Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay.

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Neonatal mouse assay for tumorigenicity: alternative to the chronic rodent bioassay.

The chronic rodent bioassay for tumors has been utilized systematically for 25 years to identify chemicals with carcinogenic potential in man. In general, those chemicals exhibiting tumorigenicity at multiple sites in both mice and rats have been regarded as possessing strong carcinogenic potential in humans. In comparison, the value of data collected for those test chemicals exhibiting more sporadic tumorigenicity results (e.g., single species/single sex or dose-independent) has been questioned. As knowledge of the carcinogenic process has increased, several alternative test systems, usually faster and less expensive than the 2-year bioassay, have been suggested for identification of the strongly acting, transspecies carcinogens. The International Conference on Harmonization for Technical Requirements for the Registration of Pharmaceuticals for Human Use has proposed an international standard that allows for the use of one long-term rodent carcinogenicity study, plus one supplementary study to identify potential human pharmaceutical carcinogens. The neonatal mouse assay for tumorigenicity has been used since 1959; however, relative to other alternate tests, little has been written about this system. It is clear that this assay system successfully identifies transspecies carcinogens from numerous chemical classes, thus recommending itself as a strong candidate for a supplementary study to identify potential human carcinogens. In contrast, there are decidedly less data available from this assay in response to pharmaceuticals shown to exhibit weak and/or conflicting results in the 2-year bioassay, knowledge invaluable to the regulatory process. This paper reviews the historical development and our experience with the neonatal mouse assay and includes suggestions for a standardized protocol and strategies to document its response to "weak" and/or "nongenotoxic" carcinogens.

DNA adduct levels in congenic rapid and slow acetylator mouse strains following chronic administration of 4-aminobiphenyl.

4-Aminobiphenyl (4-ABP) is a human and mouse bladder carcinogen. Epidemiological studies have shown that individuals with a slow acetylator phenotype, especially those exposed to high levels of carcinogenic aromatic amines, show an increased susceptibility to bladder cancer. In order to determine if a slow acetylator phenotype results in increased DNA damage, congenic mouse strains C57BL/6J and B6.A-Nat(s), which differ genetically at the acetyltransferase (EC 2.3.1.5) locus as homozygous rapid (Natr/Natr) and homozygous slow (Nat(s)/Nat(s)) acetylators respectively, were continuously administered 4-ABP.HCl (55-300 p.p.m.) in their drinking water for 28 days. The levels of covalently bound N-(deoxyguanosin-8-yl)-4-ABP-DNA adducts, which are believed to be critical for the initiation of tumors, were quantitated in the liver and bladder by 32P-postlabeling analysis. The levels of the hepatic DNA adduct increased with dose in both sexes, but were independent of the mouse acetylator genotype. At comparable doses, however, the levels of DNA adducts were 2-fold higher in the liver of the female as compared to the male animals. The DNA adducts also increased with dose in bladder of the male mice, but in contrast to the liver, the adduct levels were approximately 2-fold lower in the bladder DNA of the female mice. Also in contrast to the liver, the levels of bladder DNA adducts were significantly higher (P < or = 0.03) in the phenotypic rapid acetylator females compared to the slow acetylators at both 75 and 150 p.p.m. doses; the median levels of adducts were 10-20% higher in the phenotypic slow acetylator male bladders compared to their rapid acetylator counterparts. The results of these studies are consistent with the increased carcinogenicity of 4-ABP to the liver of female mice and the bladder of male mice. They further suggest that factors other than acetylator phenotype limit the extent of DNA adduct formation from 4-ABP in these mice.

Hemoglobin adduct and hepatic- and urinary bladder-DNA adduct levels in rapid and slow acetylator Syrian inbred hamsters administered 2-aminofluorene.

The levels of covalently bound arylamine-hemoglobin and DNA adduct formation were used as dosimeters to measure the effect of acetylator genotype and sex on the metabolic conversion of the carcinogen, 2-aminofluorene, to reactive intermediates. A single high dose of 2-aminofluorene (60 mg/kg b.wt. i.p.) was administered to male and female homozygous rapid (Patr/Patr) acetylator hamsters (MHA/SsLaK) and homozygous slow (Pats/Pats) acetylator hamsters (Bio. 82.73/H). By using 32P-postlabeling assay methodology, a sole nonacetylated DNA adduct, which cochromatographed with authentic N-(deoxyguanosin-8-yl)-2-aminofluorene was detected at 3, 6, 12, 18 or 24 hr postdosing in liver and urinary bladder DNA of both rapid and slow acetylator hamsters. The highest levels were detected at 18 hr post 2-aminofluorene injection at which time the average levels of hepatic 2-aminofluorene-DNA adducts were similar between male and female rapid and slow acetylators. By comparison, the levels of 2-aminofluorene-DNA adducts in the urinary bladder at 18 hr were about 4-fold lower than in the liver, and were significantly greater in homozygous rapid than in homozygous slow acetylator counterparts (P less than .01). In both the liver and urinary bladder, the levels of 2-aminofluorene-DNA adducts were independent of sex. In contrast to the DNA adduct data, the levels of 2-aminofluorene-hemoglobin adducts, evaluated by capillary gas chromatography-mass spectrometry, were significantly higher in the homozygous slow acetylators than in homozygous rapid acetylators. However, there again were no differences between males and females.(ABSTRACT TRUNCATED AT 250 WORDS)

Arachidonic acid-dependent peroxidative activation of carcinogenic arylamines by extrahepatic human tissue microsomes.

Prostaglandin H synthase (PHS), an arachidonic acid-dependent peroxidase, has been implicated in the peroxidative activation of carcinogenic aromatic amines in extrahepatic carcinogen target tissues of experimental animals. We have examined the arachidonic acid-dependent activation of [3H]benzidine to DNA-bound products by microsomal preparations from 75 normal human tissues obtained during necessary surgical procedures. For several samples of urinary bladder epithelium, prostatic epithelium, colonic mucosa, and peripheral lung tissue, an arachidonic acid-dependent, microsomal-catalyzed activation of benzidine was observed; and the activity could be inhibited appreciably by indomethacin, a known inhibitor of PHS. Little or no arachidonic acid-dependent activity was detected in human placenta, breast, or liver microsomes or the majority of colon microsomes. Substrate specificity was also examined with purified ram PHS and with human bladder and with active colon preparations. Purified PHS catalyzed the activation of benzidine much greater than 2-naphthylamine, 2-amino-6-methyldipyrido[1,2-alpha:3',2'-d]imidazole greater than 4-aminobiphenyl greater than 2-amino-3-methylimidazo[4,5-f]quinoline greater than 3-amino-1-methyl-5H-pyrido[4,3-b] indole. In comparison, human bladder and colon microsomes catalyzed the activation of benzidine greater than 4-aminobiphenyl, 2-amino-6-methyldipyrido[1,2-alpha:3',2'-d]imidazole, 2-naphthylamine greater than 2-amino-3-methylimidazo[4,5-f]quinoline, 3-amino-1-methyl-5H-pyrido[4,3-b]indole. To confirm the occurrence of PHS antigen in human extrahepatic tissues, an avidin/biotin-amplified competitive enzyme-linked immunoabsorbent assay was developed with purified ram PHS and a commercially available monoclonal antibody known to cross-react with human platelet PHS. The avidin/biotin-amplified enzyme-linked immunosorbent assay, which detected ng quantities of ram PHS, clearly established the presence of the PHS protein in human bladder, prostate, and lung microsomes. In contrast, PHS antigen was not detected in the liver or placental microsomes. The interindividual and tissue-dependent variability of PHS and its role in aromatic amine carcinogenesis are discussed.

Immunochemical quantitation of DNA adducts derived from the human bladder carcinogen 4-aminobiphenyl.

To assess target-tissue exposure to the human urinary bladder carcinogen 4-aminobiphenyl (ABP), we have developed a sensitive immunochemical method for measuring the major arylamine-DNA adduct formed, N-(guan-8-yl)-ABP (Gua-C8-ABP). High-affinity polyclonal antisera from rabbits immunized with N-(guanosin-8-yl)-ABP coupled to keyhole limpet hemocyanin were characterized and shown to have high specificity for antigenic determinants on the purine and biphenyl rings of Gua-C8-ABP and minimal cross-reactivity with ABP, deoxyguanosine, or hydrolyzed DNA. Assay standards containing ABP-modified DNA were prepared by reacting [3H]N-hydroxy-ABP with calf thymus DNA. DNA samples were hydrolyzed with trifluoroacetic acid and dried under vacuum, and the residues were dissolved in dimethyl sulfoxide under argon. Using a streptavidin-biotin amplified enzyme-linked immunosorbent assay, DNA hydrolysates competed at 25 micrograms DNA/microtiter well for a limiting amount of anti-keyhole limpet hemocyanin-(Gua-C8-ABP) in the presence of excess solid-phase bovine serum albumin-(Gua-C8-ABP) coating antigen. The limit of sensitivity for this assay using 25 micrograms DNA was 2 adducts/10(8) nucleotides. Gua-C8-ABP adducts in liver and bladder epithelial DNAs were readily quantified after p.o. administration of 5 mg/kg ABP to dogs. This methodology is capable of detecting adducts at levels of biological significance and should be applicable to human target-tissue dosimetry.

Acetylator genotype-dependent metabolic activation of carcinogenic N-hydroxyarylamines by S-acetyl coenzyme A-dependent enzymes of inbred hamster tissue cytosols: relationship to arylamine N-acetyltransferase.

A genetic polymorphism in S-acetyl coenzyme A (AcCoA)-dependent N-acetyltransferase has been associated with a differential risk for certain cancers in humans. In this study, several tissues from the inbred Syrian hamster with a genetically defined AcCoA-dependent N-acetyltransferase polymorphism (homozygous rapid acetylator, Bio. 87.20; homozygous slow acetylator, Bio. 82.73/H; and heterozygous acetylator, Bio. 87.20 X Bio. 82.73/H F1), were investigated for the relationship of arylamine N-acetyltransferase to the AcCoA-dependent metabolic activation of carcinogenic N-hydroxy (N-OH)-arylamines to bind to DNA (O-acetyltransferase). The levels of both 2-aminofluorene (AF) N-acetyltransferase and N-OH-AF O-acetyltransferase activity reflected the N-acetylator genotype in liver, intestine, kidney and lung cytosols. A significant acetylator gene--dose response for AF N-acetyltransferase and N-OH-AF O-acetyltransferase activities was observed in liver and lung cytosols. In contrast, acetylator genotype was not consistently expressed for the AcCoA-dependent N-acetylation of 4-aminobiphenyl (ABP), nor for the AcCoA-dependent metabolic activation of N-OH-ABP and N-OH-3,2'-dimethyl-4-aminobiphenyl in these same tissue cytosols. Two peaks of acetyltransferase activity were partially purified by ion exchange FPLC chromatography from the hepatic cytosol of both the homozygous rapid and homozygous slow acetylator hamster. In contrast to unfractionated cytosol, the isozyme(s) eluting first clearly demonstrated levels of AcCoA-dependent arylamine N-acetyltransferase and N-OH-arylamine O-acetyltransferase activities that were consistent with N-acetylator genotype (polymorphic) for all substrates tested. In contrast, the slower eluting isozyme(s) in each acetylator cytosol showed levels of AcCoA-dependent N- and O-acetyltransferase activities that did not vary with N-acetylator genotype (monomorphic). The AcCoA-dependent O-acetyltransferase activity of both the monomorphic and polymorphic peaks was paraoxon resistant. These studies demonstrate acetylator genotype-dependent control of AcCoA-dependent metabolic activation of N-OH-arylamines(O-acetylation) by polymorphic isozyme(s) similar to that for AcCoA-dependent N-acetylation of arylamines in the hamster. The polymorphic genetic control of N-OH-arylamine O-acetyltransferase may be an important risk factor for arylamine-induced cancer, in those species and tissues expressing appreciable levels of O-acetyltransferase activity.

The S-acetyl coenzyme A-dependent metabolic activation of the carcinogen N-hydroxy-2-aminofluorene by human liver cytosol and its relationship to the aromatic amine N-acetyltransferase phenotype.

A genetic polymorphism in the enzymatic N-acetylation of sulfamethazine and other drugs in humans is well known and has been related to differential susceptibility to drug toxicities. Carcinogenic aromatic amines such as 2-aminofluorene also undergo N-acetylation, and phenotypic slow acetylator individuals have been suggested to be at increased risk to arylamine-induced urinary bladder cancer. However, acetyltransferases have also been shown to catalyze a final metabolic activation step in the conversion of both hydroxamic acid (e.g. N-hydroxy-N-acetyl-2-aminofluorene N,O-acyltransferase) and N-hydroxy-arylamine (e.g. N-hydroxy-2-aminofluorene O-acetyltransferase) metabolites to DNA-bound adducts. In this regard, rapid acetylators have recently been reported to be at higher risk for colorectal cancer. In this study, we examined the enzymatic activity of 35 human liver cytosol samples (obtained surgically from organ donors) for sulfamethazine and 2-aminofluorene N-acetyltransferase activities, N-hydroxy-N-acetyl-2-aminofluorene N,O-acyltransferase activity, and the acetyl coenzyme A (CoA)-dependent O-acetylation of N-hydroxy-2-aminofluorene to form DNA-bound products. The results with sulfamethazine indicated that about two-thirds of the human liver samples were of the slow acetylator phenotype; the same individuals also exhibited levels of 2-aminofluorene N-acetylation that were consistent with their respective sulfamethazine-N-acetylation activity. N-Hydroxy-N-acetyl-2-aminofluorene N,O-acyltransferase activity was not detected. However, the acetyl CoA-dependent activation of N-hydroxy-2-aminofluorene was observed for nearly all of the samples and was consistently higher in the fast acetylator group. These data support the hypothesis that phenotypic rapid acetylator individuals are likely to be at higher risk to aromatic amine-induced cancers in those tissues containing appreciable levels of N-hydroxy arylamine O-acetyltransferase.

Role of aromatic amine acetyltransferase in human colorectal cancer.

Hepatic arylamine acetyltransferase phenotype has been suggested to be an important risk factor for urinary bladder carcinogenesis in individuals with known exposure to aromatic amines. This study was performed to evaluate the relative distribution of fast- and slow-acetylator phenotypes both in a population of men, 45 to 75 years of age, with a history of colorectal cancer and in a matched control group. Acetyltransferase activity was determined by administration of sulfamethazine and by subsequent analysis of blood and urine samples for N-acetylsulfamethazine and sulfamethazine using high-pressure liquid chromatography. The control group was composed of 28 slow-, two intermediate-, and 11 fast-acetylator individuals, while the group of patients with a history of cancer consisted of 20 slow-, three intermediate-, and 20 fast-acetylator phenotypes. This higher relative proportion of fast acetylators in the patients with a cancer history was highly significant and is consistent with the hypothesis that aromatic amines could play a role in the etiology of human colorectal cancer.

Acetyl coenzyme A-dependent metabolic activation of N-hydroxy-3,2'-dimethyl-4-aminobiphenyl and several carcinogenic N-hydroxy arylamines in relation to tissue and species differences, other acyl donors, and arylhydroxamic acid-dependent acyltransferases.

The metabolic activation of several carcinogenic N-hydroxy (N-OH)-arylamines by cytosolic S-acetyl coenzyme A (AcCoA)-dependent enzymes was examined in tissues and species susceptible to arylamine carcinogenesis. Comparisons of the AcCoA-dependent activity were also made with known cytosolic arylhydroxamic acid-dependent acyltransferases and with the ability of different acyl donors to mediate the binding of N-OH-arylamines to DNA. With rat hepatic cytosol, AcCoA-dependent DNA binding was demonstrated for several [3H]N-OH-arylamines, in the order: N-OH-3,2'-dimethyl-4-aminobiphenyl (N-OH-DMABP), N-OH-2-aminofluorene (N-OH-AF) greater than N-OH-4-aminobiphenyl greater than N-OH-N'-acetylbenzidine greater than N-OH-2-naphthylamine; N-OH-N-methyl-4-amino-azobenzene was not a substrate. No activity was detected in dog hepatic or bladder cytosol with any of the N-OH-arylamines tested. Using either N-OH-DMABP or N-OH-AF and rat hepatic cytosol, activation to DNA-bound products was also detected with acetoacetyl- and propionyl-CoA but not with folinic acid or six other acyl CoA's. However, p-nitrophenyl acetate which is known to generate acetyl-enzyme intermediates effectively replaced AcCoA. Subcellular fractionation of rat liver showed that the AcCoA-dependent DNA-binding of N-OH-DMABP with cytosol was 5 times greater than that obtained with the microsomal or mitochondrial/nuclear fractions. Furthermore, the cytosolic activity was insensitive to inhibition by the esterase/deacetylase inhibitor, paraoxon; while the activity of the other subcellular fractions was completely inhibited (greater than 95%). AcCoA-dependent activation of N-OH-DMABP was also detected with rat tissue cytosols from intestine, mammary gland and kidney, which like the liver, are targets for arylamine-induced tumorigenesis. Using N-OH-DMABP, AcCoA-dependent DNA-binding activity was also detected in the hepatic cytosols from several species in the order: rabbit greater than hamster greater than rat, human greater than guinea pig greater than mouse. In contrast, the arylhydroxamic acid, N-OH-N-acetyl-DMABP, was not activated to a DNA-binding metabolite by the hepatic cytosolic N,O-acyltransferase of any of these species, thus suggesting that the AcCoA-mediated binding of N-OH-DMABP results from the direct formation of N-acetoxy-DMABP. With N-OH-AF as the substrate, the AcCoA-dependent activation was in the order: rabbit greater than guinea pig, hamster greater than mouse greater than human, rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of an avidin-biotin amplified enzyme-linked immunoassay for detection of DNA adducts of the human bladder carcinogen 4-aminobiphenyl.

4-Aminobiphenyl (ABP) is a known human urinary bladder carcinogen which is present in tobacco smoke and may be ubiquitous in the environment. As a biological monitor of carcinogen exposure, we have developed an immunological method for measuring the predominant carcinogen-DNA adduct of ABP, N-(deoxyguanosin-8-yl)-ABP (dG-C8-ABP). Rabbits were immunized with keyhole limpet hemocyanin (KLH) conjugate prepared by a periodate oxidation and coupling of N-(guanosin-8-yl)-ABP (rG-C8-ABP) to the protein. The resulting polyclonal antisera was systematically characterized using dual inhibitor methodology augmented by specialized computer and software support; and a competitive avidin-biotin enzyme-linked immunoassay (A-B ELISA) assay employing polyclonal rabbit anti-KLH-(rG-C8-ABP) was developed. Under the assay conditions described, the detection limit for dG-C8-ABP was 18 fmol/well. The relative lack of reactivity toward ABP, N-acetyl-4-aminobiphenyl, N-(deoxyadenosin-8-yl)-ABP, N-(deoxyguanosin-8-yl)-2-aminofluorene and deoxyguanosine as inhibitors indicated that primary specificity involves epitopes found on the purine and biphenyl rings. Results emphasize the need to define polyclonal anti-adduct sera operationally in the context of the antigen/assay system used to evaluate it. Assay sensitivity was achieved by decreasing the amount of antibody and solid-phase antigen in the competitive portion of the assay and the use of avidin-biotin as well as enzymatic amplification. This methodology is a useful alternative to other ultrasensitive techniques and should be directly applicable to the detection of ABP-DNA adducts in exposed human populations.

Hepatic N-oxidation, acetyl-transfer and DNA-binding of the acetylated metabolites of the carcinogen, benzidine.

The metabolic N-oxidation, N-acetylation and N-deacetylation of the carcinogen benzidine (BZ) and its N-acetylated metabolites were examined in vitro with rat and mouse liver subcellular fractions. N-Oxidation of N-acetylbenzidine (ABZ) and N,N'-diacetylbenzidine (DABZ) was found to occur with NADPH-, NADH-fortified microsomes, although total oxidation at both nitrogens of ABZ was substantially faster than the N-oxidation of DABZ (four times for the mouse and 48 times for the rat). In both species, N-oxidation of ABZ to the arylhydroxylamine, N'-hydroxy-N-acetylbenzidine (N'-OH-ABZ), was somewhat faster than the formation of the arylhydroxamic acid, N-hydroxy-N-acetylbenzidine (N-OH-ABZ). N-Acetylation of BZ and ABZ by liver cytosol was quite efficient for both species (0.7-2.9 nmol/min/mg cytosolic protein), and these rates were found to be 3-10 times faster than their corresponding rates of N-oxidation. N-Deacetylation of ABZ and DABZ by mouse liver microsomes occurred at a rate that was comparable with N-acetylation; while N-deacetylation by rat liver microsomes was relatively slow, only 1-2% of the rate of N-acetylation. In the case of N-hydroxylated derivatives, N-OH-ABZ and N'-OH-ABZ, hepatic cytosolic N-acetylation by both rats and mice to form N-OH-DABZ was quite rapid (0.5-1.9 nmol/min/mg cytosol protein). Hepatic microsomal deacetylation of N-OH-DABZ also occurred with both species and was 2-4 times the rate of N-acetylation. These studies indicate that a significant concentration of potentially electrophilic monoacetylated N-oxidized metabolites may accumulate within the liver cell, and that they may serve as intermediates in the synthesis of the highly toxic metabolite, N-OH-DABZ. A major metabolic pathway for the formation of N-OH-DABZ is proposed as: BZ----ABZ----N'-OH-ABZ----N-OH-DABZ. The activation of N-OH-DABZ by cytosolic N,O-acyltransferase and N'-OH-ABZ by cytosolic sulfotransferase and O-acetyltransferase (acetyl CoA-dependent binding to DNA) were also examined. N-OH-DABZ N,O-acyltransferase and N'-OH-ABZ O-acetyltransferase were found to be significant pathways for rat and mouse liver, respectively. In addition, the DNA adduct formed from N-OH-DABZ in the presence of partially-purified rat hepatic N,O-acyltransferase was shown to be N'-(deoxyguanosin-8-yl)-N-acetylbenzidine, which is identical to that formed in rat liver in vivo and in the direct reaction of N'-OH-ABZ with DNA in vitro under acidic conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA adducts formed from the probable proximate carcinogen, N-hydroxy-3,2' -dimethyl-4-aminobiphenyl, by acid catalysis or S-acetyl coenzyme A-dependent enzymatic esterification.

The arylamine carcinogen 3,2'-dimethyl-4-aminobiphenyl (DMABP) has been proposed to be metabolically activated to DNA-binding derivatives through the formation of an N-hydroxy intermediate. In this study, the subsequent activation of N-hydroxy-DMABP through acid catalysis or enzymatic esterification was examined. [Ring-3H]N-hydroxy-DMABP was reacted with calf thymus DNA at pH 4.6 for 15 min to yield 370 arylamine residues per 10(6) nucleotides, while at pH 7.4 the binding was only two residues per 10(6) nucleotides. The DNA modified under acidic conditions was enzymatically hydrolyzed and analyzed by h.p.l.c. which indicated the presence of three major adducts. The products were identified by spectral and chemical properties as N-(deoxyguanosin-8-yl)-DMABP (60-70%), 5-(deoxyguanosin-N2-yl)-DMABP (2-3%) and N-(deoxyadenosin-8-yl)-DMABP (1-3%). The same adducts have previously been detected in the liver and colon of rats administered DMABP or its hydroxamic acid. Incubation of rat hepatic or intestinal cytosol at pH 7.4 for 15 min with [ring-3H]N-hydroxy-DMABP in the presence of S-acetyl coenzyme A (AcCoA) and calf thymus DNA resulted in DNA binding at levels of 30-80 arylamine residues per 10(6) nucleotides. H.p.l.c. analysis of the DNA modified in the presence of AcCoA indicated the formation of the same adducts detected in the acid-catalyzed reactions. When arylhydroxamic acid N,O-acyltransferase assays were conducted with rat liver cytosol and N-acetyl-N-hydroxy-DMABP as the substrate, binding to nucleic acids was not observed. Similarly, 3'-phosphoadenosine-5'-phosphosulfate-dependent sulfotransferase-mediated DNA binding could not be demonstrated. These data indicate that in a suitable acidic environment, N-hydroxy-DMABP will react with DNA to yield the same adducts found in vivo. Under neutral conditions, however, N-hydroxy-DMABP appears to undergo AcCoA-dependent transacetylation to an electrophilic acetoxy ester which will spontaneously react with DNA.

Comparison of the in vitro and in vivo hepatic metabolism of the carcinogen 1-nitropyrene.

[4,5,9,10(-3)H]1-nitropyrene was incubated with NADH- or NADPH-fortified rat liver microsomes under an argon atmosphere. Residual substrate and metabolites were extracted with ethyl acetate and analyzed by high pressure liquid chromatography. Both reduced and oxidized products were formed: namely, 1-aminopyrene, trans-4,5-dihydroxy-4,5-dihydro-1-nitropyrene, and 3-, 6- and 8-hydroxy-1-nitropyrene. When incubations were conducted with rat liver cytosol, only the reduced products 1-nitrosopyrene and 1-aminopyrene were detected. In parallel experiments, [4,5, 9,10-3H]1-nitropyrene was administered to rats by intravenous injection or gavage and the bile was collected. After 4 h, approximately one-third of the intravenously-administered compound appeared in the bile as O-glucuronides of 3-, 6- and 8-hydroxy-1-nitropyrene, the O-glucuronide of trans-4,5-dihydroxy-4,5-dihydro-1-nitropyrene, and unidentified glutathione conjugates. The same metabolites were found in rats treated with 1-nitropyrene by gavage; however, only 10% of the dose appeared in the bile within 12 h. These studies indicate that both nitroreduction and ring oxidation are involved in the hepatic metabolism of 1-nitropyrene. The importance of these pathways in the etiology of 1-nitropyrene tumorigenesis is discussed.

Formation of DNA adducts in vivo in rat liver and intestinal epithelium after administration of the carcinogen 3,2'-dimethyl-4-aminobiphenyl and its hydroxamic acid.

Administration of the 3H-labeled colon carcinogen, 3,2'-dimethyl-4-aminobiphenyl (DMABP) and its hydroxamic acid derivative, N-hydroxy-N-acetyl-DMABP, to male F344 rats resulted in high levels of covalent binding to hepatic and intestinal DNA, RNA and protein. For both compounds, binding to hepatic macromolecules was 2-4 times higher than in the intestine. High pressure liquid chromatographic analysis of the enzymatically hydrolyzed DNA from liver and intestinal epithelium indicated the presence of two carcinogen-DNA adducts: 5-(deoxyguanosin-N2-yl)-DMABP (15%), N-(deoxyguanosin-8-yl)-DMABP (50%), and a decomposition product of the latter (15%). N-acetylated adducts were not detected. When measured after 7 days, all adducts in the intestinal DNA had decreased by 70%, while only a 29% decrease had occurred in the hepatic DNA. To determine if the loss of DMABP products was a consequence of cell turnover or repair, rats were treated with [3H]thymidine and DMABP, and the specific activity of hepatic liver and intestinal DNA was measured. Between 1 and 7 days only a slight decrease in [3H]thymidine content occurred in hepatic DNA as compared with a 95% reduction in intestinal DNA. Thus, the higher rate of DNA synthesis in the intestine versus that in the liver may serve to promote fixation of the initiating lesion and account for the preferential induction of intestinal cancer by DMABP. Furthermore, comparison of these data with metabolic activation pathways reported earlier strongly suggest that N-hydroxy-DMABP is the proximate carcinogenic metabolite of both DMABP and N-hydroxy-N-acetyl-DMABP.

Alteration of urinary levels of the carcinogen, N-hydroxy-2-naphthylamine, and its N-glucuronide in the rat by control of urinary pH, inhibition of metabolic sulfation, and changes in biliary excretion.

The hepatic metabolism of arylamine bladder carcinogens to N-hydroxy arylamine N-glucuronides, their excretion in the urine, and their subsequent acidic hydrolysis to highly carcinogenic and reactive N-hydroxy arylamines have been proposed as essential steps in arylamine-induced urinary bladder carcinogenesis. In this study, alteration of urinary pH, inhibition of metabolic sulfation, and blockage of biliary disposition were shown to profoundly affect the urinary excretion of the probable ultimate bladder carcinogen, N-hydroxy-2-naphthylamine (N-HO-2-NA) and its N-glucuronide conjugate. The normal pH of rat urine (6.7) was altered to 5.7 or 7.7 by administration of NH4Cl or NaHCO3 in the drinking water. Subsequent treatment with either 2-naphthylamine (2-NA) or 2-nitronaphthalene (2-NN) resulted in increased urinary levels of free N-HO-2-NA (relative to its N-glucuronide) in acidic urines and decreased relative amounts of free N-HO-2-NA in alkaline urines. In addition, 2-NN yielded 5--10-fold greater levels of urinary N-HO-2-NA and its N-glucuronide than rats given 2-NA; and 2-NA was not detected as a urinary metabolite of 2-NN. Some 12 additional metabolites of 2-NA and 2-NN were also found. Of these, 2-amino-1-naphthol and its sulfate and glucuronide conjugates were quantitated. From these data, 2-NA and 2-NN appear to share common metabolic pathways which yield free N-HO-2-NA as a putative ultimate urinary bladder carcinogen. Pentachlorophenol, a known inhibitor of hepatic sulfotransferases, was shown to cause a 2--3-fold increase in the urinary levels of N-HO-2-NA N-glucuronide and N-HO-2-NA from 2-NA-treated rats. Similarly, inhibition of the biliary excretion of 2-NA by bile duct ligation resulted in a 6-fold increase in total urinary N-HO-2-NfA. Furthermore, analyses of bile revealed that substantial amounts of N-HO-2-NA N-glucuronide, but not free N-HO-2-NA, were present. The role of urinary versus biliary excretion of N-hydroxy arylamines in relation to bladder and colon carcinogenesis is discussed.

Formation of 3-(glutathion-S-YL)-N-methyl-4-aminoazobenzene and inhibition of aminoazo dye-nucleic acid binding in vitro by reaction of glutathione with metabolically-generated N-methyl-4-aminoazobenzene-N-sulfate.

The reaction of glutathione (GSH) with metabolically-formed N-methyl-4-aminoazobenzene-N-sulfate (MAB-N-sulfate), a presumed ultimate carcinogenic metabolite of N,N-dimethyl-4-aminoazobenzene (DAB), was investigated using a hepatic sulfotransferase incubation mixture containing GSH and the proximate carcinogen, N-hydroxy-N-methyl-4-aminoazobenzene (N-HO-MAB). Under these conditions, 6--16% of the MAB-N-sulfate formed could be trapped as an aminoazo dye-GSH adduct. Upon subsequent purification, the adduct was shown to be chromatographically and spectrally identical to 3-(glutathion-S-yl)-N-methyl-4-aminoazobenzene (3-GS-MAB), a known biliary metabolite of DAB and a product of the reaction of the synthetic ultimate carcinogen, N-benzoyloxy-N-methyl-4-aminoazobenzene(N-BzO-MAB), with GSH. Neither 2'- nor 4'-GS-MAB, both products of the latter reaction, were detected in the sulfotransferase incubation mixture. GSH-S-transferases did not appear to be involved in the reaction of MAB-N-sulfate of N-BzO-MAB with GSH. The addition of triethyltin, a potent GSH-S-transferase inhibitor, had no effect on the yield of 3-GS-MAB in (N-HO-MAB sulfotransferase)-GSH incubations; and the addition of cytosol or purified GSH transferases A and B to a (N-BzO-MAB)-GSH reaction mixture did not increase the amount of 3-GS-MAB formed.