An arylamine acetyltransferase (AT-I) from Syrian golden hamster liver: cloning, complete nucleotide sequence, and expression in mammalian cells

ArylamineN-Acetyltransferases: What We Learn from Genes and Genomes

Arylamine N-acetyltransferases (NATs) are phase II xenobiotic metabolizing enzymes, catalyzing acetyl-CoA-dependent N- and O-acetylation reactions. All NATs have a conserved cysteine protease-like Cys-His-Asp catalytic triad inside their active site cleft. Other residues determine substrate specificity, while the C-terminus may control hydrolysis of acetyl-CoA during acetyltransfer. Prokaryotic NAT-like coding sequences are found in >30 bacterial genomes, including representatives of Actinobacteria, Firmicutes and Proteobacteria. Of special interest are the nat genes of TB-causing Mycobacteria, since their protein products inactivate the anti-tubercular drug isoniazid. Targeted inactivation of mycobacterial nat leads to impaired mycolic acid synthesis, cell wall damage and growth retardation. In eukaryotes, genes for NAT are found in the genomes of certain fungi and all examined vertebrates, with the exception of canids. Humans have two NAT isoenzymes, encoded by highly polymorphic genes on chromosome 8p22. Syntenic regions in rodent genomes harbour two Nat loci, which are functionally equivalent to the human NAT genes, as well as an adjacent third locus with no known function. Vertebrate genes for NAT invariably have a complex structure, with one or more non-coding exons located upstream of a single, intronless coding region. Ubiquitously expressed transcripts of human NAT1 and its orthologue, murine Nat2, are initiated from promoters with conserved Sp1 elements. However, in humans, additional tissue-specific NAT transcripts may be expressed from alternative promoters and subjected to differential splicing. Laboratory animals have been widely used as models to study the effects of NAT polymorphism. Recently generated knockout mice have normal phenotypes, suggesting no crucial endogenous role for NAT. However, these strains will be useful for understanding the involvement of NAT in carcinogenesis, an area extensively investigated by epidemiologists, often with ambiguous results.

Tissue distribution of N-acetyltransferase 1 and 2 catalyzing the N-acetylation of 4-aminobiphenyl and O-acetylation of N-hydroxy-4-aminobiphenyl in the congenic rapid and slow acetylator Syrian hamster

N-acetyltransferase 1 (NAT1) and 2 (NAT2) enzymes catalyzing both deactivation (N-acetylation) and activation (O-acetylation) of arylamine carcinogens such as 4-aminobiphenyl (ABP) were investigated in a Syrian hamster model congenic at the NAT2 locus. NAT2 catalytic activities (measured with p-aminobenzoic acid) were significantly (P < 0.001) higher in rapid than slow acetylators in all tissues (except heart and prostate where activity was undetectable in slow acetylators). NAT1 catalytic activities (measured with sulfamethazine) were low but detectable in most tissues tested and did not differ significantly between rapid and slow acetylators. ABP N-acetyltransferase activity was detected in all tissues of rapid acetylators but was below the limit of detection in all tissues of slow acetylators except liver where it was about 15-fold lower than rapid acetylators. ABP N-acetyltransferase activities correlated with NAT2 activities (r2 = 0.871; P < 0.0001) but not with NAT1 activities (r2 = 0.132; P > 0.05). Levels of N-hydroxy-ABP O-acetyltransferase activities were significantly (P < 0.05) higher in rapid than slow acetylator cytosols for many but not all tissues. The N-hydroxy-ABP O-acetyltransferase activities correlated with ABP N-acetyltransferase activities (r2 = 0.695; P < 0.0001) and NAT2 activities (r2 = 0.521, P < 0.0001) but not with NAT1 activities (r2 = 0.115; P > 0.05). The results suggest widespread tissue distribution of both NAT1 and NAT2, which catalyzes both N- and O-acetylation. These conclusions are important for interpretation of molecular epidemiological investigations into the role of N-acetyltransferase polymorphisms in various diseases including cancer.

Eukaryotic arylamine N-acetyltransferase

Arylamine N-acetyltransferases (NAT; EC 2.3.1.5) catalyse the transfer of acetyl groups from acetylCoA to xenobiotics, including drugs and carcinogens. The enzyme is found extensively in both eukaryotes and prokaryotes, yet the endogenous roles of NATs are still unclear. In order to study the properties of eukaryotic NATs, high-throughput substrate and inhibitor screens have been developed using pure soluble recombinant Syrian hamster NAT2 (shNAT2) protein. The assay can be used with a wide range of compounds and was used to determine substrate specificity of shNAT2. We describe the expression and characterisation of shNAT2 and also purified recombinant human NAT1 and NAT2, including the use of the assay to explore the substrate specificities of each of the enzymes. Hamster NAT2 has similar substrate specificity to human NAT1, acetylating para-aminobenzoate but not arylhydrazine and hydralazine compounds. The overlapping but distinct substrate-specific activity profiles of human NAT1 and NAT2 were clearly observed from the screen. Naturally occurring compounds were tested as substrates or inhibitors of shNAT2 and succinylCoA was found to be a potent inhibitor of shNAT2.

Prostate expression of N-acetyltransferase 1 (NAT1) and 2 (NAT2) in rapid and slow acetylator congenic Syrian hamster

Arylamine carcinogens induce prostate tumours in rodent models and may contribute to the aetiology of human prostate cancers. N-acetylation and O-acetylation, catalysed by N-acetyltransferase 1 (NAT1) and 2 (NAT2), activate and/or deactivate arylamines to electrophilic intermediates that bind DNA and initiate tumours in target organs. NAT1 and NAT2 are both subject to genetic polymorphism in humans, and molecular epidemiological investigations suggest that NAT1 and/or NAT2 acetylator genotype modifies risk for prostate cancers. A Syrian hamster model congenic at the NAT2 locus was used to investigate the role of acetylator genotype in N- and O-acetylation of aromatic and heterocyclic amine carcinogens in the liver and prostate. A gene dose-response (NAT2*15/*15>NAT2*15/*16A>NAT2*16A/*16A) relationship was observed in liver and prostate cytosol towards the N-acetylation of p-aminobenzoic acid, 2-aminofluorene, beta-napthylamine, 4-aminobiphenyl, and 3,2'-dimethyl-4-aminobiphenyl. NAT1 and NAT2 were separated and partially purified from liver and prostate cytosol. NAT1 and NAT2 in liver and prostate catalysed -acetylation of the arylamines above and O-acetylation of N-hydroxy derivatives of 2-aminofluorene, 4-aminobiphenyl and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. Rates were higher in rapid versus slow acetylators when catalysed by NAT2 but not when catalysed by NAT1. Partially purified prostate NAT2 exhibited higher apparent K(m) and V(max) than NAT1. Prostate NAT1 mRNA levels were higher than NAT2 and neither NAT1 nor NAT2 mRNA level differed with NAT2 acetylator genotype. The results provide mechanistic support for a role of NAT1 and/or NAT2 acetylator polymorphism(s) in human prostate cancer risk related to aromatic and/or heterocyclic amine carcinogens.

Cytosolic arylamine N-acetyltransferase (NAT) deficiency in the dog and other canids due to an absence of NAT genes

The purpose of this study was to determine the molecular basis in the dog for an unusual and absolute deficiency in the activity of cytosolic N-acetyltransferase (NAT), an enzyme important for the metabolism of arylamine and hydrazine compounds. NAT activity towards two NAT substrates, p-aminobenzoic acid and sulfamethazine, was undetectable in dog liver cytosol, despite substrate concentrations ranging from 10 microM to 4 mM and a wide range of incubation times. Similarly, no protein immunoreactive to NAT antibody was evident on western blot analysis of canine liver cytosol. Southern blot analysis of genomic DNA from a total of twenty-five purebred and mixed bred dogs, and eight wild canids, probed with a full-length human NAT2 cDNA, suggested an absence of NAT sequences in all canids. Polymerase chain reaction amplification of genomic DNA using degenerate primers designed to mammalian NAT1 and NAT2 consensus sequences generated products of the expected size in human, mouse, rabbit, and cat DNA, but no NAT products in any dog or wild canids. These results support the conclusion that cytosolic NAT deficiency in the domestic dog is due to a complete absence of NAT genes, and that this defect is shared by other canids.

Evidence for the lack of hepatic N-acetyltransferase in suncus (Suncus murinus)

The abilities of liver cytosol fractions from the suncus and Sprague-Dawley (SD) rats to N-acetylate aniline, p-aminobenzoic acid, p-aminosalicylic acid and 2-aminofluorene (AF) were compared. The cytosol from rats N-acetylated these substrates at efficient rates, whereas the cytosol from the suncus did not N-acetylate these substrates at detectable rates. When AF was given to the suncus, 2-acetylaminofluorene (AAF), a metabolite of AF formed by N-acetyltransferase (NAT), was not detectable in serum, whereas the metabolite was seen clearly in rats. Northern blot and Southern blot analyses, using cDNAs coding for human NATs as probes, indicated that not only the transcripts but also the genes of the enzymes were undetectable in suncus. These results suggest that the suncus is among the few species known to lack NATs.

Complementary DNAs for two arylamine N-acetyltransferases with identical 5' non-coding regions from rat pineal gland

A cDNA library was constructed from the pineal gland of rats injected with isoproterenol and screened with 32P-labeled cDNAs encoding arylamine N-acetyltransferases from rabbit and human liver. Two types of cDNAs for arylamine N-acetyltransferases (A-type and B-type) were isolated. Expression of the cDNAs in Chinese hamster ovary cells indicated that A-type N-acetyltransferase acetylates both arylamines and arylalkylamines, while the B-type enzyme acetylates only arylamines. Therefore, neither the A-type nor the B-type of enzyme seems to be the arylalkylamine N-acetyltransferase involved in melatonin synthesis in the pineal gland. Nucleotide sequence analysis revealed that both A-type and B-type cDNAs code for 290 amino acids, and that they showed 82.8% similarity in the coding region. However, the nucleotide sequence in the 5' non-coding region was identical in the A-type and B-type cDNAs. In addition, the 5' non-coding region contained another possible open reading frame for 79 amino acids. Data base research revealed that the complementary sequence of the 5' non-coding region has high similarity with the coding regions of cDNAs for high-mobility-group proteins (HMG) 1 and 2, which are thought to regulate mRNA transcription.

Polymorphic arylamine N-acetyltransferase encoding gene (NAT2) from homozygous rapid and slow acetylator congenic Syrian hamsters

The nucleotide (nt) and deduced amino acid (aa) sequences were determined for polymorphic arylamine N-acetyl-transferase (NAT2) and its gene, NAT2, from homozygous rapid and slow acetylator congenic Syrian hamsters. The slow acetylator (NAT2s) allele contained three point mutations which differed from the rapid acetylator allele (NAT2r); two mutations were silent, and the third mutation resulted in a premature stop codon. The NAT2s allele contained a truncated open reading frame of 726 nt encoding a 242-aa protein, which is 48-aa shorter than NAT2r.

A new Salmonella tester strain expressing a hamster acetyltransferase shows high sensitivity for arylamines

A hamster acetyltransferase, AT-I, has high activities for N-acetylation of arylamines, O-acetylation of N-hydroxyarylamines and N,O-acetyltransfer of N-hydroxyarylacetamides. In the present study, the cDNA was expressed in Salmonella typhimurium TA1538. The new SAT138 strain expressing high levels of AT-I showed remarkably high sensitivity (> 10,000 fold) for a carcinogenic intermediate, N-hydroxy-2-acetylaminofluorene, in an Ames mutagenesis test as compared to the parental TA1538 strain. SAT138 had 650-1,600-fold higher sensitivities for mutagenesis induced by 2-acetylaminofluorene and benzidine in the presence of S9 mix. Higher sensitivities (32-560-fold) were also observed with N-hydroxy-2-aminofluorene, N-hydroxy-4-aminobiphenyl, N-hydroxy-4-acetylaminobiphenyl, N-hydroxy-4-propionylaminobiphenyl and N-hydroxy-phenacetin in the absence of S9 mix. These high sensitivities to arylamines and the related chemicals are accounted for by the efficient expression of AT-I in the cytosol of this bacterium. The unique characteristics of SAT138 having high N-hydroxyarylacetamide N,O-transacetylating activity, which is defective in Salmonella acetyltransferase, provide broadened and high sensitivities for the detection of mutagenic N-substituted chemicals in the Salmonella mutagenesis test.

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.