Bioactivation of N-arylhydroxamic acids by rat hepatic N-acetyltransferase. Detection of multiple enzyme forms by mechanism-based inactivation

Human arylamine N-acetyltransferase 1: in vitro and intracellular inactivation by nitrosoarene metabolites of toxic and carcinogenic arylamines

Arylamines (ArNH 2) are common environmental contaminants, some of which are confirmed risk factors for cancer. Biotransformation of the amino group of arylamines involves competing pathways of oxidation and N-acetylation. Nitrosoarenes, which are products of the oxidation pathway, are electrophiles that react with cellular thiols to form sulfinamide adducts. The arylamine N-acetyltransferases, NAT1 and NAT2, catalyze N-acetylation of arylamines and play central roles in their detoxification. We hypothesized that 4-nitrosobiphenyl (4-NO-BP) and 2-nitrosofluorene (2-NO-F), which are nitroso metabolites of arylamines that are readily N-acetylated by NAT1, would be potent inactivators of NAT1 and that nitrosobenzene (NO-B) and 2-nitrosotoluene (2-NO-T), which are nitroso metabolites of arylamines that are less readily acetylated by NAT1, would be less effective inactivators. The second order rate constants for inactivation of NAT1 by 4-NO-BP and 2-NO-F were 59200 and 34500 M (-1) s (-1), respectively; the values for NO-B and 2-NO-T were 25 and 23 M (-1) s (-1). Densitometry quantification and comparisons of specific activities with those of homogeneous recombinant NAT1 showed that NAT1 constitutes approximately 0.002% of cytosolic protein in HeLa cells. Treatment of HeLa cells with 4-NO-BP (2.5 microM) for 1 h caused a 40% reduction in NAT1 activity, and 4-NO-BP (10 microM) caused a 50% loss of NAT1 activity within 30 min without affecting either glyceraldehyde 3-phosphate dehydrogenase (GAPDH) or glutathione reductase (GR) activities. 2-NO-F (1 microM) inhibited HeLa cell NAT1 activity by 36% in 1 h, and a 10 microM concentration of 2-NO-F reduced NAT1 activity by 70% in 30 min without inhibiting GAPDH or GR. Mass spectrometric analysis of NAT1 from HeLa cells in which NAT1 was overexpressed showed that treatment of the cells with 4-NO-BP resulted in sulfinamide adduct formation. These results indicated that exposure to low concentrations of nitrosoarenes may lead to a loss of NAT1 activity, thereby compromising a critical detoxification process.

Identification and Characterization of Functional Rat Arylamine N-Acetyltransferase 3: Comparisons with Rat Arylamine N-Acetyltransferases 1 and 2

Arylamine N-acetyltransferases (NATs; EC 2.3.1.5) catalyze both the N-acetylation and O-acetylation of arylamines and N-hydroxyarylamines. Humans possess two functional N-acetyltransferase genes, NAT1 and NAT2, as well as a nonfunctional pseudogene, NATP. Previous studies have identified Nat1 and Nat2 genes in the rat. In this study, we identified and characterized a third rat N-acetyltransferase gene (Nat3) consisting of a single open reading frame of 870 base pairs encoding a 290-amino acid protein, analogous to the previously identified human and rat N-acetyltransferase genes. Rat Nat3 nucleotide sequence was 77.2 and 75.9% identical to human NAT1 and NAT2, respectively. Rat Nat3 amino acid sequence was 68.6 and 67.2% identical to human NAT1 and NAT2, respectively. Rat Nat1, Nat2, and Nat3 were each cloned and recombinantly expressed in Escherichia coli. Recombinant rat Nat3 exhibited thermostability intermediate between recombinant rat Nat1 and Nat2. Recombinant rat Nat3 was functional and catalyzed the N-acetylation of several arylamine substrates, including 3-ethylaniline, 3,5-dimethylaniline, 5-aminosalicylic acid, 4-aminobiphenyl, 4,4'-methylenedianiline, 4,4'-methylenebis(2-chloroaniline), and 2-aminofluorene, and the O-acetylation of N-hydroxy-4-aminobiphenyl. The relative affinities of arylamine carcinogens such as 4-aminobiphenyl, N-hydroxy-4-aminobiphenyl, and 2-aminofluorene for N- and O-acetylation via recombinant rat Nat3 were comparable with recombinant rat Nat1 and higher than for recombinant rat Nat2. This study is the first to report a third arylamine N-acetyltransferase isozyme with significant functional capacity.

Characterization of hamster recombinant monomorphic and polymorphic arylamine N-acetyltransferases: bioactivation and mechanism-based inactivation studies with N-hydroxy-2-acetylaminofluorene

The purified hamster recombinant arylamine N-acetyltransferases (NATs), rNAT1-9 and rNAT2-70D, were characterized for their capabilities to bioactivate N-hydroxy-2-acetylaminofluorene (N-OH-AAF) to DNA binding reactants and for their relative susceptibilities to mechanism-based inactivation by N-OH-AAF. The rate of DNA adduct formation resulting from rNAT1-9 bioactivation of [14C]N-OH-AAF was more than 30 times greater than that of rNAT2-70D-catalyzed bioactivation of [14C]N-OH-AAF. This result is consistent with substrate specificity data indicating that N-OH-AAF is a much better acetyl donor for hamster NAT1 than NAT2. Previous studies indicated that N-OH-AAF is a mechanism-based inactivator of hamster and rat NAT1. In the presence of N-OH-AAF, both rNAT1-9 and rNAT2-70D underwent irreversible, time-dependent inactivation that exhibited pseudo first-order kinetics and was saturable at higher N-OH-AAF concentrations. The enzymes were partially protected from inactivation by the presence of cofactor and substrates. The limiting rate constants (ki) and dissociation constants (Ki) for inactivation by N-OH-AAF were determined. The second-order rate constants (ki/KI) were 22.1 min-1 mM-1 for rNAT1-9 and 1.0 min-l mM-1 for rNAT2-70D, indicating that rNAT1-9 is approximately 20 times more susceptible than rNAT2-70D to inactivation by N-OH-AAF. The kinetic parameters for rNAT1-9 were nearly identical to values previously reported for partially purified hamster NAT1. Partition ratios were 504 for inactivation of rNAT1-9 by N-OH-AAF and 137 for inactivation of rNAT2-70D. Thus, a turnover of almost 4 times as many N-OH-AAF molecules is required to inactivate each molecule of rNAT1-9 than is needed to inactivate rNAT2-70D. The partition ratio data are consistent with the finding that rNAT1-9 catalyzes a higher rate of DNA adduct formation by N-OH-AAF than rNAT2-70D. The combined results indicate that the recombinant enzymes are catalytically and functionally identical to hamster NATs and, therefore, will be a useful resource for studies requiring purified NATs.

Higher frequency of aberrant crypt foci in rapid than slow acetylator inbred rats administered the colon carcinogen 3,2'-dimethyl-4-aminobiphenyl

Humans and other mammals such as rats exhibit a genetic polymorphism in acetyltransferase (NAT2) capacity, yielding rapid and slow acetylator phenotypes. The rapid acetylator phenotype has been associated with increased incidence of human colorectal cancer in some, but not all, epidemiological studies. In order to investigate this possible association, a rapid (F-344) and slow (WKY) acetylator inbred rat model was utilized to investigate the role of the acetylator genotype (NAT2) in the formation of aberrant crypt foci (ACF) following administration of colon carcinogens. Age-matched (retired breeder) female rapid and slow acetylator inbred rats received two weekly injections (50 or 100 mg/kg, sc) of 3,2'-dimethyl-4-aminobiphenyl (DMABP) or a single 50 mg/kg, sc, injection of 1,2-dimethyl-hydrazine (DMH). The rats were euthanized at 10 weeks and ACF were evaluated in the cecum, ascending, transverse, and descending colon, and rectum. ACF were observed in the colon and rectum, but not the cecum of rapid and slow acetylator inbred rats administered DMABP or DMH. ACF were more concentrated in the descending colon. ACF frequencies were significantly higher in colons of rapid than slow acetylator inbred rats administered DMABP, a colon carcinogen which is activated via O-acetylation catalyzed by polymorphic acetyltransferase (NAT2). At 50 mg/kg, ACF frequency in the distal colon was 2.29 +/- 0.57 in rapid acetylators versus 0.38 +/- 0.18 in slow acetylators. At 100 mg/kg, ACF frequency was 4.11 +/- 1.06 in rapid versus 1.57 +/- 0.48 in slow acetylators. ACF frequency did not differ significantly between rapid and slow acetylator inbred rats administered DMH, a colon carcinogen which is not metabolized by polymorphic acetyltransferase. The two inbred rat strains did not differ in hepatic microsomal phenacetin deethylase activity, which is a marker for CYP1A2 activity important for the activation of aromatic amines. These results support the hypothesis that rapid acetylator (NAT2) genotype is a risk factor in aromatic amine-induced colon carcinogenesis.

Longitudinal distribution of arylamine N-acetyltransferases in the intestine of the hamster, mouse, and rat. Evidence for multiplicity of N-acetyltransferases in the intestine

Experimental and clinical evidence indicates that AcCoA:arylamine N-acetyltransferases (NATs; EC 2.3.1.5) are involved in the bioactivation and inactivation of a wide variety of arylamine, hydrazine, and carcinogenic arylamine xenobiotics. Longitudinal distribution of NATs in the intestine of the hamster, mouse, and two strains of rat was examined utilizing the model arylamine substrates procainamide(PA) and p-aminobenzoic acid (PABA) for the monomorphic (NAT1) and polymorphic (NAT2) enzymes in the rodent. NAT1 and NAT2 were distributed quite differently in each species examined. In particular, rat intestinal NATs were distributed equally throughout the intestinal tract. In contrast, hamster intestinal NATs decreased in activity from the proximal small intestine to the distal large intestine. Mouse NAT2 activity was highest in the cecum, whereas NAT1 was highest in the proximal small intestine. Although these model substrates have been shown to be selective for NATs, they are not specific. Therefore, a series of biochemical studies were undertaken to evaluate NAT multiplicity in the intestine of the F-344 rat. To assess multiplicity of NAT expression, selective inhibition, differential sensitivity to heat inactivation, and kinetic analysis were performed on intestinal cytosol. Eadie-Hofstee transformation of PA N-acetylation yielded a curvilinear plot indicative that a low affinity-high capacity enzyme aside from NAT1 (presumably NAT2) was contributing to PA N-acetylation activity. PA activity was found to exhibit approximately 4- to 5-fold greater thermostability than PABA activity. Furthermore, PA acetylation could be inhibited selectively with vinyl fluorenyl ketone (2.5 to 5 microM) but not with methotrexate (up to 2 mM). Taken together, these studies suggest the expression of both NAT1 and NAT2 in the intestine of the F-344 rat.

Development of high sensitive umu test system: rapid detection of genotoxicity of promutagenic aromatic amines by Salmonella typhimurium strain NM2009 possessing high O-acetyltransferase activity

A highly sensitive umu test system for the detection of carcinogenic/mutagenic aromatic amines has been developed utilizing a new tester strain, Salmonella typhimurium NM2009, possessing an elevated O-acetyltransferase (O-AT) level. NM2009 was constructed by subcloning the bacterial O-AT gene into a plasmid vector pACYC184 and introducing the plasmid into the original strain S. typhimurium TA1535/pSK1002 harboring an umuC'-'lacZ fusion gene. The system is based on the ability of DNA-damaging agents (genotoxins) to induce umuC gene expression and monitored by measuring the cellular beta-galactosidase activity evoked by the fusion gene. Twenty-two aromatic amine compounds including arylamines, aminoazo dyes, and heterocyclic aromatic amines were tested for inducibility of DNA damage after metabolic activation by rat liver S9 in strain NM2009 and the sensitivity was compared with those of the parent strain TA1535/pSK1002 and the O-AT-defective strain NM2000. NM2009 had about 400 times higher O-AT activity than the parent strain. It was found that NM2009 was much more sensitive to aromatic amines than other strains to induce umuC gene expression after metabolic activation; the chemicals which were extremely sensitive in strain NM2009 include 2-aminoanthracene, 2-aminofluorene, 2-acetylaminofluorene, benzidine, 6-aminochrysene, 2,4-diaminotoluene, 2,6-diaminotoluene, 1-naphthylamine, o-tolidine, 3-MeO-AAB, o-aminoazotoluene, Glu-P-1, Trp-P-1, MeA alpha C, A alpha C, MeIQ, MeIQx, and IQ. In contrast, Trp-P-2 and PhIP showed almost similar sensitivities in three tester strains used in this study. These results suggest that strain NM2009 with high O-acetyltransferase activity is very useful to detect the genotoxic activities of potential mutagenic aromatic amine compounds, which require metabolic activation via the cytochrome P-450/acetyltransferase system.

Inactivation of hamster monomorphic N-acetyltransferase by vinyl fluorenyl ketone

Arylamine N-acetyltransferases (NATs) are cytosolic enzymes that play important roles in the detoxification and activation of xenobiotic arylamines and their metabolites. Vinyl fluorenyl ketone (VFK) is a selective and potent active site-directed irreversible inhibitor of rat liver monomorphic NAT. The present study demonstrated that VFK is an active site-directed affinity label for hamster liver monomorphic NAT, but is a much less effective inactivator of the polymorphic N-acetyltransferase isozyme. The potency, irreversibility and selectivity of VFK make it a potentially valuable tool for characterization of NATs that exhibit acetyl donor specificity similar to that of hamster monomorphic NAT.

Effect of group-selective modification reagents on arylamine N-acetyltransferase activities

Two forms of hamster hepatic arylamine N-acetyltransferase (NAT; EC 2.3.1.5), designated NAT I and NAT II, were purified 200- to 300-fold by sequential 35-50% ammonium sulfate fractionation, Sephadex G-100 gel filtration chromatography, AAB affinity chromatography, DEAE ion exchange chromatography, and P-200 gel filtration chromatography. Treatment of either NAT I or NAT II with N-ethylmaleimide (NEM), a cysteine selective reagent, caused a concentration-dependent loss of enzymatic activities. Acetyl coenzyme A (AcCoA) protected NAT I against inactivation by NEM, whereas both 2-acetylaminofluorene (2-AAF) and AcCoA protected NAT II against inactivation. Incubation of either NAT I or NAT II with phenylglyoxal (PG), an arginine selective reagent, caused a time-dependent and a concentration-dependent loss of both NAT I and NAT II activities; the inactivations followed pseudo first-order kinetics. The reaction order with respect to PG was approximately two for each enzyme, consistent with the expected stoichiometry for the reaction of PG with arginine. The presence of AcCoA provided full protection of NAT I against inactivation by PG. However, neither AcCoA nor 2-AAF provided protection of NAT II against inactivation by PG. Diethylpyrocarbonate (DEPC), a histidine selective reagent, caused time-dependent and concentration-dependent pseudo first-order inactivation of both NAT I and NAT II. Neither AcCoA nor products of NAT-catalyzed reactions protected NAT I and NAT II against inactivation by DEPC. These results suggest that cysteine, arginine and histidine residues are essential to the catalytic activity of both NAT I and NAT II; the cysteine(s) is located at or near the binding site of NAT I and NAT II, and the arginine residue appears to be located in the AcCoA binding site of NAT I. In contrast, the essential arginine residue(s) of NAT II and the essential histidine residue(s) of both NAT I and NAT II are not likely to reside in the binding site of the enzymes.

Acetylator genotype-dependent N-acetylation of arylamines in vivo and in vitro by hepatic and extrahepatic organ cytosols of Syrian hamsters congenic at the polymorphic acetyltransferase locus

Our laboratory recently reported the successful construction of homozygous rapid (Bio. 82.73/H-Patr) and homozygous slow (Bio. 82.73/H-Pat(s)) acetylator congenic Syrian hamsters. These hamsters are isogenic except for the polymorphic acetylator gene locus (Pat) and perhaps other closely linked loci. The purpose of the present investigation was to assess the expression of acetylator genotype both in vivo and in vitro in a variety of hepatic and extrahepatic organ cytosols. Levels of arylamine N-acetyl-transferase were generally high and in the relative order: liver greater than colon greater than kidney greater than pancreas greater than prostate, urinary bladder, and lung. However, an acetylator gene dose-response was clearly expressed in each tissue, with highest levels in homozygous Patr acetylators, intermediate levels in heterozygous Patr/Pat(s) acetylators, and lowest levels in homozygous Pat(s) acetylators. The magnitude of the acetylator genotype-dependent differences in N-acetyltransferase activity were substrate specific, wherein p-aminobenzoic acid showed the largest differences and p-aminophenol the smallest. The N-acetylation of p-aminobenzoic acid in vivo also reflected acetylator genotype in the congenic hamsters. These results further document the successful construction of rapid and slow acetylator congenic hamsters which should prove very valuable in future studies to assess the role of acetylator genotype in the toxicity and carcinogenicity of arylamine chemicals.

Interaction with microsomal lipid as a major factor responsible for S9-mediated inhibition of 1,8-dinitropyrene mutagenicity

1,8-Dinitropyrene (1,8-DNP), present in polluted air, is a rodent carcinogen and a potent, direct-acting mutagen in salmonella typhimurium TA98. This mutagenicity is markedly reduced in the presence of mammalian hepatic S9 or microsomes. We demonstrate that at least a substantial part of this effect is attributable to non-enzymatic processes. The microsomal-dependent inhibition was unaffected by omission of an NADPH-generating system or when the cytochrome P-450 inhibitor, SKF-525A, or the cytochrome P-448 inhibitor, ellipticine, was incorporated in the metabolic activation system, suggesting that mixed function oxidases are not involved. Heat inactivation partially decreased the ability of induced S9 to reduce DNP mutagenicity. Substitution of S9 with a similar concentration of bovine serum albumin did not affect DNP activity. Thus non-specific binding to microsomal protein is not involved. However, when lipids, derived from uninduced microsomes, were added to incubations of DNP and S. typhimurium TA98, mutagenicity was decreased. Furthermore, substitution of microsomal lipids with a suspension of phosphatidylcholine (PC), a major lipid constituent of microsomes, affected DNP mutagenicity similarly. An increase in PC concentration resulted in a greater inhibitory effect. The reduction in DNP mutagenicity observed with microsomal lipids or with PC was less than that detected with uninduced S9, whilst the mutagenicity of 2-nitrofluorene was reduced to an approximately equal extent by lipids and S9. This phenomenon may be responsible for the S9-mediated detoxification of other mutagenic nitroaromatic compounds and may have important implications for mutagenicity testing.

Drug metabolism in carcinogenesis and cancer chemotherapy

Cytotoxic drugs have become invaluable for the clinical oncologist in the treatment of neoplastic disease. Frequently, these therapeutic agents are used in combination in order to combat the heterogeneity imposed by the variable tumor cell biochemistry of the neoplastic cell population. Hence, one could argue polypharmacy has become the rule rather than the exception in cancer chemotherapy. The use of such regimens obviously increases the potential for drug-drug interactions and also may potentiate the effects of interindividual variation in drug metabolism. Altered expression of drug metabolizing enzymes may also predispose certain individuals to cancer through enhanced metabolic activation and decreased detoxication of environmental, dietary and possibly endogenous procarcinogens. Many anticancer drugs can be considered as prodrugs which require metabolic activation to exert their selective cytotoxic effects. Recent molecular and biochemical advances have increased our understanding of the factors which govern the regulation of drug metabolizing enzymes and have improved our knowledge of the metabolism and action of anticancer agents. The aim of this review is not to exhaustively document all the work in the area of drug metabolism in relation to cancer, but to provide a comprehensive update of some of the recent advances in drug metabolism which have helped to rationalize the mechanism of action of some anticancer drugs and which may help to optimize future patient selection for certain novel chemotherapeutic regimens. This review also discusses some of the more recent breakthroughs in the area of carcinogenesis and highlights directions for future studies.

An isozyme-selective affinity label for rat hepatic acetyltransferases

Affinity chromatography of an ammonium sulfate precipitate obtained from rat hepatic cytosol resulted in the separation of two fractions of N-acetyltransferase (NAT) activity. NATI catalyzed the S-acetylcoenzyme A (AcCoA)-dependent acetylation of p-aminobenzoic acid (PABA); NAT II catalyzed the N-hydroxy-2-acetylaminofluorene (N-OH-AAF)-dependent acetylation of 4-amino-azobenzene (AAB) (N,N-acetyltransferase), the AcCoA-dependent acetylation of procainamide (PA), and the N-arylhydroxamic acid N,O-acyltransferase (AHAT) activity that results in the conversion of N-OH-AAF and related hydroxamic acids to electrophilic reactants. 1-(Fluoren-2-yl)-2-propen-1-one (vinyl fluorenyl ketone, VFK) was shown to be a potent and irreversible inactivator of NAT II activities. A 200-fold higher concentration of VFK was required to inactivate NAT I activity than was required for inactivation of NAT II activities. Similar selectivity in the inactivation of the isozymes was observed when experiments were conducted with enzyme preparations that contained both NAT I and NAT II activities. The presence of substrates and products of the NAT II-catalyzed reactions such as AcCoA, 2-acetylaminofluorene (2-AAF), and N-acetyl-4-aminoazobenzene (N-Ac-AAB) protected NAT II from the inactivating effects of VFK, providing evidence that VFK is an active site directed inhibitor (affinity label) of NAT II. Studies with 1-(fluoren-2-yl)-2-propan-1-one (EFK), an analogue of VFK in which the alpha, beta-unsaturated vinyl ketone group of VFK has been replaced with an ethyl ketone group, demonstrated that the conjugated ketone of VFK is required for inactivation of enzyme activity. The results of these studies suggest that agents such as VFK should have utility as probes of acetyltransferase multiplicity and in the investigation of the active site topography of the enzymes.

Analogues of N-hydroxy-4-acetylaminobiphenyl as substrates and inactivators of hamster hepatic acetyltransferases

1. A series of analogues of N-hydroxy-4-acetylaminobiphenyl (1, N-OH-AAB) has been synthesized and evaluated in vitro as substrates and inactivators of hamster hepatic N,N-acetyltransferase (N,N-AT) activity. The analogues of 1 are N-arylhydroxamic acids in which an atom or small functional group has been incorporated between the phenyl rings of 1. 2. The structural and molecular properties of the atoms between the two phenyl rings had little influence on the ability of the compounds to serve as acetyl donors in the N-arylhydroxamic acid-dependent transacetylation of 4-aminoazobenzene (AAB) catalysed by N,N-AT. An exception was the SO2 analogue (6) which was inactive. 3. All of the compounds except 6 were mechanism-based inactivators (suicide inhibitors) of hamster hepatic N,N-AT. The inhibition of N,N-AT by the hydroxamic acids was irreversible. The properties of the atom or functional group between the phenyl rings had a substantial influence on the relative effectiveness of the compounds as inactivators of N,N-AT. trans-N-Hydroxy-4-acetylaminostilbene (N-OH-AAS, 7) was the most potent and effective mechanism-based inactivator among the compounds studied. The ketone analogue (2) was the least effective among the compounds that exhibited inactivating activity. 4. The presence of the nucleophile cysteine in the incubation mixtures reduced the extent of inactivation of N,N-AT by 1 and by the ether (4) analogue but had little effect on the inactivation caused by 7. The inactivation of N,N-AT by N-OH-AAS (7) does not appear to involve electrophiles that are released from the active site and subsequently become covalently bound to the enzyme.