Biomedical subjects

P Hrelia

Publications and source records attributed to P Hrelia.

At least 37 records · Page 2Linked to original sources

PubMed1996

Cytogenetic effects of Metalaxyl on human and animal chromosomes.

The purpose of this study was to assess the cytogenetic effects in vitro and in vivo of a commonly used fungicide, Metalaxyl. Chromosome damage in vitro, quantified by cultured human peripheral blood lymphocytes, demonstrated dose-related effects not associated with mitotic inhibition or cell death. Significant induction of chromosomal aberrations was observed with between 300 and 1000 micrograms/ml Metalaxyl in the absence of microsomal activation. Incubation in the presence of S9 mix produced less cytogenetic damage. Single i.p. injections of 75-300 mg/kg Metalaxyl had no effect on the frequency of micronuclei, detected in murine polychromatic erythrocytes. Micronuclei results were not compromised by direct evidence of cytotoxicity in the bone marrow of treated animals. The results in the present study indicated that genotoxicity of Metalaxyl was detected only in vitro and not in vivo. Available data reported that Metalaxyl was non-carcinogenic and gave negative results in a battery of genotoxicity tests. So, clastogenicity of Metalaxyl may not be evidence for DNA reactivity, but it may indicate alterations in cell homeostasis which are well implicated in the process of carcinogenesis.

Alanine↗

PubMed1996

Genetic and non-genetic biomarkers related to carcinogenesis in evaluating toxicological risk from Fenarimol.

A multibiomarker approach based on the study of toxicity mechanisms at both genetic and metabolic levels has been applied to Fenarimol. With regard to genotoxicity, particular attention was given to assays for chromosomal aberration and micronuclei; clastogenic potential was assessed in human peripheral blood lymphocytes in vitro, while the induction of micronuclei was studied in male CD1 mouse bone marrow polychromatic erythrocytes (PCE). Fenarimol did not induce any significant dose-related increase in micronucleated PCEs, up to 4-fold above the control level at a single dose of 75 mg/kg b.w., was observed 24 h after treatment. Using selective biochemical markers of effect Fenarimol was found to induce CYP 2B1 isoforms in liver, kidney and lung microsomes of Swiss Albino CD1 male and female mice, as shown by the significant increase in specific 2B1-probe pentoxyresorufin O-dealkylase activity. On the contrary, CYP 3A, probed by N-demethylation of aminopyrine, were only induced in the liver. Results were corroborated by means of Western immunoblotting using rabbit polyclonal antibodies anti-CYP 2B1 and 3A. Northern blotting analysis with CYP 2B1 and 3A cDNA biotinylated probes showed that the expression of such isoforms is regulated at mRNA level. Taken as a whole, these data indicate the possible (mutagenic) cotoxic/cocarcinogenic and promoting potential of this fungicide.

Animals↗

PubMed1996

Interferons in precancer and cancer prevention: where are we?

Clinical research on interferons (IFN) still focuses on the treatment of advanced cancer. The research strategy eventually must be reevaluated. The cellular minimal deviations that are seen in early phases of carcinogenesis might be the most rational target for immune interventions. That biologic response modifiers have considerable capacity to prevent induction and development of malignant neoplasias has been demonstrated in several animal systems. Even the few clinical studies available at present on the treatment of preneoplastic lesions with IFN have definitely shown more success than those involving treatment of advanced tumors. In addition, there is experimental evidence that IFN might be suitable candidates for immunoprevention. The major problems hampering a wider application of IFN in immunoprevention is that they cause adverse effects. Unfortunately, we do not know much about the specific mechanisms involved in the immune control of human tumor development during the initial and the latency phases of carcinogenesis. More research is needed in this area. In this article the state of the art of using IFN for treating preneoplastic lesions is reviewed, and also we report some of our experimental results on IFN and anticancerogenesis.

Animals↗

PubMed1996

The genetic and non-genetic toxicity of the fungicide Vinclozolin.

The mutagenic/cocarcinogenic potential of the fungicide Vinclozolin was assessed by a comprehensive examination of toxicity mechanisms at both the genetic and the metabolic level. Vinclozolin did not induce any significant increase in chromosomal aberrations in human pheripheral blood lymphocytes cultured in vitro, both in the presence and in the absence of metabolic activation. However, significant dose-related increases in micronucleated erythrocytes (up to 4-fold over the control) were found in the bone marrow cells of mice 24 h after treatment with the fungicide over a range of concentrations from 312.5 to 1250 mg/kg. The morphology and the size of micronuclei induced was suggestive of a predominantly clastogenic mode of action. Several cytochrome P450 (CYP)-dependent reactions have been monitored in liver, kidney and lung microsomes of male and female Swiss Albino CD1 mice in order to ascertain certain toxic non-genetic properties (related to carcinogenesis) of Vinclozolin. It was found to be a selective inducer towards CYP 3A (liver, kidney) and 2E1 (liver), as exemplified by the significant increases of the demethylation of aminopyrine (APND, up to 2.3-fold, female liver), and hydroxylation of p-nitrophenol (pNPH, up to 5.6-fold, male liver). In general, however, Vinclozolin has a complex pattern of induction and suppression of CYP-dependent enzymes, as shown from the reduced expression of various monooxygenases depending upon dose, sex or organ considered. For example, pNPH activity was suppressed in kidney (up to 48% loss, averaged between male and female), whereas ethoxycoumarin O-deethylase was reduced in lung up to 53% in male (at the highest dose). These data were sustained by means of Western immunoblotting using rabbit polyclonal antibodies anti-CYP 3A and 2E1. Northern blotting analysis using CYP 3A1/2 and 2E1 cDNA biotinylated probes showed that the expression of such isozymes is regulated at the mRNA level. Taken together, the findings indicate the clastogenic activity and the possible cotoxic, cocarcinogenic and promoting potential of Vinclozolin.

Animals↗

PubMed1995

Analysis of metabolism and genotoxicity of 5-nitro-3-thiophenecarboxanilides in bacterial, mammalian and human cells.

5-nitro-3-thiophenecarboxanilide (NTCA3) was clearly mutagenic in Salmonella typhimurium strains TA98, YG1021 (the strain with elevated nitroreductase) and YG1024 (the strain with elevated O-acetyltransferase) and only slightly mutagenic at the gpt locus in AS52 cells. Clastogenic activity in human lymphocytes was dependent on the length of exposure: detectable chromosome aberrations were observed following a 24 h treatment period, but not after 3 h exposure. S9 increased genotoxicity in both mammalian cells and human lymphocytes. Metabolites formed by incubation of NTCA3 with the different cell systems were examined. A time-course study in cell whole extracts showed that bacterial and mammalian cells can acetylate NTCA3 forming 5-acetylamino-3-thiophene-carboxanilide. The formation of this metabolite in human lymphocyte extracts was not confirmed. These data support the conclusions that: (i) both bacterial and mammalian activation pathways play a role in mutations by NTCA3; (ii) the N-acetylated derivative is generated by acyl-transferase after reduction and is the end product of the metabolism in both bacterial and mammalian cells; and (iii) different levels of reductase and acetyltransferase activity may contribute to the differential sensitivity of the different cellular species to the genotoxicity of NTCA3. The fact that NTCA3 serves as substrate for enzymatic activities of importance also in human metabolism needs consideration in assessing the potential risk posed by NTCAs.

Acetylesterase↗

PubMed1994

Genetic safety evaluation of pesticides in different short-term tests.

Cyanazine, cyhexatin, dicamba and DNOC are pesticides commonly and broadly used in agriculture pest control. However, there is little information on their toxicity and mutagenicity in human cells and in whole animals. Therefore, UDS assay and SCE assay in human peripheral lymphocytes, and chromosome aberration analysis in bone marrow of rats have been used to assess the DNA-damaging activity of the above pesticides. Cyanazine proved non-genotoxic in all the test systems. Cyhexatin showed only weakly positive results for SCE induction in human lymphocytes, providing no concern for genotoxicological hazard. While dicamba did not show clastogenic effects in rodents, DNOC gave significant dose-related increases of structural chromosome aberrations in rat bone marrow cells. Female animals showed increased sensitivity to the toxic effects by DNOC at the highest dose. The results provide further information on the intrinsic genotoxic activity of the tested pesticides, which may contribute to the toxicological assessment of the risk associated with human exposure.

Animals↗

PubMed1994

Interactive effects between trichloroethylene and pesticides at metabolic and genetic level in mice.

A combined cytogeneticurine metabolite analysis approach was used to assess potential interactive effects between Fenarimol (FN), a fungicide, and trichloroethylene (TRI), a halogenated solvent. FN was demonstrated to selectively induce P450-2B1 isoforms in different organs of treated mice. Since the rate of metabolism and the stereospecificity of metabolism are dependent on the types and amount of P450s available, FN might drastically alter the metabolic activation of a precarcinogen, such as TRI, and its toxicological consequences. Male CD1 mice were divided into untreated, vehicle control, and experimental groups. Animals of the latter groups were treated ip with 150 mg/kg bw FN in corn oil, 457 mg/kg bw TRI in corn oil, TRI plus FN separated by different time intervals. Bone marrow cells were harvested for determination of micronuclei (MN) frequencies in polychromatic erythrocytes (PCE). The presence of the known metabolite of TRI, trichloroethanol (TCE), was quantitated in collected urine by gas chromatography using an electron-capture detector. Linear regression analysis shows that MN frequency by TRI is correlated with TCE concentration in urine. Observed potentiation of genotoxicity of TRI by FN pretreatment (1 hr before TRI treatment) apparently reflects changes in the spectra of enzymes involved in TRI metabolism, and altered toxicokinetic, as witnessed by the 20% difference in TCE excretion from combined treated mice. However, no increased genetic or metabolic effects were observed when FN was administered 3 hr before TRI. No significant interactive effects were observed at a genetic level when FN was administered 1 hr and 3 hr after TRI whereas a 33 to 47% loss in TCE excretion was recorded.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Interaction of licorice on glycyrrhizin pharmacokinetics.

The effects of components of aqueous licorice root extract (LE) on the pharmacokinetics of glycyrrhizin (G) and glycyrrhetic acid (GA) were investigated in rats and humans. The aim of this work was to define the role of pharmacokinetics in G toxicity. In the procedure, G and GA were detected in biological fluids by means of recently improved HPLC methods. Significantly lower G and GA plasma levels were found in rats and humans treated with LE compared to the levels obtained with those in which G alone was administered. The pharmacokinetic curves showed significant differences in the areas under the plasma-time curve (AUC), Cmax, and Tmax parameters. The data obtained from urine samples are in agreement with the above results and confirm a reduced bioavailability of G present in LE compared to pure G. This should be attributed to the interaction during intestinal absorption between the G constituent and the several components in LE. The modified bioavailability could explain the various clinical adverse effects resulting from the chronic oral administration of G alone as opposed to LE.

Administration, Oral↗

PubMed1993

Murine alpha/beta interferons inhibit benzo(a)pyrene activation and mutagenesis in mice.

In addition to their antiviral and immune regulatory properties, interferons (IFNs) are known to depress hepatic cytochrome P450-dependent metabolism. As many chemical mutagens and carcinogens require bioactivation by the mixed-function monooxygenase (MFO) system in order to be genotoxic, a combined genetic and biochemical approach was used to establish whether IFNs could inhibit the activation of benzo(a)pyrene (BaP) to the ultimate clastogenic metabolite(s) in vivo. Treatment of mice with murine IFN-alpha/beta depressed cytochrome P450 content, as well as ethoxyresorufin O-deethylase activity (EROD), as a probe of class IA1 P450 isozymes, for 24 hrs and delayed the attainment of normal levels to approximately 30 hrs. After IFNs plus BaP treatment, EROD activity showed a reduction up to 70% after 24 hrs with an enhancement in activity at 30 hrs. A positive correlation exists between the rate of inhibition of oxidative BaP hepatic metabolism and inhibition of clastogenic effects in vivo, as scored in the bone marrow chromosome aberration assay.

Animals↗

PubMed1993

Mechanism of genotoxicity and electron density distribution by NMR of 5-nitro-3-thiophenecarboxamides, a novel group of direct-acting mutagens in Salmonella typhimurium.

The mutagenic activity of 23 5-nitro-3-thiophenecarboxanilides and of 5-nitro-3-thiophenecarboxamide, the prototype, (NTCAs) have been evaluated in the Ames test on Salmonella typhimurium strains TA100 ad TA98 with and without metabolic activation. Effects of different substituents (electron-donating and electron-withdrawing) were studied to evaluate structural features that affect the metabolism and the bacterial mutagenic potency. All the derivatives were direct-acting mutagens, the mutagenic potency ranging from 0.7 to 142 revertants (rev.)/nmol in TA100 and from 0.09 to 68 rev./nmol in TA98 strain. Results obtained with strains TA98NR and TA98/1,8-DNP6 indicated that the mutagenic activity was largely dependent on bacterial nitroreductase, whereas the O-acetylation step was not critical for mutagenic potency. Superoxide (O2-.) and hydroxyl (OH.) scavengers as well as other radical scavengers and enzymes inhibited NTCAs mutagenicity to different extents. In particular, O2-. seemed to be involved in NTCAs mutagenicity, showing a free radical pathway for NTCA metabolism. [1H]- and [13C]NMR data indicated that the effects of different substituents on genotoxicity are probably not exerted on the electron density distribution. The importance of factors such as extent of nitration, reduction potential, orientation of nitrosubstituent and planarity of the molecule are discussed.

Acetyltransferases↗

PubMed1993

Synthesis of a series of 5-nitro-(benzimidazoles and indoles) as novel antimycotics and evaluation as genotoxins in the Ames test.

Nitrobenzimidazole and nitroindole derivatives, related to oxiconazole and characterized by an oxyiminic function, have been synthesized as novel antimycotics and their mutagenic activity tested in Salmonella typhimurium strains TA100 and TA98 with and without an exogenous metabolizing system. TA98NR and TA98/1,8-DNP6 strains were employed to identify a specific metabolic reaction which governs the mutagenic potency. Active compounds are weak direct-acting mutagens. Only derivatives bearing a nitro group on the phenyl ring linked to the oxyiminic function and lacking halogenated substituents show mutagenic activity. Metabolism by bacterial enzyme systems is important to the expression of genotoxicity. The reductive activation of nitrobenzimidazoles and nitroindoles carried out by the 'classical' nitroreductase of Salmonella, which is defective in TA98NR, is required of mutagenicity. Similarly, the O-acetyltransferase defective in TA98/1,8-DNP6 is required for the efficient production of the ultimate electrophilic nitrogen species, which react with DNA. The role of bacterial metabolism in mutation induction needs careful consideration to assess the potential risk to humans from nitrobenzimidazole and nitroindole antimycotics.

Antifungal Agents↗

PubMed1993

Multiple end point procedure to evaluate risk from pesticides.

Because of the potential environmental impact of pesticides and the large population potentially exposed, the effects of chronic exposure to pesticides need to be determined. Mutagenicity studies have been used to identify specific agents as potential carcinogens or other human health hazards. However, short-term tests are only theoretically correlated to carcinogenesis because their end points can measure only the genotoxic potential of chemicals, i.e., their activities as initiating agents in multistep carcinogenesis. The objective of our research presented here is to provide a comprehensive examination of the mechanism of toxicity of a series of pesticides. These are substances for which toxicity, at both the genetic and metabolic level, has not been adequately described. Preliminary results on a broad series of compounds belonging to different biological classes (herbicide, insecticide, fungicide) seem to indicate that pesticides are toxic but are poor initiating agents, as shown by negative or weak positive results on different genetic end points (gene mutations, DNA effects, and chromosome aberrations in vitro and in vivo). Immunochemical and biochemical studies, however, seem to indicate the cocarcinogenic and promoting potential of these chemicals. As an example, the genotoxic and biochemical effects induced by Fenarimol (a fungicide) are discussed. The results reported stress the importance of identifying chemicals that act at different levels of the multistep carcinogenesis process to ascertain the risk associated with exposure.

Animals↗

PubMed1993

Quantitative and molecular analyses of genetic risk: a study with ionizing radiation.

Mammalian cells in culture have been used to study the genetic effects of physical and chemical agents. We have used Chinese hamster ovary (CHO) cells, clone K1-BH4, to quantify mutations at the X-linked, large (35 kb) hypoxanthine-guanine phosphoribosyltransferase (hprt) locus (the CHO/HPRT assay) induced by environmental agents. By transfecting an hprt-deletion mutant CHO cell line with the plasmid vector pSV2gpt, we isolated a transformant, AS52. AS52 cells carry a single functional copy of an autosomal, small (456 bp) xanthine-guanine phosphoribosyltransferase (gpt) gene (the bacterial equivalent of the mammalian hprt gene; AS52/GPT assay). We found that ionizing radiations such as X-rays and neutrons and oxidative genotoxic chemicals such as Adriamycin, bleomycin, hydrogen peroxide, and potassium superoxide are much more mutagenic to the gpt gene in AS52 cells than to the hprt locus in K1-BH4 cells. The hypermutability of the gpt gene probably results from a higher recovery of multilocus deletion mutants in AS52 cells than in K1-BH4 cells, rather than a higher yield of induced mutants. These results demonstrate that the use of the hprt locus alone could lead to an underestimate of the genetic risk of these agents. Analyses of the mutation spectrum using a polymerase chain reaction-based deletion screening and DNA sequencing procedure showed that a high proportion of HPRT- and GPT- mutants induced by X-rays carry deletion mutations. Thus, both the mutant frequency and mutation spectrum need to be considered in assessing the genetic risk of ionizing radiation and oxidative genotoxic chemicals.

Animals↗

PubMed1991

Pharmacokinetics of ursodeoxycholic acid in rat.

The pharmacokinetic behaviour and metabolism of ursodeoxycholic acid (UDCA) have been studied in the rat. After oral administration of both 3H-labelled (4 muCi/kg body wt) and unlabelled (20 mg) UDCA, UDCA appeared in serum almost entirely in conjugated form (taurine conjugated); UDCA was present in bile mostly as taurine conjugated; the more relevant metabolite is 3 alpha,6 alpha, 7 beta-trihydroxycholanoic acid which represents 10% of the total bile acid pool. UDCA increased bile flow and selectively decreased biliary cholesterol secretion, while phospholipid secretion was unaffected. Faecal UDCA excretion was 15-20% while the urinary extraction was 1.5% during 24 h. The data show that UDCA, when administered in high dose, is promptly secreted into bile almost entirely metabolized to tauroursodeoxycholic acid, where it (1) desaturates the cholesterol in bile, (2) exerts choleretic properties.

Administration, Oral↗

PubMed1991

Wide spectrum detection of precarcinogens in short-term bioassays by simultaneous superinduction of multiple forms of cytochrome P450 isoenzymes.

The use of Aroclor 1254 to induce S9 liver fractions is a standard method for conducting short-term genotoxicity assays. An alternative induction procedure, using beta-naphthoflavone (beta-NF), as a safe (non-carcinogenic) substitute for polychlorinated biphenyls, combined with sodium phenobarbital (PB), was found to be equally effective. The aim of this work is to realize a novel schedule of induction for the preparation of metabolizing systems containing a wider spectrum of induced cytochrome P450s. Five inducers of different 'classes' such as PB (class IIB P450s), beta-NF (IA), isosafrol (IA2), ethanol (IIE1) and pregnenolone 16 alpha-carbonitrile (IIIA) were injected daily both separately (to achieve maximal monooxygenase induction) in male and female mice. Induction was monitored using specific P450-linked activities. In the optimal schedule for complete induction, the various monooxygenases were greater (2- to 4-fold) than those achieved by the classical schedule. More than a 14-fold increase of total P450 and 3.3-fold increase of NADPH-cytochrome (P450) c-reductase activity, over those uninduced, account for the above increase. For example, there was a marked increase in the deethylation of ethoxyresorufin (37-fold) compared to the uninduced mice that was considerably higher than classical induction (8-fold over uninduced). On the contrary, phase II reactions i.e. epoxide hydrolase, glutathione S-transferase, glutathione S-epoxide transferase and UDP-glucuronosyl transferase, examined to compare the phase I/phase II ratios in the traditional and proposed procedures, were increased to a lesser extent (2-fold over uninduced). No significant sex differences were seen. Five precarcinogens specifically metabolized by each of the induced P450s elicited a higher mutagenicity response in the presence of superinduced fractions with respect to the classical one, when tested on Salmonella typhimurium (cyclophosphamide, benzo[alpha]pyrene, 2-naphthylamine and dimethylnitrosamine) or Saccharomyces cerevisiae D7 strain (diethylstilbestrol). These novel metabolizing biosystems, with an enhanced spectrum of induced P450s and oxidative/post-oxidative reaction rates, are recommended for detecting unknown xenobiotics in genotoxicity studies.

Animals↗

PubMed1990

Molecular analysis of reactive oxygen-species-induced mammalian gene mutation.

We have studied the mutagenicity and toxicity of physical and chemical agents in the Chinese hamster ovary (CHO) cell line K1-BH4 and its transformant, AS52. The AS52 cells lack the normal X-linked mammalian hypoxanthine-guanine phosphoribosyltransferase (hprt) gene but instead contain a single autosomally integrated copy of the bacterial equivalent, the xanthine-guanine phosphoribosyltransferase (gpt) gene. We found that X-rays and neutrons appear to be equitoxic to both cell types; however, these physical agents are approximately 10 times more mutagenic to the gpt gene of AS52 cells than to the hprt gene of K1-BH4 cells. We reasoned that if reactive oxygens were to mediate the mutagenic effects of both radiomimetic chemicals and radiation, then reactive oxygen-producing chemicals, such as streptonigrin and bleomycin, and oxidizing agents such as potassium superoxide and hydrogen peroxide, would exhibit similar levels of toxicity but different frequencies of mutants when assayed with the two cell lines. Our experiments fulfill such predictions. We postulate that the apparent hypermutability of AS52 cells probably results from a higher recovery of multi-locus deletion mutants in AS52 cells than in K1-BH4 cells, rather than a higher yield of induced mutants. Preliminary studies, using Southern blot and the polymerase chain reaction to analyze the mutational spectrum of the mutants, support our hypothesis that reactive oxygens induce deletion mutations in mammalian cells.

Animals↗

PubMed1990

Cytogenetic assays in genotoxic studies: somatic cell effects of benzene and germinal cell effects of dibromochloropropane.

We have reviewed the results of our cytogenetic studies to evaluate the usefulness of these assays in genotoxic studies. In one study, we observed unusual dose-response in lymphocyte chromosome aberration frequencies after exposure of mice to low doses of a chemical mixture (benzene, chloroprene, epichlorohydrin, and xylene). The frequency in the high dose group is lower than those of the medium and low dose groups. This reduction of genotoxicity is correlated with a significant induction of a detoxifying enzyme, glutathione-S-transferase. The data also suggest that extrapolation of effects from high to low doses for risk assessment may be erroneous. Using benzene as a model clastogen, we found that the clastogenic effect persists for a long time after termination of exposure in mice. This phenomenon is probably due to a gradual release of benzene from absorption of the chemical in body fat. In an inhalation study, we observed that chromosome aberrations are induced in mice after exposure to benzene at below the occupational exposure limit of 1 ppm. Since benzene is a ubiquitous environmental contaminant, it may interact with other environmental agents to modify its genotoxic effects. We found that a nongenotoxic drug, praziquantel, and a free radical scavenger, DMSO, can enhance or reduce respectively the clastogenic activities of benzene in mice. Both modifying agents acted by altering the metabolic pathways of benzene. In a study with rats, we showed that carcinogenic doses of benzene can induce chromosome aberrations in lymphocytes of rats long before the expression of cancer. With dibromochloropropane, we observed that this chemical can induce dominant lethality in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Dimethylsulfoxide as modifier of the organospecific mutagenicity of metronidazole in mice.

Mutagenicity and carcinogenicity of metronidazole (MT) are imputable to the formation of toxic intermediates, which include radical forms derived from the nitroreductive process. Since dimethylsulfoxide (DMSO), the "universal" solvent, can quench free radicals in vitro, it was suggested that DMSO might protect by scavenging free radical generation in vivo. This study wanted to evaluate if DMSO (given concomitantly or prophylactically) protects against the organospecific mutagenicity of MT in vivo by means of the intrasanguineous host-mediated assay. DMSO used as solvent showed a 20%-30% reduction in the mutation frequencies by MT. Prophylactic administration of DMSO for 3 d caused a suppression of the organospecific mutagenicity. However, some increases in the spontaneous mutation frequency and enhancement of MT mutagenicity in kidney were observed. The protective effect was paralleled by a decrease in NADPH cytochrome c (P450) reductase in liver, kidney, and to a lesser extent in lung microsomes from pretreated mice. Inhibition of mutagenic activity might be related to scavenging of radical species as supported by the lack of tissue specificity and no appreciable changes in specific enzyme activity. However, changes in reductase content in prophylactically pretreated mice can affect the quantitative biotransformation of MT to the proximal mutagen contributing to the observed suppression in mutation frequencies.

Animals↗