mammalian organism. Most prominent has been the ability to detect and quantify somatic mutation and relate the nature of the mutation to the specific type of chemical damage. Building upon the foundation of the human lymphocyte hypoxanthine guanine phosphoribosyl transferase (hprt) system, and later, the mouse hprt system, methods for the detection and quantification of hprt mutations in rat lymphocytes were developed. These methods are described in this report as is the ongoing validation of the assay. Additionally, the characterization of the recovered mutants and a comparison of the mutation spectrum in the rat lymphocyte system to the spectrum in cancer genes, such as H-ras and p53, and the spectrum in transgenic systems, such as lacI, are included. The development of the rat lymphocyte hprt system and validation of the assay at the molecular level, provide an effective and reliable measure of genetic damage in an in vivo system which is readily comparable to measurement of genetic damage in the human.

Effects of prenatal exposure to 5-bromo-2'-deoxyuridine on reproductive function in male mouse offspring

The effect of the halogenated pyrimidine analog 5-bromo-2'-deoxyuridine (BrdU), on reproductive functions of male mouse offspring treated prenatally was studied. BrdU was administered intraperitoneally to pregnant ICR mice at 100 mg/kg/d on days 8 through 13 of gestation, and at 100 or 200 mg/kg/d on days 14 through 18 of gestation. Dams were allowed to deliver naturally. Male offspring were aged for 10 weeks and then cohabited with untreated female mice for assessment of reproductive performance. Histopathologic examination of the testes and pituitary, sperm analysis, and determination of plasma testosterone concentrations of offspring at 12 weeks of age were performed. In the subsequent study, pregnant ICR mice were treated with 200 mg BrdU/kg on day 10, 13, or 15 of gestation. The embryos or fetuses were obtained from mothers from 6 to 48 h after treatment, and pyknotic cells in the ventricular zone of the telencephalon were counted. There was a significant decrease in body weight gain of offspring in all of the BrdU-treated groups. A marked decrease in copulation rate was noted in the male offspring of dams treated on days 8 through 13 of gestation, whereas no significant decreases in copulation and fertility rates were found in the male offspring of dams treated on days 14 through 18 of gestation. Neither histopathologic examination of testes nor sperm analyses revealed adverse effects of this compound, whereas cysts in the pars distalis of the pituitary were observed in the male offspring treated on days 8 through 13 of gestation. Dilatation of the lateral ventricles was also observed in male offspring at 12 weeks of age in the group treated on days 8 through 13 of gestation. The incidence of pyknotic cells in the ventricular zone of embryos was markedly increased 24 h after treatment on day 10 of gestation. These results indicate that the impaired fertility of the male offspring of dams treated with BrdU on days 8 through 13 of gestation may be due to the effects of BrdU exposure on central nervous system function that result in loss of libido rather than to the direct effects of this compound on the male reproductive organs.

Lymphocyte mutant frequency in relation to DNA adduct formation in rats treated with tumorigenic doses of the mammary gland carcinogen 7,12-dimethylbenz[a]anthracene

The ability of the rat lymphocyte hprt assay to detect tissue-specific carcinogens was evaluated using 7,12-dimethylbenz[a]anthracene (DMBA) administered under conditions that result in mammary gland tumors. Fifty-day-old female Sprague-Dawley rats were given single doses of 5 and 20 mg/kg DMBA by gavage, and the frequency of 6-thioguanine-resistant (TGr) T-lymphocytes was measured over a period of 21 weeks. A time- and dose-dependent increase in mutant frequency was found, with a maximum frequency found 9-15 weeks after treatment with 20 mg/kg of DMBA. Rats were also dosed with 1, 2.5, 5, 10, 15 and 20 mg/kg of DMBA and assayed for TGr mutant frequency 10 weeks after treatment. A significant linear dose-response was found, with all the DMBA doses resulting in significant increases in mutant frequency. To determine whether or not DMBA-induced mutants in rat lymphocytes reflected the DNA damage in the target tissue, rats were treated with 5 and 20 mg/kg of DMBA and spleen lymphocytes and mammary gland tissue were assayed for DNA adduct formation 1, 3 and 7 days later. A similar pattern of 32P- postlabeled adducts, involving both dG and dA nucleotides, was found in DNA from both the target tissue and the surrogate lymphocytes. Adduct formation was dose responsive in both tissues, with a 2.3- to 4-fold higher concentration in mammary gland as compared with lymphocytes. These results indicate that the rat lymphocyte hprt assay is sensitive to a mammary gland carcinogen and that similar types of DNA adducts are associated with both the lymphocyte mutants and the mammary gland tumors induced by DMBA.

Characterization of rat lymphocyte primary culture for the development of an in-vitro mutagenesis assay: effect of interleukin-2 and 2-mercaptoethanol on the activities of intermediary metabolism enzymes and cell proliferation

Efficient energy utilization is essential for cell growth; in an attempt to improve the growth conditions of the rat T-lymphocyte culture model for potential use in studying the mutagenic activity of carcinogens in vitro, we have investigated the effects of phytohemagglutinin (PHA), interleukin-2 (IL-2) and 2-mercaptoethanol (2-ME) on the activities of intermediary metabolism enzymes and cell proliferation. Isolated lymphocytes were cultured in the presence and absence of PHA, IL-2, or 2-ME. The intermediary metabolism enzymes investigated were glutamate dehydrogenase, glutamate-pyruvate transaminase, malate dehydrogenase, isocitrate dehydrogenase, lactate dehydrogenase, pyruvate kinase, and fatty acid synthetase (FAS). Measurable activity of all enzymes investigated, except for FAS, was detected in PHA-stimulated cells cultured with IL-2 or 2-ME. The unstimulated lymphocytes had significantly lower enzyme activity than stimulated cells. The combination of all three agents showed increased enzyme activity. This increase in activity brought about by the combination of the three agents was not reproduced by either agent acting alone. In general, the increase in enzyme activity correlated with cell proliferation as measured by [3H]thymidine uptake in PHA-stimulated cultures containing IL-2 and/or 2-ME. The results suggest that the addition of exogenous IL-2 and 2-ME enhances metabolic function and may be beneficial in in vitro culture of rat lymphocytes.

Comparison of the cell cycle regulated synthesis and phosphorylation of stress proteins, actin isoforms and a novel actin-like protein following drug administration in cultured rat lymphocytes

Administration of phytohemagglutinin initiated cycling of rat lymphocytes in vitro, and following treatment with this drug and other drugs in combination, lymphocytes were pulse labeled with [3H] leucine of [32P] phosphate. The nuclei were isolated from lymphocytes and collected from partitions of the cell cycle, and the proteins analyzed from fluorographs following gel electrophoresis for protein biomarkers after drug exposure. Stress proteins (sps) were dependent on a specific drug or drugs in combination (i.e., interleukin-2, bleomycin) for their synthesis that occurred only during the G1-phase of the cell cycle. An "actin-like" protein (A4) with electrophoretic mobilities similar to the actin complex, was synthesized in S and G2 phases and phosphorylated in all phases of the cell cycle only following the administration of drugs in combination. A4 exhibited a binding affinity for sp 24 that was cell cycle regulated (i.e., A4 from S phase did not bind with sp 24, but A4 from G2 phase did bind with the sp. Protein A4 appeared similar in some structural aspects to the nonmuscular actin isoform family but differed in epitope, suggesting a unique relationship and represented a stable protein, perhaps a product from the mutation of an actin gene. The dependence of certain sps and protein A4 for their induction by drugs in combination may serve as biomarkers of chemical interaction and toxicity.

Sister-chromatid exchanges in lymphocytes from methimazole-induced hypothyroid mice

The inhibitory effect of an antithyroid drug on mouse T lymphocytes was investigated. Inbred C57BL/6 mice were provided with an antithyroid drug, methimazole, for 2, 4 and 6 weeks and the in vitro responses of the lymphocytes were studied. The proliferative responses of T lymphocytes from the spleen of methimazole (MMI)-treated mice significantly (p < 0.05) decreased following concanavalin A stimulation, and the inhibitory effect became prominent with the increased duration of MMI treatment. A concomitant increase in the frequency of induced sister-chromatid exchanges was also observed in these T lymphocytes. When the splenocytes were stimulated with concanavalin A for 24 h, their ability to produce interleukin-2 (IL-2) was significantly decreased (p < 0.05). The results indicated that methimazole interfered with the normal proliferation of T lymphocytes by suppressing the production of IL-2, a cytokine also known as T cell growth factor, as well as inducing a higher incidence of sister-chromatid exchange during cell division.

Analysis of in vivo mutation induced by N-ethyl-N-nitrosourea in the hprt gene of rat lymphocytes

The rat lymphocyte hprt assay measures in vivo mutagenicity by quantifying the frequency of 6-thioguanine-resistant (TGr) spleen lymphocytes cultured in vitro. In this study we have examined the types of mutations induced in the hprt gene of TGr lymphocyte clones from female Fischer 344 rats exposed to 100 mg/kg N-ethyl-N-nitrosourea (ENU). Hprt exons 3 and 8 were amplified from DNA extracted from each of 249 clones, and the resulting products were screened for mutant:wild-type heteroduplex formation by denaturing gradient gel electrophoresis. The analysis revealed 59 clones with mutations in exon 3, and 20 clones with mutations in exon 8. DNA sequence analysis of the heteroduplexes identified 84 mutations: all of the mutations were base pair substitutions, and 88% were mutations of A:T base pairs. At least 82% were induced independently. These results suggest that the mutations found in TGr rat lymphocytes from ENU-treated rats were due mainly to ethylthymidine adducts. In addition, a comparison of these results with previously reported in vivo ENU mutational profiles indicates that the types of mutation detected by heteroduplex screening of rat hprt exons 3 and 8 are representative of mutation in the entire protein coding sequence.

Analysis of in vivo mutation in exon 8 of the rat hprt gene

We have analyzed mutations in exon 8 of the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene in T-lymphocytes from the spleens of ethylnitrosourea-treated female rats. Presumptive hprt- mutants were isolated by clonal growth in the presence of 6-thioguanine. DNA from 6-thioguanine-resistant colonies was amplified by the polymerase chain reaction using intronic primers flanking hprt exon 8. The identification of mutant sequences and the separation of mutant DNA from contaminating wild-type DNA was accomplished by denaturing gradient gel electrophoresis. Of 118 clones analyzed, 19 contained mutations and DNA sequence analysis identified eight unique sequence alterations. We also used single-strand conformation polymorphism analysis to screen for mutations in the same fragment of the hprt gene. This analysis was less successful than denaturing gradient gel electrophoresis in detecting the eight unique mutations. The procedures described here may represent a useful approach for studying the mechanisms of in vivo mutation.

Ascorbic acid (vitamin C) modulates the mutagenic effects produced by an alkylating agent in vivo

Recent reports suggest that ascorbic acid (vitamin C) inhibits tumorigenesis as well as exerts a protective effect against mutagenesis in vitro; however, there is no information on its ability to affect gene mutations induced in vivo. In this study, we have investigated the antimutagenic effects of ascorbic acid on the frequency of 6-thioguanine-resistant (6-TGr) T-lymphocytes produced in Fischer 344 rats dosed with the direct-acting alkylating agent, N-ethyl-N-nitrosourea (ENU). The frequency of 6-TGr T-lymphocytes from the spleen measured five weeks after ENU treatment indicated that ENU produced a substantial mutagenic response. Pretreatment and/or post-treatment of rats with ascorbic acid administered in the drinking water appeared to inhibit the response, but the inhibition was statistically significant only when data from the various dosing schedules were pooled. In addition, there was no clear dose-dependency to the inhibitory effect of ascorbic acid. To further evaluate the time effects of the vitamin supplement on ENU mutagenicity, rats were exposed to the mutagen together with ascorbic acid, which was given continuously for the entire duration of the experiment. At specific times after ENU treatment, the frequency of 6-TGr T-cells was determined in lymphocytes isolated from the spleen and the thymus. Time-dependent increases in the frequency of 6-TGr T-cells were observed with ENU treatment; ascorbic acid significantly reduced the ENU-mediated mutagenic responses, most dramatically in the spleen at weeks 6 and 8 (P < 0.0001), and to a lesser extent in the thymus (P < 0.01 at week 6 and P < 0.006 at week 8). Our data suggest that ascorbic acid intake affects the in vivo mutagenicity of ENU, a direct-acting mutagen/carcinogen, and that the reported inhibitory effects of the antioxidant on carcinogenesis may be partially mediated by its effects on mutagenesis. Although it is difficult to extrapolate from rodent studies to humans, the results presented suggest an explanation for epidemiological data that link vitamin C ingestion with decreased cancer risk.

The effect of time after treatment, treatment schedule and animal age on the frequency of 6-thioguanine-resistant T-lymphocytes induced in Fischer 344 rats by N-ethyl-N-nitrosourea

The persistence of 6-thioguanine-resistant (TGr) T-lymphocytes was investigated in Fischer 344 rats treated with N-ethyl-N-nitrosourea (ENU) using two schedules. Male rats, aged 3 months, were given i.p. injections containing a total of 0, 50 or 100 mg ENU/kg either as a single treatment (single-dose group) or divided among 10 weekly treatments (split-dose group). At 1, 3, 5, 10, 20, 30 and 50 weeks after the single-dose treatment, and 10, 20, 30 and 50 weeks after beginning the split-dose regimen, animals were assayed for the frequency of TGr spleen lymphocytes. ENU produced significant dose- and time-dependent responses in the single- and the split-dose treatment groups. Although a few of the 50 mg/kg split-dose treatments were significantly higher than the comparative single-dose groups, the number of TGr lymphocytes produced by the two dosing regimens were generally similar. The frequency of TGr cells for control animals increased with the age of the animals. The mode of ENU administration did not greatly influence the percent cloning efficiency (%CE) of the non-selection cultures, although the %CE declined in animals over 10 months of age. To investigate the relationship between the frequency of TGr cells and the age of the animals at the time of ENU administration, additional rats aged 17 months were treated with a single dose of ENU and at 1, 5 and 10 weeks following exposure, the frequencies of TGr cells were determined from the isolated lymphocytes. No difference in mutagen sensitivity between rats treated at 3 months of age and those treated at 17 months of age was detected at the time points evaluated. The data demonstrate the persistence of ENU-induced TGr T-lymphocytes in the rat and suggest that the dose and possibly the treatment schedule, but not the age of the animal at the time of treatment, affect the response.

Flow cytometric analysis of the cell-cycle distribution of spleen lymphocytes isolated from Fischer 344 rats exposed to ethyl nitrosourea

Current studies in our laboratory are designed to determine the frequency of genotoxic responses induced in lymphocytes isolated from Fischer 344 rats. To evaluate the effect of a model compound, N-ethyl-N-nitrosourea (ENU), on the cell-cycle distribution of spleen lymphocytes, 8-week old, female Fischer 344 rats were injected i.p. with ENU and sacrificed 1, 2, 4, and 6 weeks after exposure. Four replicate cultures per dose per exposure period were established and cells were cultured for 66 hr. Colcemid, an agent which blocks cells in mitosis and induces an accumulation of cells in the G2 + M peak, was added to two of the four cultures as a positive control. After a 3 hr incubation, the cells were harvested, the nuclei stained with propidium iodide, and the DNA content of the individual nuclei was quantified by flow cytometry. As expected, exposure to Colcemid resulted in an accumulation of cells in the G2 + M phase of the cell cycle, which was accompanied by a decrease in the G0 + G1 population. The increase in the G2 + M population was significant (p < 0.05) in cultures of lymphocytes assayed at 4 and 6 weeks after exposure. The effect of increasing ENU concentration was an increase in the percentage of S-phase cells (p = 0.05) and a decrease (p < 0.02) in the percentage of G0 + G1 cells. This finding was observed only in those lymphocytes isolated 1 week after exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetic toxicology of 5-bromodeoxyuridine in mammalian cells

The thymidine analog, BrdUrd, induces many biological responses which are of importance to the field of genetic toxicology and related disciplines. These include the induction of SCE, specific-locus mutations, and toxicity, inhibition of cell proliferation, and the expression of fragile sites in the human genome. In early models which addressed the mechanisms of the biological effects of BrdUrd exposure, two pathways were proposed to account for the induction of the biological responses. Incorporation of the enol form of BrdUrd into the nascent DNA strand after pairing with deoxyguanosine was proposed as one pathway, whereas the incorporation of BrdUrd opposite adenosine in place of thymidine was proposed as the second pathway. Many novel and sophisticated techniques have been applied to the study of the mechanism of the induction of biological effects by BrdUrd leading to a substantial increase in our understanding of these mechanisms. However, the experimental evidence clearly supports the contention that BrdUrd exerts its effects on eukaryotic cells through mechanisms similar to those originally proposed to explain the genotoxicity of BrdUrd.

Comparative study of intracellular glutathione content in rat lymphocyte cultures treated with 2-mercaptoethanol and interleukin-2

The level of intracellular glutathione (GSH) in mitogen-stimulated mouse lymphocytes is increased in the presence of 2-mercaptoethanol (2-ME), an enhancer of lymphocyte activation and proliferation. Since proliferation of lymphocytes in response to mitogens involves direct activation by a mitogen followed by continued proliferation in response to interleukin-2 (IL-2), we have investigated the effect of 2-ME and exogenous IL-2 on the GSH content and cell proliferation of rat lymphocytes stimulated with phytohemagglutinin (PHA). PHA stimulation increased both GSH content and the magnitude of the proliferative response, as measured by thymidine incorporation into cellular DNA. However, incubation of stimulated lymphocytes with 2-ME or IL-2 for 72 hr produced a significant further elevation of GSH levels and thymidine incorporation. 2-ME also increased the GSH content in unstimulated cultures, but it had little effect on thymidine incorporation. IL-2 increased GSH content and decreased thymidine incorporation in unstimulated lymphocytes. Exposure of cells to DL-buthionine-(S,R)-sulfoximine (BSO), an inhibitor of GSH biosynthesis, significantly depleted GSH and lowered the proliferative response, suggesting a crucial role of de novo GSH synthesis for lymphocyte activation. The data suggest that both 2-ME and IL-2 promote lymphocyte proliferation, although the mechanisms by which intracellular GSH levels are increased by the agents are apparently different.

Analysis of rat lymphocyte activation of benzo[a]pyrene, 2-acetylaminofluorene, and several of their metabolites to mutagenic and DNA-damaging species in vitro

Rat lymphocytes are a potentially useful and convenient cell system for monitoring the genotoxic effects of chemicals in vivo, but little is known about the ability of these cells to metabolize promutagens to genotoxic species. In this study, Fischer 344 rat lymphocytes were treated in vitro with benzo[a]pyrene (BaP), 2-acetylaminofluorene (2-AAF), and several of their metabolites, and DNA damage was measured using nucleoid sedimentation analysis. Of the BaP derivatives, BaP 4,5-oxide and BaP 7,8-diol-9, 10-epoxide decreased lymphocyte nucleoid sedimentation, whereas BaP and BaP 7,8-dihydrodiol had little effect. Among the 2-AAF derivatives, N-acetoxy-2-AAF, N-hydroxy-2-AAF, and N-hydroxy-2-aminofluorene damaged rat lymphocyte nucleoids, whereas 2-AAF, 2-aminofluorene, and fluorene produced little detectable damage. The decrease in nucleoid sedimentation produced by N-hydroxy-2-AAF was not inhibited by paraoxon, salicylamide, or 2-aminofluorene, whereas paraoxon inhibited damage produced by N-acetoxy-2-AAF. In co-culture assays, rat lymphocytes increased the mutagenicity of N-hydroxy-2-AAF in Salmonella typhimurium strain TA98, but mediated little or no mutagenic response with BaP, BaP 7,8-dihydrodiol, and 2-AAF. Also, human lymphocytes, but not rat lymphocytes, mediated a positive mutagenic response with BaP 7,8-dihydrodiol in Chinese hamster ovary UV5 cells. Although rat lymphocytes may metabolize certain proximal genotoxic chemicals to DNA-damaging species (e.g., N-hydroxy-2-AAF), these data suggest that in vivo lymphocyte DNA damage is more likely to result from lymphocytes encountering reactive chemical derivatives produced by other cells. It is also clear that differences exist between the ability of human and rat lymphocytes to activate promutagens, and this may impact on the use of the rat model to predict the genotoxicity of chemicals in humans.

Evaluation of the genotoxicity of gentian violet in bacterial and mammalian cell systems

Previous studies indicate that gentian violet (GV), a triphenylmethane dye used in agriculture and human medicine, is a clastogen in vitro and a carcinogen in chronically exposed mice and rats. Data on its genotoxic activity, however, have been incomplete and partly contradictory. Mutagenesis and DNA damage experiments were conducted to re-evaluate the genotoxic potential of GV in both bacterial and mammalian cell systems. GV was mutagenic in Salmonella typhimurium tester strains TA97 and TA104, but there was little mutagenic activity detected in strains TA98 and TA100. A rat liver homogenate fraction (S9) tended to increase mutagenicity. The major microsomal metabolites of GV, pentamethylpararosaniline and N,N,N',N'-tetramethylpararosaniline were less mutagenic in TA97 and TA104, while N,N,N',N"-tetramethylpararosaniline was a weak mutagen in Salmonella. GV was not mutagenic in Chinese hamster ovary (CHO) cell strain CHO-K1-BH4, and was a questionable mutagen in CHO-AS52 cells. While GV produced DNA damage as measured by sedimentation of nucleotids derived from B6C3F1 mouse lymphocytes treated in vitro, no damage was found in lymphocytes isolated from mice dosed with GV. GV was also a weak producer of gene amplification in an SV40-transformed Chinese hamster cell line. The results indicate that GV is a point mutagen in bacteria; however, since similar exposure conditions produced weak mutagenic activity in mammalian cells, GV may be carcinogenic by virtue of its clastogenic activity.