Quantitative comparison between in vivo DNA adduct formation from exposure to selected DNA-reactive carcinogens, natural background levels of DNA adduct formation and tumour incidence in rodent bioassays

This study aimed at quantitatively comparing the occurrence/formation of DNA adducts with the carcinogenicity induced by a selection of DNA-reactive genotoxic carcinogens. Contrary to previous efforts, we used a very uniform set of data, limited to in vivo rat liver studies in order to investigate whether a correlation can be obtained, using a benchmark dose (BMD) approach. Dose-response data on both carcinogenicity and in vivo DNA adduct formation were available for six compounds, i.e. 2-acetylaminofluorene, aflatoxin B1, methyleugenol, safrole, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline and tamoxifen. BMD(10) values for liver carcinogenicity were calculated using the US Environmental Protection Agency BMD software. DNA adduct levels at this dose were extrapolated assuming linearity of the DNA adduct dose response. In addition, the levels of DNA adducts at the BMD(10) were compared to available data on endogenous background DNA damage in the target organ. Although for an individual carcinogen the tumour response increases when adduct levels increase, our results demonstrate that when comparing different carcinogens, no quantitative correlation exists between the level of DNA adduct formation and carcinogenicity. These data confirm that the quantity of DNA adducts formed by a DNA-reactive compound is not a carcinogenicity predictor but that other factors such as type of adduct and mutagenic potential may be equally relevant. Moreover, comparison to background DNA damage supports the notion that the mere occurrence of DNA adducts above or below the level of endogenous DNA damage is neither correlated to development of cancer. These data strongly emphasise the need to apply the mode of action framework to understand the contribution of other biological effect markers playing a role in carcinogenicity.

History and rationale of genetic toxicity testing: an impersonal, and sometimes personal, view

Genetic toxicity testing is a necessary and pivotal component of product development and registration. This article traces the historical development and evolution of genetic toxicity testing, and the rationale for such testing, and identifies some of the individuals who played key roles in this process. The evolution of the present test batteries and some of the research and rationales behind the decisions to accept or reject tests are described.

Structure-activity relationships of several anisidine and dibenzanthracene isomers in the w/w+ somatic assay of Drosophila melanogaster

Several structurally related anisidine and dibenzanthracene isomers were evaluated for genotoxic effects in the somatic w/w+ assay of Drosophila melanogaster employing insecticide-susceptible (IS) and insecticide-resistant (IR) tester strains. In addition, and in order to find whether or not at the genetic level a regulatory effect is found, crosses between ISxIR strains and IRxIS strains were done. Chemicals tested were the aromatic amines (AAs) ortho-anisidine (o-AN), meta-anisidine (m-AN), and para-anisidine (p-AN) and the polycyclic aromatic hydrocarbons (PAHs) 1,2;3,4-dibenzanthracene (1,2;3,4-DBA) and 1,2;5,6-dibenzanthracene (1,2;5,6-DBA). As positive control N-nitrosodimethylamine (DMN) was used. Our results show that the genotoxic activity of DMN was higher in the IR than in the IS strain. There seems to be a tendency for slightly lower values as measured by clone induction in crosses between ISxIR and IRxIS. o-AN was positive in the IS strain and in crosses between ISxIR and IRxIS but negative in the IR strain. m-AN, p-AN and 1,2;3,4-DBA proved to be not recombinogenic in all strains and crosses while 1,2;5,6-DBA was positive at the highest concentration tested in all the crosses assayed. These findings show that the recombinogenic activity of the anisidine isomers depends on the position of the chemical group relative to one another and that the position of the benzene ring seems to be structurally relevant for genotoxicity of DBA isomers. With respect to IR and IS strains it remains to be determined to what extent the spectrum of metabolizing capacity really differs between the strains of the test assay. Thus more information is needed about the regulation and expression of the cytochrome-P450 genes and action at the molecular level taking place in the eye imaginal disc as well as between insecticide susceptible and resistant strains after exposure to genotoxic chemicals.

Activity of aromatic amines in the eye: w/w+ somatic assay of Drosophila melanogaster

Aromatic amines represent a category of classical environmental hazards which require biotransformation in order to exert their mutagenic and carcinogenic effects. The mutagenic activity of several aromatic amines was tested by means of the w/w+ somatic assay of Drosophila melanogaster employing a wild-type strain Leiden Standard (LS) and an insecticide-resistant stock Hikone-R (HK-R). Four monocyclicortho-anisidine (O-AN), ortho-toluidine (O-TO), 2,4-diaminoanisole (2,4-DAA) and 4-nitro-o-phenylenediamine (4-N-o-PDA), and one bicyclic- 4,4'-oxidianiline (4,4'-ODA) aromatic amines were tested by chronic exposure. Two separate experiments were run, and concurrent controls were treated with the solvent mixture. Results show that the ortho-anilines (O-AN and O-TO) were active in the assay, while the diamines containing a methoxy (2,4-DAA) or a nitro group (4-N-o-PDA) were inactive. The bicyclic aromatic amine, 4,4'-ODA, was the most potent. The genotoxic responsiveness of both strains was very similar.

Large-bowel mucosal biotransformation activity in persons at high risk for colorectal cancer. A preliminary report

The transformation of xenobiotics and endogenous compounds to active carcinogens and their subsequent deactivation as an aid to eradication may be important in the etiology of some gastrointestinal cancers. In mammals the gastrointestinal tract has been shown to be an important site of inducible enzyme systems active in mucosal biotransformation, but few data are available in man. The mucosal activity of CYPIA1 (formerly aryl hydrocarbon hydroxylase), a potential carcinogen-activating enzyme, and catechol-O-methyl transferase, a potential carcinogen-inactivating enzyme were determined in colonic tissue obtained by biopsy. There were no significant differences in activity rates in normal mucosa between colorectal cancer and healthy persons, but significant differences are seen in patients with a history of neoplasia with no evidence of recurrence. The levels of activity of these carcinogen-inductive and -protective enzymes may be prognostic markers, in that the balance or imbalance could play a role in the recurrence of neoplasia. This will require confirmation and prospective studies.

Micronucleus incidence and hematological effects in bullfrog tadpoles (Rana catesbeiana) exposed to 2-acetylaminofluorene and 2-aminofluorene

The micronucleus technique used with Rana catesbeiana tadpoles is useful for the evaluation of aromatic amine toxicity. When exposed to increasing concentrations of the aromatic amine 2-acetylaminofluorene (2-AAF 1 x 10(-8) M to 5 x 10(-5) M), R. catesbeiana tadpoles responded with an increasing frequency of micronuclei (MN) in the peripheral blood. Micronuclei increased fourfold with the 5 x 10(-5) M 2-AAF exposure. Micronuclei frequency significantly increased with concentrations as low as 5 x 10(-8) M. The time from initial mutagen exposure to the time of maximum number of MN present in the peripheral blood occurred at two separate times: 2 d and 14 d after exposure. The occurrence of two MN frequency peaks may be due to the presence of two erythrocyte populations from two hemopoietic sources (liver and kidneys) in young tadpoles. The occurrence of immature erythrocytes, as well as general degenerative changes [such as fragmentation (schistocytosis)] in the erythrocytes and vacuolated cytoplasms suggested 2-AAF may contribute to hemolytic anemia effects. 2-AAF also increased the frequency of rubricytes found in the peripheral blood, but had no significant effect on the hematocrit values of animals exposed to concentrations up to 1 x 10(-5) M. The tadpoles may have been producing rubricytes to replace the damaged red blood cells (RBCs). The direct-acting mutagen 2-aminofluorene (2-AF) was considerably more toxic to peripheral RBCs than the indirect-acting 2-AAF.(ABSTRACT TRUNCATED AT 250 WORDS)

Diethyldithiocarbamate suppresses the plant activation of aromatic amines into mutagens by inhibiting tobacco cell peroxidase

Diethyldithiocarbamate is an antimutagen and repressed the activation of promutagens by plant systems. Earlier work implicated the involvement of tobacco cell (TX1) peroxidases in the plant cell activation of aromatic amines. We now present data that diethyldithiocarbamate represses the activation of 2-aminofluorene and m-phenylenediamine by inhibiting intracellular TX1 peroxidases under in vivo conditions. Concentrations of diethyldithiocarbamate that caused a 50% repression of TX1 cell activation of 2-aminofluorene and m-phenylenediamine also induced a 50% inhibition of TX1 cell peroxidase activity. Diethyldithiocarbamate in a concentration range between 25 and 500 microM directly inhibited peroxidase activity in TX1 cell homogenates in a concentration-dependent manner. Similar results were observed with purified horseradish peroxidase. The kinetics of peroxidase activity were studied in homogenates from control cells and cells treated with 750 microM and 25 mM diethyldithiocarbamate. There was no significant difference among the Km values among the three groups with a mean (+/- standard error) Km of 2.58 +/- 0.23 mM. However, the Vmax differed from 4.02 to 2.12 nmoles tetraguaiacol/min/micrograms protein, in the control and in the 25 mM diethyldithiocarbamate treatment group, respectively. These data indicate that diethyldithiocarbamate is a non-competitive inhibitor of TX1 cell peroxidase.

Role of mixed-function oxidases and non-protein sulfhydryl content in [14C]-2-chloro-4-acetotoluidide binding to liver and kidney in starlings

The compound 2-chloro-4-acetotoluidide (CAT) is highly toxic to many avian species, including the starling. In our earlier work, we demonstrated the covalent binding of radioactivity from [14C]-CAT to liver and kidneys of the starling. In the present study, the effects of inducers of mixed-function oxidase (MFO) and non-protein sulfhydryl (NPSH) depletor on the total and covalent binding of [14C]-CAT radioactivity to liver and kidney of the starling were examined. The total and covalently bound radioactivity from [14C]-CAT to liver and kidney were decreased significantly in the starling pretreated with the MFO inducer, 3-methylcholanthrene. However, pretreatment with phenobarbital, another inducer of MFO, had no effect. Pretreatment with the inhibitor of MFO, SKF 525-A, reduced the covalent binding of [14C]-CAT radioactivity to liver but not to kidney. There was no reduction in the NPSH content of liver or kidney following intravenous administration of CAT (3.5 mg kg-1). Reduction of NPSH levels in the liver or kidney following treatment with diethyl maleate caused a significant increase in the covalent binding of [14C]-CAT to kidney but not to liver.

Effects of specific monooxygenase and oxidase inhibitors on the activation of 2-aminofluorene by plant cells

Using specific inhibitors, a plant cell/microbe coincubation assay was employed to investigate biochemical mechanisms of plant activation. The biological endpoints of mutation induction, inhibition of mutagenicity and viability of the plant-activating system as well as viability of the microbiological indicator were simultaneously assayed from the same reaction tube. We investigated six inhibitors of monooxygenases and oxidases (diethyldithiocarbamate, methimazole, metyrapone, (+)-catechin, 7,8-benzoflavone and potassium cyanide). The activation of 2-aminofluorene by TX1 cells was mediated by an enzyme system(s) that was inhibited by microM amounts of diethyldithiocarbamate or 7,8-benzoflavone. (+)-Catechin (at low concentrations) or methimazole enhanced the activation of 2-aminofluorene while higher concentrations of (+)-catechin were inhibitory. These data indicate that a significant pathway of the plant activation of 2-aminofluorene is via a cytochrome P-448-type N-hydroxylase. The presence of a FAD-dependent monooxygenase was not detected.

The plant cell/microbe coincubation assay for the analysis of plant-activated promutagens

The preincubation and suspension procedures of the plant cell/microbe coincubation assay are described and the activation of 2-aminofluorene and m-phenylenediamine by cultured tobacco, cotton, carrot and maize cells is compared. The assay measures the plant activation of promutagens into genotoxins detected in Salmonella typhimurium as well as toxicity in plant and microbial cells. At concentrations of 2-aminofluorene 0-0.5 mumoles/reaction tube, the rank order of the efficiency of activation by plant cells was tobacco much greater than cotton greater than carrot. Cultured maize cells did not activate 2-aminofluorene. The tobacco cell activation of 2-aminofluorene was inhibited 50% by 750 microM diethyldithiocarbamate under conditions that did not affect the cell viability. Tobacco cells were also the most efficient plant cells in activating m-phenylenediamine (0-5 mumoles/reaction tube). The 'biological affinity' of m-phenylenediamine for the activation system in tobacco cells was approximately 100 microM.

Effect of retinoic acid, butylated hydroxytoluene, propylgallate and selenium on 2-acetylaminofluorene induced hepatotoxicity

Rats of the Sprague-Dawley strain were administered 2-acetylaminofluorene (2-AAF) in different diet regimens to ascertain the effect of retinoic acid(RA), butylated hydroxytoluene (BHT), propylgallate (PG), and selenium on induced hepatotoxicity. In the first study, groups of young male albino rats of the Sprague-Dawley strain were fed a diet containing 0.05% 2-AAF. Carcinogen was added to the diet for 3 weeks, omitted 1 week, added 2 weeks, omitted 2 weeks, added 3 weeks, and omitted for the final 4 weeks. Animals were terminated at the end of 15 weeks. Supplementation of the diet by 0.5% BHT, PG or 4 ppm selenium as sodium selenite in the drinking water either throughout the entire feeding period or only during the administration of the carcinogen-free diet greatly inhibited the hepatotoxicity of 2-AAF. In contrast, addition of 0.02% RA to the diet in the above feeding regimens enhanced the hepatotoxicity of 2-AAF. In the second study, male and female Sprague-Dawley rats were administered a diet containing 0.03% 2-AAF for 16 weeks or a diet containing 0.03% 2-AAF supplemented with 0.02% RA for the first 4 weeks and the last 6 weeks of the 16-week feeding period. Addition of the 0.02% RA to the diet enhanced the hepatotoxicity of 2-AAF, especially in the female rats.

Effects of vitamin A on the metabolism and mutagenicity of 2AAF in vitro and on the covalent binding to rat liver DNA/RNA in vivo

Vitamin A has been shown to affect the in vitro metabolism of 2AAF. At low concentrations of retinol or retinyl palmitate, a decreased production of ring-hydroxylated as well as deacetylated and N-hydroxylated metabolites was observed, measured by high performance liquid chromatography. The increased mutagenicity of 2AAF observed after addition of vitamin A in the Ames test cannot therefore be explained as a result of stimulated N-hydroxylation. However, the addition of retinol was found to enhance the mutagenicity of the metabolite N-OH-2AAF in the presence of an S-9 fraction of rat liver homogenate. No differences with regard to the covalent binding of 2AAF or its metabolites to rat liver DNA/RNA in vivo could be demonstrated in animals fed diets with normal or high vitamin A content.

Rapid and sensitive method for the microassay of nitrosobenzene plus phenylhydroxylamine in blood

An assay method has been developed for the determination of the combined concentration of nitrosobenzene plus phenylhydroxylamine (as nitrosobenzene) in small volumes of blood. The initial step in the procedure consisted of the simultaneous oxidation of phenylhydroxylamine to nitrosobenzene and of ferrous hemoglobin to methemoglobin by ferricyanide. Nitrosobenzene in the ferricyanide-treated blood samples was then extracted into ethyl acetate, and separated and quantitated by reversed-phase high-performance liquid chromatography with electrochemical detection. The sensitivity limit for nitrosobenzene in blood was in the pmol/ml concentration range, less than 100 microliter of blood was required for assay, and the procedure was convenient for routine multisample use. In comparison with previous assays, this method was more sensitive, had a lower coefficient of variation, and required 25-40 fold smaller blood sample volumes. The method was combined with the orbital sinus bleeding technique in order to follow the nitrosobenzene time course in vivo using small serial blood samples from rats treated with intraperitoneal injections of phenylhydroxylamine or aniline.

Toxicokinetics, covalent binding and histopathological features of [14C]2-chloro-4-acetotoluidide toxicity in the starling after intravenous administration

The toxicokinetics and subcellular covalent binding of the avicide 2-chloro-4-acetotoluidide (CAT) were examined in the starling. Intravenous administration of 4.5 mg/kg ring-labeled [14C]-CAT produced biphasic decay curves of the radioactivity in plasma and other tissues. The alpha phase of the decay curves ranged from 17 min to 1 hr while the beta phase of the decay curves varied widely from 4 hr up to 23 hr. Significant amounts of tissue covalent binding were found only in the plasma, liver, and kidney. The amount of covalently bound material found in plasma and liver varied from 30 to 55 pmole/mg protein and from 800 to 1100 pmole/mg protein in the kidney. A subcellular distribution study indicated that there is a high affinity for covalently bound radioactivity in the light mitochondrial and microsomal fractions of the liver and kidney. The kidneys of treated birds, 18 hr postinjection, had areas of necrosis, swelling of the proximal tubule epithelium, and dilation of the distal tubules. No lesions were found in the livers of these birds.

Influence of disulfiram on mutagenicity of the urinary metabolites of 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene in the Salmonella test system

Male Sprague-Dawley rats were fed control diet or 0.65% disulfiram (DS) in the diet for up to 20 days. Groups of these animals were injected with 2-acetylaminofluorene (AAF) or N-hydroxy-2-acetylaminofluorene (N-OH-AAF) after 3, 10 and 20 days, in order to study the effects of DS administration on the mutagenic activity of urinary metabolites of the carcinogens. Administration of DS alone for 20 days did not increase significantly the mutagenicity of urine. Urine from rats administered either of the carcinogens showed mutagenic activity which was increased appreciably in the presence of S9 and beta-glucuronidase. Urine from rats which also were fed DS showed an even greater increase in revertants under these conditions. The results indicate that DS increased the excretion of glucuronide metabolites of the carcinogens.

The metabolism of N-acetyl-2-aminofluorene to a mutagen in L5178Y/TK+/-mouse lymphoma cells

Direct treatment of L5178Y mouse lymphoma TK+/- cells with N-acetyl-2-aminofluorene (AAF) from two commercial sources produced small, but reproducible increases in mutant frequency over background in the absence of exogenous microsomal enzymes. Unlike most direct-acting mutagens which typically produce regular, dose-dependent increases in mutant frequency; AAF treatment caused very slight dose-related increases or a saturation phenomenon which could be overcome by increased exposure time. Direct mutagenicity following prolonged (24h) exposure was confirmed when a third highly purified (99.9%) AAF sample was tested. Microsomal enzyme analyses of disrupted L5178Y cell preparations revealed negligible benzo[a]pyrene hydroxylase but measurable AAF-N-hydroxylase activity. These data demonstrate that L5178Y mouse lymphoma cells are capable of limited metabolism of AAF to an active mutagen.

Mechanisms of action of carcinogenic aromatic amines: an investigation using mutagenesis in bacteria

The mutagenicities of groups of N-acetoxy-N-arylacetamides, nitroarenes, arylamides and arylamines were determined in the Salmonella typhimurium tester stains TA98, TA1538, TA100, TA1535 and TA1537. Three broad classes of mutagenic activity were found, interpreted as follows: class A, including 2-naphthylamine, produced essentially only base-pair substitution without induction of error-prone repair; class B, including 4-aminobiphenyl, caused consideration induction of error-prone repair, accompanied by a lower level of frame shifting; class C, including N-acetoxy-2-acetamidofluorene, produced high levels of frame shifting, with some induction of error-prone repair. Correlation of these results with known reactions of certain aromatic amine derivatives with nucleosides and nucleic acids, and with molecular orbital calculations, suggests that the effect of class A is produced by small aromatic groups attached to extranuclear heteroatoms in DNA bases, the effect of class B is caused by large aromatic groups attached to extranuclear heteroatoms or by arylamines attached to C-8 of guanine, while the effect of class C is caused by arylamides attached to C-8 of guanine, probably rotating into the helix, as proposed by others. The data also suggest that the N-acetoxy-N-arylacetamides are generally useful models for ultimate metabolites derived in vivo, even if the in vivo metabolites do not carry an acetyl group. Finally, there is a rough correlation between the sum of reversions induced in TA98 and TA100 by the N-acetoxy-N-arylacetamides and their previously determined local carcinogenicities. There is a poor correlation between mutagenicity in any one tester strain and carcinogenicity.

The binding of metabolites formed from aminostilbene derivatives to nucleic acids in the liver of rats

Carcinogenic trans-4-dimethylaminostilbene (trans-DAS) and trans-4-acetylaminostilbene (trans-AAS) as well as inactive cis-DAS and DABB were highly and specifically labeled with tritium and administered orally to female Wistar rats. Covalent binding to liver rRNA and DNA was measured and found to be higher for the carcinogenic compounds. Digests from these nucleic acids were chromatographed on Sephadex LH-20 and 16 different nucleoside adducts were characterised by their retention volumes. Labeled trans-DAS was administered in doses ranging from 0.025--250 mumol/kg. Binding to nucleic acids was directly proportional to the dose at low doses (0.025--2.5 mumol/kg) and less than proportional at higher doses (25--250 mumol/kg). The pattern of nucleoside adducts remained practically constant over the wide range of doses. A pharmacokinetically determined threshold of metabolic activation thus could not be demonstrated for this compound. A modified procedure is described to simultaneously isolate pure liver rRNA and DNA from nonfasted rats in high yields.

Effect of acetylator phenotype on the rate at which procainamide induces antinuclear antibodies and the lupus syndrome

To investigate the relation between acetvlator phenotype and the development of procainamide-induced lupus, we determined the rate of development of antinuclear antibodies in 20 patients of known acetylator phenotype receiving chronic procainamide therapy. The duration of therapy required to induce antibodies in 50 per cent of slow (11) and rapid (nine) acetylators was 2.9 and 7.3 months respectively. The median total dose that produced ant;bodies was 1.5 g per kilogram and 6.1 g per kilogram respectively. After one year antibodies had developed in 18 patients. Retrospective studies of patients in whom procainamide lupus had developed revealed that the duration of therapy required for induction in 14 slow and seven rapid acetylators was 12 +/- 5 and 48 +/- 22 months respectively (P less than 0.002). We conclude that acetylator phenotype influences the rate at which procainamide induces antinuclear antibodies and probably the lupus syndrome. Antibody production is probably related to the parent compound or a non-acetylated metabolite.

A human subject with a new defect in repair of ultraviolet damage

The subject under study (11961) is a child with extreme sun sensitivity. Fibroblasts derived from the child's skin, like those from patients with the disorder xeroderma pigmentosum were hypersensitive to the lethal effects of 254 nm and 310 nm UV-irradiation. Unlike xeroderma pigmentosum cells, however, fibroblasts from our subject were not hypersensitive to the chemical mutagen N-hydroxyacetylaminofluorene but they were hypersensitive to ethylmethanesulfonate. Furthermore, despite the ultra violet light sensitivity, no defects could be detected either in excision or postreplication repair of damaged DNA after UV-irradiation of 11961 cells. This again contrasts with xeroderma pigmentosum cells, which are defective in one or the other of these repair processes. On the basis of these characteristics and the clinical symptoms, we are not at present able to classify this patient as having any of the known sun-sensitive syndromes.

N,N-diethyl-4-aminoazobenzene (DEAB): acute actions with respect to possible carcinogenicity as well as the role of solvents. Morphological and pharmacological investigations

The acute action of the azo dye DEAB was investigated in Sprague-Dawley (SD) and Wistar (Wi) rats. The substance was dissolved both in DMSO and sunflower oil and was administered once by stomach tube. Cytochrome P-450-DEPENDENT N-demethylation of ethylmorphine and dimethylnitrosamine are differentially altered depending on the solvent used. The excretion of DEAB as well as of N,N-dimethyl-4-amino-azobenzene (DAB) is delayed and diminished if the substances are dissolved in DMSO. Beside these effects the mitotic number in the adrenal cortex is significantly elevated in both strains of rats. But, in SD rats only DMSO-solution of DEAB is effective. In Wi rats both are effective, the oily solution more than that in DMSO. In this respect DEAB resembles DAB and various other carcinogens which are efficient in stimulating adrenocortical cell division. Considering the positive short-term assay after three other substances which revealed carcinogenic properties in long-term experiments we conclude that also DEAB may be carcinogenic in adequate long-term examination.

Inhibition of 1,2-dimethylhydrazine metabolism by disulfiram

1. The effects of disulfiram on the metabolism of 1,2-dimethylhydrazine were studied in CDF rats. 2. Treatment with disulfiram causes enhanced elimination of azomethane in the expired air, inhibition of CO2 production, and decreased levels of 1,2-dimethylhydrazine metabolites in the urine. 3. These results suggest that disulfiram inhibits the N-oxidation of azomethane to azoxymethane, thus preventing further metabolism to the ultimate carcinogenic species, and provide an explanation for the observations that disulfiram inhibits 1,2-dimethylhydrazine-induced neoplasia of the large intestine.

Enzymic deacetylation of carcinogenic arylacetamides by tissue microsomes of the dog and other species

The relative ability of arylacetamide deacetylase enzyme systems of dog liver to carry out the deacetylation of the carcinogens, 4-acetylaminobiphenyl, 2-acetylaminofluorene, and 2-acetylaminaphthalene, was examined. The arylacetamides were incubated with unfortified dog liver microsomes, and enzyme activity (nmol arylamine/mg protein/hr) was estimated by colorimetric quantitation of the resulting arylamines. The dog liver enzyme system displayed characteristics similar to those described for the rodent liver enzyme system in that enzyme activity was greatest in liver tissue, was localized in the microsomal subcellular fraction, required no cofactors, and was inhibited by heat, sodium fluoride, and thiol reagents. In five replicate assays, the relative rates of deacetylation were about 10, 6, and 1 with 4-acetylaminobiphenyl (84.8 +/- 12.4), 2-acetylaminofluorene (52.5 +/- 5.1), and 2-acetylaminonaphthalene (8.8 +/- 3.3), respectively. As a canine urinary bladder carcinogen, 4-acetylaminobiphenyl is considered more potent than 2-acetylaminofluroene, while 2-acetylaminonaphthalene is devoid of detectable carcinogenic activity, despite the fact that 2-aminoaphthalene is a well-established canine urinary bladder carcinogen. Removal of the acetyl group may be a requirement for urinary bladder carcinogenesis; accordingly, the present studies demonstrate the appearance of a direct relationship between dog liver deacetylase enzyme specificity and urinary bladder susceptibility to these carcinogenic arylacetamides.

The nature of the alkylation lesion in mammalian cells

Methylating agents may produce as many as nine alkylated purine and pyrimidine adducts in DNA, as well as forming phosphotriesters and inducing apurinic sites and strand breaks. Although some of these products are formed in proportionately small amounts, there are sufficient sites affected in the DNA of a mammalian cell to make even the most minor product of potential biological significance. It is not possible to specify the exact reaction sites resulting in biological damage, but it is possible to quantitate the excisiion-repair of such damage both in the bulk of the DNA and at DNA growing points. Excision-repair can be measured in the bulk of the DNA by determining the specific activity of the NaCl eluate of a benzoylated naphthoylated DEAE-cellulose column of extracts of cells after treatment and incubation in the presence of hydroxyurea and labeled thymidine. The average number of nucleotides inserted per methyl methanesulfonate-induced methyl group is 0.1, per apurinic site is 9. Repair in growing-point regions after methyl methanesulfonate treatment occurs to approximately the same extent as in the bulk of the DNA.