The pharmacogenetics of NAT: structural aspects

Risk of Carcinogenicity Associated with Synthetic Hair Dyeing Formulations: A Biochemical View on Action Mechanisms, Genetic Variation and Prevention

Article tries to visualize the potential for carcinogenic trigger in humans with a preference for oxidative synthetic of hair dyeing formulations, especially which belong to the category of permanent colours. According to the International Agency for Cancer, hair dyes for personal use are not strictly classified as carcinogen to humans. However, some controversy exists that requires clarification. Some epidemiological studies support the association between the risk of cancer development and personal use of hair dyes (pooled relative risk RR = 1.50. 95% CI: 1.30-1.98). The world-wide sale of hair dyeing cosmetics have exceeded 15 billion dollars by the year 2012 and has maintained an annual growth rate of 8-10%. This raises concerns and need to be addressed. The review article briefly discusses about the different hair dye components based on their chemical nature, permanence, interaction of dye components with different parts of the hair shaft, action mechanisms, health risk assessment, associated challenges and possible alternatives. There appears variability towards the pathological changes incurred in the human system upon the use of synthetic hair formulations. This probably appears due to the presence of interindividual genetic variation of enzymes handling these xenobiotics. The redox mechanism of major hair dye components appears to be involved in the carcinogenic trigger. Most of the hair dye constituents pose serious health issues. However, we do have few better alternatives to prevent the toxicity associated with hair dye constituents without compromising the need of today's fashion statement and expectations of the youth.

Biomarkers of Potential Harm: Summary of an FDA-Sponsored Public Workshop

Introduction

Since 2009, the United States (US) Food and Drug Administration (FDA) Center for Tobacco Products (CTP) has had the authority to regulate the manufacture, distribution, and marketing of tobacco products in order to reduce the death and disease caused by tobacco use. Biomarkers could play an important role across a number of FDA regulatory activities, including assessing new and modified risk tobacco products and identifying and evaluating potential product standards.

Methods

On April 4-5, 2016, FDA/CTP hosted a public workshop focused on biomarkers of potential harm (BOPH) with participants from government, industry, academia, and other organizations. The workshop was divided into five sessions focused on: (1) overview of BOPH; (2) cardiovascular disease (CVD); (3) chronic obstructive pulmonary disease (COPD); (4) cancer; and (5) new areas of research.

Results and Conclusions

The deliberations from the workshop noted some promising BOPH but also highlighted the lack of systematic effort to identify BOPH that would have utility and validity for evaluating tobacco products. Research areas that could further strengthen the applicability of BOPH to tobacco regulatory science include the exploration of composite biomarkers as predictors of disease risk, "omics" biomarkers, and examining biomarkers using existing cohorts, surveys, and experimental studies.

Implications

This paper synthesizes the main findings from the 2016 FDA-sponsored workshop focused on BOPH and highlights research areas that could further strengthen the science around BOPH and their applicability to tobacco regulatory science.

Pharmacogenomics in Papua New Guineans: unique profiles and implications for enhancing drug efficacy while improving drug safety

Papua New Guinea (PNG) can be roughly divided into highland, coastal and island peoples with significant mitochondrial DNA differentiation reflecting early and recent distinct migrations from Africa and East Asia, respectively. Infectious diseases such as tuberculosis, malaria and HIV severely impact on the health of its peoples for which drug therapy is the major treatment and pharmacogenetics has clinical relevance for many of these drugs. Although there is generally little information about known single nucleotide polymorphisms in the population, in some instances, their frequencies have been shown to be higher than anywhere worldwide. For example, CYP2B6*6 is over 50%, and CYP2C19*2 and *3 are over 40 and 25%, respectively. Conversely, CYP2A6*9, 2B6*2, *3, *4 and *18, and 2C8*3 appear to be much lower than in Whites. CYP2D6 known variants are unclear, and for phase II enzymes, only UGT2B7 and UGT1A9 data are available, with variant frequencies either slightly lower than or similar to Whites. Although almost all PNG people tested are rapid acetylators, but which variant(s) define this phenotype is not known. For HLA-B*13:01, HLA-B*35:05 and HLA-C*04:01, the frequencies show some regioselectivity, but the clinical implications with respect to adverse drug reactions are not known. There are minimal phenotype data for the CYPs and nothing is known about drug transporter or receptor genetics. Determination of genetic variants that are rare in Whites or Asians but common in PNG people is a topic of both scientific and clinical importance, and further research needs to be carried out. Optimizing the safety and efficacy of infectious disease drug therapy through pharmacogenetic studies that have translation potential is a priority.

Estrogen sulfotransferase in the metabolism of estrogenic drugs and in the pathogenesis of diseases

INTRODUCTION

Biotransformation is important in the metabolism of endobiotics and xenobiotics. This process comprises the activity of phase I and phase II enzymes. Estrogen sulfotransferase (SULT1E1 or EST) is a phase II conjugating enzyme that belongs to the family of cytosolic sulfotransferases. The expression of SULT1E1 can be detected in many tissues, including the liver. SULT1E1 catalyzes the transfer of a sulfate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to any available hydroxyl group in estrogenic molecules. The substrates of SULT1E1 include the endogenous and synthetic estrogens. Upon SULT1E1-mediated sulfation, the hydrosolubility of estrogens increases, preventing the binding between the sulfated estrogens and the estrogen receptor (ER). This sulfated state of the estrogens is not irreversible, as the steroid sulfatase (STS) can convert sulfoconjugated estrogens to free estrogens. The expression of SULT1E1 is inducible by several diseases that involve tissue inflammation, such as type 2 diabetes, sepsis, and ischemia-reperfusion injury. Areas covered: This systematic literature review aims to summarize the role of SULT1E1 in the metabolism of estrogenic drugs and xenobiotics, and the role of SULT1E1 in the pathogenesis of several diseases, including cancer, metabolic disease, sepsis, liver injury, and cystic fibrosis. Meanwhile, ablation or pharmacological inhibition of SULT1E1 can affect the outcomes of the aforementioned diseases. Expert opinion: In addition to its role in metabolizing estrogenic drugs, SULT1E1 is unexpectedly being unveiled as a mediator for the disease effect on estrogen metabolism and homeostasis. Meanwhile, because the expression and activity of SULT1E1 can affect the outcome of diseases, the same sulfotransferase and the reversing enzymes STS can be potential therapeutic targets to prevent or manage diseases. Accumulating evidence suggest that the physiological and pathophysiological effects of SULT1E1 can be estrogen-independent and it is necessary to elucidate what other possible substrates may be recognized by the enzyme. Moreover, human studies are paramount to confirm the human relevance of the animal studies.

Acute murine colitis reduces colonic 5-aminosalicylic acid metabolism by regulation of N-acetyltransferase-2

Pharmacotherapy based on 5-aminosalicylic acid (5-ASA) is a preferred treatment for ulcerative colitis, but variable patient response to this therapy is observed. Inflammation can affect therapeutic outcomes by regulating the expression and activity of drug-metabolizing enzymes; its effect on 5-ASA metabolism by the colonic arylamine N-acetyltransferase (NAT) enzyme isoforms is not firmly established. We examined if inflammation affects the capacity for colonic 5-ASA metabolism and NAT enzyme expression. 5-ASA metabolism by colonic mucosal homogenates was directly measured with a novel fluorimetric rate assay. 5-ASA metabolism reported by the assay was dependent on Ac-CoA, inhibited by alternative NAT substrates (isoniazid, p-aminobenzoylglutamate), and saturable with Km (5-ASA) = 5.8 μM. A mouse model of acute dextran sulfate sodium (DSS) colitis caused pronounced inflammation in central and distal colon, and modest inflammation of proximal colon, defined by myeloperoxidase activity and histology. DSS colitis reduced capacity for 5-ASA metabolism in central and distal colon segments by 52 and 51%, respectively. Use of selective substrates of NAT isoforms to inhibit 5-ASA metabolism suggested that mNAT2 mediated 5-ASA metabolism in normal and colitis conditions. Western blot and real-time RT-PCR identified that proximal and distal mucosa had a decreased mNAT2 protein-to-mRNA ratio after DSS. In conclusion, an acute colonic inflammation impairs the expression and function of mNAT2 enzyme, thereby diminishing the capacity for 5-ASA metabolism by colonic mucosa.

Genetic selection of volunteers and concomitant dose adjustment leads to comparable hydralazine/valproate exposure

WHAT IS KNOWN AND OBJECTIVE

Hydralazine is an inhibitor of DNA methyltransferases, whereas valproate interferes with histone deacetylation. In combination, they show a marked synergism in reducing tumour growth as well as development of metastasis and inducing cell differentiation. Hydralazine is metabolized by the highly polymorphic N-acetyltransferase 2. The current pilot study was performed to analyse the pharmacokinetic parameters of a single dose of hydralazine in 24 h (one tablet with 83 mg for slow acetylators and one tablet with 182 mg for fast acetylators) and three fixed doses of valproate (one tablet of controlled liberation with 700 mg every 8 h) in healthy genetically selected volunteers. Selection was performed based on their NAT2 activity as deduced from their genotype.

METHODS

An open label non-randomized single arm study was conducted in two groups of six healthy volunteers of both genders aged 20-45 years with a body mass index 22·2-26·9 which were classified as fast or slow acetylators after genotyping 3 SNPs that cover 99·9% of the NAT2 variants in the Mexican population. Blood samples were collected predose and serially post-dose in an interval of 48 h. Hydralazine and valproate concentrations were determined by ultra-high performance liquid chromatography (UPLC) coupled to tandem mass spectroscopy (MS/MS).

RESULTS AND DISCUSSION

The AUC0-48 h and Cmax of hydralazine were almost identical (1410 ± 560 vs. 1446 ± 509 ng h/mL and 93·4 ± 16·7 vs. 112·5 ± 42·1 ng/mL) in both groups with NAT2 genotype-adjusted doses, whereas the multidose parameters of valproate were not significantly affected neither by the selection of the NAT2 genotype (AUC0-48 h 2064 ± 455 vs. 1896 ± 185 μg h/mL; Cmax 96·4 ± 21·1 vs. 88·8 ± 7·2 μg/mL, for the fast and slow acetylators, respectively) nor the co-administration of 83 or 182 mg of hydralazine.

WHAT IS NEW AND CONCLUSION

Comparable hydralazine exposures (differences in AUC0-inf of only 7%) were observed in this study with genetic selection of volunteers and concomitant dose adjustment. However, the conclusions have yet to be confirmed with a full-powered 2 × 2 crossover study.

Biosolid-borne tetracyclines and sulfonamides in plants

Tetracyclines and sulfonamides used in human and animal medicine are released to terrestrial ecosystems from wastewater treatment plants or by direct manure application. The interactions between plants and these antibiotics are numerous and complex, including uptake and accumulation, phytometabolism, toxicity responses, and degradation in the rhizosphere. Uptake and accumulation of antibiotics have been studied in plants such as wheat, maize, potato, vegetables, and ornamentals. Once accumulated in plant tissue, organic contaminants can be metabolized through a sequential process of transformation, conjugation through glycosylation and glutathione pathways, and ultimately sequestration into plant tissue. While studies have yet to fully elucidate the phytometabolism of tetracyclines and sulfonamides, an in-depth review of plant and mammalian studies suggest multiple potential transformation and conjugation pathways for tetracyclines and sulfonamides. The presence of contaminants in the vicinity or within the plants can elicit stress responses and defense mechanisms that can help tolerate the negative effects of contaminants. Antibiotics can change microbial communities and enzyme activity in the rhizosphere, potentially inducing microbial antibiotic resistance. On the other hand, the interaction of microbes and root exudates on pharmaceuticals in the rhizosphere can result in degradation of the parent molecule to less toxic compounds. To fully characterize the environmental impacts of increased antibiotic use in human medicine and animal production, further research is essential to understand the effects of different antibiotics on plant physiology and productivity, uptake, translocation, and phytometabolism of antibiotics, and the role of antibiotics in the rhizosphere.

Effects of single nucleotide polymorphisms on human N-acetyltransferase 2 structure and dynamics by molecular dynamics simulation

Background

Arylamine N-acetyltransferase 2 (NAT2) is an important catalytic enzyme that metabolizes the carcinogenic arylamines, hydrazine drugs and chemicals. This enzyme is highly polymorphic in different human populations. Several polymorphisms of NAT2, including the single amino acid substitutions R64Q, I114T, D122N, L137F, Q145P, R197Q, and G286E, are classified as slow acetylators, whereas the wild-type NAT2 is classified as a fast acetylator. The slow acetylators are often associated with drug toxicity and efficacy as well as cancer susceptibility. The biological functions of these 7 mutations have previously been characterized, but the structural basis behind the reduced catalytic activity and reduced protein level is not clear.

Methodology/Principal Findings

We performed multiple molecular dynamics simulations of these mutants as well as NAT2 to investigate the structural and dynamical effects throughout the protein structure, specifically the catalytic triad, cofactor binding site, and the substrate binding pocket. None of these mutations induced unfolding; instead, their effects were confined to the inter-domain, domain 3 and 17-residue insert region, where the flexibility was significantly reduced relative to the wild-type. Structural effects of these mutations propagate through space and cause a change in catalytic triad conformation, cofactor binding site, substrate binding pocket size/shape and electrostatic potential.

Conclusions/Significance

Our results showed that the dynamical properties of all the mutant structures, especially in inter-domain, domain 3 and 17-residue insert region were affected in the same manner. Similarly, the electrostatic potential of all the mutants were altered and also the functionally important regions such as catalytic triad, cofactor binding site, and substrate binding pocket adopted different orientation and/or conformation relative to the wild-type that may affect the functions of the mutants. Overall, our study may provide the structural basis for reduced catalytic activity and protein level, as was experimentally observed for these polymorphisms.

The role of N-acetyltransferase 2 polymorphism in the etiopathogenesis of inflammatory bowel disease

BACKGROUND AND PURPOSE

Inflammatory bowel disease (IBD) consists of ulcerative colitis (UC) and Crohn's disease (CD), which are complex genetic disorders resulting from the interplay between several genetic and environmental risk factors. The arylamine N-acetyltransferase 2 (NAT2) enzyme detoxifies a wide spectrum of naturally occurring xenobiotics including carcinogens and drugs. Acetylation catalyzed by NAT2 is an important process in metabolic activation of arylamines to electrophilic intermediates that initiate carcinogenesis. The aim of our study was to determine whether there is any association between the susceptibility to inflammatory bowel disease among the variations of NAT2 genotypes.

METHODS

This study was carried out in 80 patients with IBD. The control group consisted of 100 healthy volunteers. The most common mutations found in the Caucasian population are at the positions 481T, 803G, 590A and 857A on the NAT2 gene. This was determined using the polymerase chain reaction-restriction fragment length polymorphism method with DNA extracted from peripheral blood.

RESULTS

Risk of IBD development was 3.86 for the carriers of the NAT2*5/NAT2*7 genotype and 2.53 for the carriers with NAT2*6/NAT2*7, but it was not statistically significant. A statistically significant correlation between the NAT2*7 allele prevalence and the risk for developing IBD was found (OR = 5.8; P = 0.005).

CONCLUSIONS

Higher prevalence of the NAT2*7 allele in patients with IBD and the obtained OR values could suggest that this mutation has the effect of increasing IBD development. Future studies are needed to confirm our assumptions on larger group of patients.

Genetic variation in N-acetyltransferases 1 and 2, cigarette smoking, and risk of non-Hodgkin lymphoma

Cigarette smoke contains many carcinogens that are metabolically activated through xenobiotic metabolism by phase I and II enzymes, including N-acetyltransferases 1 and 2 (NAT1 and NAT2). We investigated non-Hodgkin lymphoma risk in general and by subtype in relation to NAT1 and NAT2 genotypes and cigarette smoking in a population-based case-control study in Connecticut. Of the 535 controls, 53.1% reported ever smoking, and of the 461 cases, 55.7% reported ever smoking. We found a two-fold increased risk of T-cell lymphoma among those possessing the NAT1*10 genotype compared to those with other NAT1 genotypes; including an OR of 2.0 (95% CI: 1.0-2.4) for those heterozygous or homozygous for NAT1*10 genotypes. Rapid acetylator NAT2 phenotype increased the risk of both T-cell lymphoma (OR = 3.2; 95% CI: 1.1-9.5) and marginal zone lymphoma (OR = 3.0; 95% CI: 1.0-8.7), though these results were based on a small number of cases. When smoking status and risk of NHL was stratified by NAT1 and NAT2 genotypes, an increased risk of NHL overall was observed in current (OR = 1.7; 95% CI: 1.2-2.4) smokers without the NAT1*10 genotype but not among smokers with the NAT1*10 genotype (p-interaction < 0.01). No association between history of cigarette smoking and risk of NHL overall was observed with any NAT2 genotype. Our results present modest evidence that acetylation rate is associated with risk of NHL for specific subtypes and that the NAT1*10 genotype is an "at-risk" allele. Additionally, our results suggest that the relationship between NHL and smoking status may be modified by common genetic variation in NAT1 but not NAT2. We conclude that these findings require replication in larger studies and ultimately in pooled analyses.

Effects of dietary factors and the NAT2 acetylator status on gastric cancer in Koreans

Environmental dietary carcinogens and genetic polymorphisms in metabolic enzymes have been reported to be the risk factors for gastric cancer. This study was undertaken to investigate the effects of the diet, the N-acetyltransferase (NAT) 2 acetylation status and their interaction on gastric cancer risk. The study population consisted of 471 gastric cancer patients and 471 age- and sex-matched control subjects. NAT2 genotypes were identified using single-nucleotide primer extension reaction methods. Thirty-one alleles related to 12 polymorphism sites were assayed in this study. Significantly increased odds ratios were observed in former smokers (OR = 2.39, 95% CI = 1.57-3.62), heavy drinkers (OR = 1.28, 95% CI = 1.06-1.55) and individuals who eat well-done meat (OR = 1.24, 95% CI = 1.09-1.41). The odds ratios (95% CI) for high intake of kimchi, stews and soybean paste were 3.27 (2.44-4.37), 1.96 (1.50-2.58) and 1.63 (1.24-2.14), respectively. The NAT2 genotype alone was not associated with gastric cancer risk. A significant gene-environment interaction was observed between environmental carcinogens and NAT2 genotypes. The odds ratios for kimchi, stews and soybean paste were higher in slow/intermediate acetylators than in rapid acetylators. The odds ratios for slow/intermediate acetylators were 2.28 (95% CI: 1.29-4.04) for light smokers and 3.42 (95% CI: 2.06-5.68) for well-done meat intake. The NAT2 acetylator genotype may be an important modifier of the effects of environmental factors on gastric cancer risk.

Polymorphism of Biotransformation Genes and Risk of Relapse in Childhood Acute Leukemia

Leukemia is a hematological malignancy that involves bone marrow. Polymorphism of biotransformation genes plays an important role in primary childhood leukemia and affects the incidence and character of acute leukemia relapse. A biochip designed to assess some polymorphisms of biotransformation genes was used to determine the frequency of the polymorphic variants ofCYP1A1, CYP2D6, GSTT1, GSTM1, MTHFR, MTRR, NQO1, CYP2C9, CYP2C19andNAT2in 332 children with acute lymphoblastic leukemia (ALL) and 71 children with acute myeloblastic leukemia (AML). TheCYP1A1 *1/*2A, GSTT1non null andGSTM1non null genotypes were more frequent in patients with primary leukemia than in relapse. Analysis of theNAT2genotype frequency revealed a characteristic genotype for each type of leukemia, which prevailed in patients with relapse: the genotype341C/-, 481T/-, 590G/G, 857G/Gprevailed in ALL patients with relapse, and the genotype341T/T, 481C/C, 590A/- in AML patients with relapse when compared with patients having primary ALL or AML, respectively. Thus, the polymorphisms ofCYP1A1, GSTT1, GSTM1andNAT2genes can be considered as markers for risk of relapse in childhood acute leukemia and can be used for the prognosis and individualization of standard therapy.

Hair dye use, genetic variation in N-acetyltransferase 1 (NAT1) and 2 (NAT2), and risk of non-Hodgkin lymphoma

BACKGROUND

Several previous studies have found non-Hodgkin lymphoma (NHL) risk to be associated with hair dye use, particularly use of permanent, dark colors and use before 1980, when hair dye formulations changed.

METHODS

We examined NHL risk in relation to reported hair dye use among 1,321 cases and 1,057 controls from a US population-based multi-center study. DNA was extracted from blood or buccal cells to identify genetic variation in N-acetyltransferase 1 (NAT1) and 2 (NAT2), which encode enzymes that metabolize aromatic amine compounds found in hair dyes.

RESULTS

Among women, 509 cases and 413 controls reported hair dye use [odds ratio (OR) = 1.2; 95% confidence interval (95% CI) = 0.9, 1.6]. Risk estimates were higher for use before 1980 than for use in 1980 or later, particularly for use of permanent, intense tone (black, dark brown, dark blonde) products (<1980-OR = 1.6; 95% CI 0.9, 2.7; >or=1980-OR = 0.6; 95% CI 0.4, 1.1). Risk estimates were increased for women who used permanent, intense tone products before 1980 if they had the rapid/intermediate NAT2 phenotype (OR = 3.3; 95% CI 1.3, 8.6) or the NAT1 10 allele (OR = 2.5; 95% CI 0.9, 7.6), but not if they were slow NAT2 acetylators (OR = 1.5; 95% CI 0.6, 3.6) or had no copies of the NAT1 10 allele (OR = 1.5; 95% CI 0.7, 3.3). NHL risk was not increased among women who began hair dye use after 1980 or among men.

CONCLUSION

Our results support previous research demonstrating elevated NHL risk among women who used dark color or intense tone permanent hair dyes before 1980. We present the first evidence suggesting that this risk may differ by genetic variation in NAT1 and NAT2.

Cloning and molecular characterization of three arylamine N-acetyltransferase genes from Bacillus anthracis: identification of unusual enzymatic properties and their contribution to sulfamethoxazole resistance

The arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes that catalyze the N-acetylation of arylamines and their N-hydroxylated metabolites. These enzymes play a key role in detoxication of numerous drugs and xenobiotics. We report here the cloning, functional expression, and characterization of three new NAT genes (termed banatA, banatB, and banatC) from the pathogen Bacillus anthracis. The sequences of the corresponding proteins are approximately 30% identical with those of characterized eukaryotic and prokaryotic NAT enzymes, and the proteins were recognized by an anti-NAT antibody. The three genes were endogenously expressed in B. anthracis, and NAT activity was found in cell extracts. The three NAT homologues exhibited distinct structural and enzymatic properties, some of which have not previously been observed with other NAT enzymes. Recombinant BanatC displayed strong NAT activity toward several prototypic NAT substrates, including the sulfonamide antibiotic sulfamethoxazole (SMX). As opposed to BanatC, BanatB also had acetyl-CoA (AcCoA) and p-nitrophenyl acetate (PNPA) hydrolysis activity in the absence of arylamine substrates, indicating that it may act as an AcCoA hydrolase. BanatA was devoid of NAT or AcCoA/PNPA hydrolysis activities, suggesting that it may be a new bacterial NAT-like protein with unknown function. Expression of BanatC in Escherichia coli afforded higher-than-normal resistance to SMX in the recombinant bacteria, whereas an inactive mutant of the enzyme did not. These data indicate that BanatC could contribute to the resistance of B. anthracis to SMX.

ArylamineN-acetyltransferases

Arylamine N-acetyltransferases (NATs), known as drug- and carcinogen-metabolising enzymes, have had historic roles in cellular metabolism, carcinogenesis and pharmacogenetics, including epidemiological studies of disease susceptibility. NAT research in the past 5 years builds on that history and additionally paves the way for establishing the following new concepts in biology and opportunities in drug discovery: i) NAT polymorphisms can be used as tools in molecular anthropology to study human evolution; ii) tracing NAT protein synthesis and degradation within cells is providing insight into protein folding in cell biology; iii) studies on control of NAT gene expression may help to understand the increase in the human NAT isoenzyme, NAT1, in breast cancer; iv) a NAT homologue in mycobacteria plays an essential role in cell-wall synthesis and mycobacterial survival inside host macrophage, thus identifying a novel biochemical pathway; v) transgenic mice, with genetic modifications of all Nat genes, provide in vivo tools for drug metabolism; and vi) structures of NAT isoenzymes provide essential in silico tools for drug discovery.

Genetic variation in N-acetyltransferase 1 (NAT1) and 2 (NAT2) and risk of non-Hodgkin lymphoma

Animal studies suggest that lymphomagenesis can be induced by exposure to carcinogenic aromatic and heterocyclic amines found in diet, cigarette smoke and the environment, but human epidemiologic investigations of these exogenous exposures have yielded conflicting results. As part of our evaluation of the role of aromatic and heterocyclic amines, which are metabolized by N-acetyltransferase (NAT) enzymes, in the etiology of non-Hodgkin lymphoma (NHL), we examined NHL risk in relation to genetic variation in NAT1 and NAT2 and exposure to cigarette smoke and dietary heterocyclic amines and mutagens. We genotyped 10 common single nucleotide polymorphisms (SNPs) in NAT1 and NAT2 among 1136 cases and 922 controls from a population-based case-control study in four geographical areas of the USA. Relative risk of NHL for NAT1 and NAT2 genotypes, NAT2 acetylation phenotype, and exposure to cigarette smoke and dietary heterocyclic amines and mutagens was estimated using odds ratios (ORs) and 95% confidence intervals (CIs) derived from unconditional logistic regression models. We observed increased risk of NHL among individuals with the NAT1*10/*10 genotype compared with individuals with other NAT1 genotypes (OR = 1.60, 95% CI = 1.04-2.46, P = 0.03). We also observed increased NHL risk in a dose-dependent model among NAT2 intermediate- and rapid-acetylators compared with slow-acetylators, although only the trend was statistically significant (intermediate: OR = 1.18, 95% CI = 0.97-1.44, P = 0.1; rapid: OR = 1.43, 95% CI = 0.97-2.14, P = 0.07; P for linear trend = 0.03). Compared with non-smokers, NHL risk estimates for current cigarette smoking were increased only among NAT2 intermediate/rapid-acetylators (OR = 2.44, 95% CI = 1.15-5.20, P = 0.02). Our data provide evidence that NAT1 and NAT2 genotypes are associated with NHL risk and support a contributory role for carcinogenic aromatic and/or heterocyclic amines in the multi-factorial etiology of NHL.

Inhibitory effects of polyphenolic compounds on human arylamine N-acetyltransferase 1 and 2

Arylamine N-acetyltransferases (NAT) are important enzymes involved in the metabolic activation of aromatic and heterocyclic amines and inhibitors of NAT enzymes may be valuable as chemopreventive agents. Phytochemicals including cinnamic acid derivatives, various classes of flavonoids and coumarins were tested for the inhibitory activity on NAT1 and NAT2 from human liver and the human cholangiocarcinoma cell line: KMBC cells. Assays were performed using p-aminobenzoic acid and sulfamethazine as selective substrates for NAT1 and NAT2, respectively. NAT1 and NAT2 activities were present in liver cytosol. However, the KMBC cells showed only NAT1 activity. There was a marked difference in the ability of the test chemicals to inhibit NAT1 and NAT2. Caffeic acid, ferulic acid, gallic acid and EGCG inhibited NAT1 but not NAT2, whereas scopuletin and curcumin inhibited NAT2 but not NAT1. Quercetin, kaemferol and other flavonoids, except epicatechin and silymarin, inhibited both enzymes. The kinetics of inhibition of NAT1 by caffeic acid, EGCG and quercetin were of the non-competitive type, whereas that of NAT2 by quercetin, curcumin and kaemferol was also of the non-competitive type. The most potent inhibitor was quercetin, which has the inhibitory constants for NAT1 and NAT2 of 48.6 +/- 17.3 and 10.0 +/- 1.8 microM, respectively.

Cloning, functional expression and characterization of Mesorhizobium loti arylamine N-acetyltransferases: rhizobial symbiosis supplies leguminous plants with the xenobiotic N-acetylation pathway

Arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes involved in the detoxification of numerous aromatic chemicals. The NAT-dependent N-acetylation pathway has not previously been detected in plants. We demonstrate here the occurrence of the NAT-dependent pathway in leguminous plants, due to symbiosis with Mesorhizobium loti. We cloned two NAT enzymes from M. loti and showed that these two recombinant enzymes catalysed the N-acetylation of several known NAT substrates, including aniline-derived pesticide residues. We also demonstrate the existence of a functional NAT-dependent acetylation pathway in the root nodules of Lotus japonicus inoculated with M. loti. M. loti is the first non-eukaryotic organism shown to express two catalytically active NAT isoforms. This work also provides the first evidence for acquisition of a xenobiotic detoxification pathway by a plant through symbiosis with a soil microbe.

Insight into the structure ofMesorhizobium lotiarylamineN-acetyltransferase 2 (MLNAT2): A biochemical and computational study

The arylamine N-acetyltransferases (NAT; EC 2.3.1.5) are xenobiotic-metabolizing enzymes (XME) that catalyze the transfer of an acetyl group from acetylCoA (Ac-CoA) to arylamine, hydrazines and their N-hydroxylated metabolites. Eukaryotes may have up to three NAT isoforms, but Mesorhizobium loti is the only prokaryote with two functional NAT isoforms (MLNAT1 and MLNAT2). The three-dimensional structure of MLNAT1 has been determined (Holton, S.J., Dairou, J., Sandy, J., Rodrigues-Lima, F., Dupret, J.M., Noble, M.E.M. and Sim, E. (2005) Structure of Mesorhizobium loti arylamine N-acetyltransferase 1. Acta Cryst, F61, 14-16). No MLNAT2 crystals have yet been produced, despite the production of sufficient quantities of pure protein. Using purified recombinant MLNAT1 and MLNAT2, we showed here that MLNAT1 was intrinsically more stable than MLNAT2. To test whether different structural features could explain these differences in intrinsic stability, we constructed a high-quality homology model for MLNAT2 based on far UV-CD data. Despite low levels of sequence identity with other prokaryotic NAT enzymes ( approximately 28% identity), this model suggests that MLNAT2 adopts the characteristic three-domain NAT fold. More importantly, molecular dynamics simulations on the structures of MLNAT1 and MLNAT2 suggested that MLNAT2 was less stable than MLNAT1 due to differences in amino-acid sequence/structure features in the alpha/beta lid domain.

Impairment of the activity of the xenobiotic-metabolizing enzymes arylamine N-acetyltransferases 1 and 2 (NAT1/NAT2) by peroxynitrite in mouse skeletal muscle cells

Reactive nitrogen species and their by-products, such as peroxynitrite, modulate many physiological functions of skeletal muscle. Peroxynitrite generation occuring under specific conditions, such as inflammation, may also lead to skeletal muscle dysfunction and pathologies. Arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes (XMEs) involved in the detoxification and/or metabolic activation of several drugs and chemicals. In addition to other XMEs, such as gluthatione S-transferases or cytochromes P450, NAT enzymes are expressed in skeletal muscle. We show here that functional NAT1 and NAT2 isoforms are expressed in mouse myotubes and that peroxynitrite may impair their activity in these cells. We show that this inactivation is likely due to the irreversible modification of NATs catalytic cysteine residue in vivo. Our results suggest that peroxynitrite-dependent inactivation of muscle XMEs such as NATs may contribute to muscle dysfunction by impairing the biotransformation activity of this key cellular defense enzyme system.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

The xenobiotic-metabolizing enzymes arylamine N-acetyltransferases in human lens epithelial cells: inactivation by cellular oxidants and UVB-induced oxidative stress

The human arylamine N-acetyltransferases NAT1 and NAT2 are important xenobiotic-metabolizing enzymes involved in the detoxification and metabolic activation of numerous drugs and chemicals. NAT activity depends on genetic polymorphisms and on environmental factors. It has been shown that low NAT-acetylation activity could increase the risk of age-dependent cataract, suggesting that NAT detoxification function may be important for lens cells homeostasis. We report here that the NAT acetylation pathway may occur in human lens epithelial (HLE) cells. Functional NAT1 enzyme was readily detected in HLE cells by reverse transcription-polymerase chain reaction, Western blotting, and enzyme activity assays. NAT2 mRNA and enzymic activity were also detected. We investigated whether oxidants, known to be produced in HLE cells during oxidative stresses and involved in age-dependent cataract formation, decreased endogenous NAT1 and NAT2 activity. The exposure of HLE cells to peroxynitrite led to the dose-dependent irreversible inactivation of both NAT isoforms. Exposing HLE cells to continuously generated H(2)O(2) gave a dose-dependent inactivation of NAT1 and NAT2, reversible on addition of high concentrations of reducing agents. UVB irradiation also induced the reversible dose-dependent inactivation of endogenous NAT1 and NAT2, reversible on addition of reducing agents. Thus, our data suggest that functional NAT1 and NAT2 are present in HLE cells and may be impaired by oxidants produced during oxidative and photooxidative stresses. Oxidative-dependent inhibition of NATs in these cells may increase exposure of lens to the harmful effects of toxic chemicals that could contribute to cataractogenesis over time.

Structure of Mesorhizobium loti arylamine N-acetyltransferase 1

The arylamine N-acetyltransferase (NAT) enzymes have been found in a broad range of both eukaryotic and prokaryotic organisms. The NAT enzymes catalyse the transfer of an acetyl group from acetyl Co-enzyme A onto the terminal nitrogen of a range of arylamine, hydrazine and arylhydrazine compounds. Recently, several NAT structures have been reported from different prokaryotic sources including Salmonella typhimurium, Mycobacterium smegmatis and Pseudomonas aeruginosa. Bioinformatics analysis of the Mesorhizobium loti genome revealed two NAT paralogues, the first example of multiple NAT isoenzymes in a eubacterial organism. The M. loti NAT 1 enzyme was recombinantly expressed and purified for X-ray crystallographic studies. The purified enzyme was crystallized in 0.5 M Ca(OAc)2, 16% PEG 3350, 0.1 M Tris-HCl pH 8.5 using the sitting-drop vapour-diffusion method. A data set diffracting to 2.0 A was collected from a single crystal at 100 K. The crystal belongs to the orthorhombic spacegroup P2(1)2(1)2(1), with unit-cell parameters a = 53.2, b = 97.3, c = 114.3 A. The structure was refined to a final free-R factor of 24.8%. The structure reveals that despite low sequence homology, M. loti NAT1 shares the common fold as reported in previous NAT structures and exhibits the same catalytic triad of residues (Cys-His-Asp) in the active site.

Inactivation of Human Arylamine N‐Acetyltransferase 1 by Hydrogen Peroxide and Peroxynitrite

Arylamine N-acetyltransferases (NAT) are xenobiotic-metabolizing enzymes responsible for the acetylation of many arylamine and heterocyclic amines. They therefore play an important role in the detoxification and activation of numerous drugs and carcinogens. Two closely related isoforms (NAT1 and NAT2) have been described in humans. NAT2 is present mainly in the liver and intestine, whereas NAT1 is found in a wide range of tissues. Interindividual variations in NAT genes have been shown to be a potential source of pharmacological and/or pathological susceptibility. Evidence now shows that redox conditions may also contribute to overall NAT activity. This chapter summarizes current knowledge on human NAT1 regulation by reactive oxygen and nitrogen species.

Screening and characterizing human NAT2 variants

Acetyl CoA:arylamine N-acetyltransferase (NAT; E.C. 2.3.1.5) enzymes play a key role in the metabolic activation of aromatic amine and nitroaromatic mutagens to electrophilic reactive intermediates. We have developed a system in which the activation of mutagens by recombinant human NAT2, expressed in Escherichia coli, can be detected by the appearance of Lac+ revertants. The mutagenesis assay is based on the reversion of an E. coli lacZ frameshift allele; the host strain for the assay is devoid of endogenous NAT activity and a plasmid vector is used for expression of human NAT2. A high-throughput version of the assay facilitates rapid screening of pools of NAT2 variants generated (for example) by random mutagenesis. Along with the methods for these assays, we present selected results of a screening effort in which mutations along the length of the NAT2 sequence have been examined. Homology modeling and simulated annealing have been used to analyze the potential effects of these mutations on structural integrity and substrate binding.

Lack of association between arylamine N-acetyltransferase 2 (NAT2) polymorphism and systemic sclerosis

OBJECTIVES

It has been shown that exposure to some environmental toxins may induce scleroderma-like illness in predisposed individuals, but the etiopathogenesis of the idiopathic form of systemic sclerosis (SSc) remains obscure. The genetic background of this illness has been confirmed in multiple studies. We investigated whether patients with SSc differ from healthy subjects with regard to the enzymatic activity of polymorphic N-acetyltransferase 2 (NAT2).

METHODS

The study was carried out in 39 patients with SSc; 15 fulfilled the criteria of diffuse SSc (dSSc) and 24 of limited SSc (lSSc); an ethnically matched control group consisted of 100 healthy volunteers. Acetylation phenotype was estimated using the isoniazid as a model drug. The most common mutations in the Caucasian population at positions 481T, 803G, 590A and 857A on the NAT2 gene were determined using the polymerase chain reaction-restriction fragment length polymorphism method with deoxyribonucleic acid (DNA) extracted from peripheral blood.

RESULTS

In the group of patients with SSc, the frequency of fast acetylator genotypes was 38.5% (95% CI 23.4-55.4), while that for the genotypes coding slow acetylator status was 51.3% (95% CI 34.8-67.6). There was a strong correlation between NAT2 phenotype and NAT2 genotype with a concordance of 97%. We did not observe a preponderance of slow acetylators among patients with SSc and in two subsets of SSc. With the sample size analyzed in the present study, there is a 90% probability of detecting significant differences in distribution of slow, fast, and intermediate phenotypes between patients with SSc and controls, there is a difference of at least 30.3, 28.7 and 21.9% in the distribution of these phenotypes in the general population, respectively.

CONCLUSION

Acetylator status does not seem to be the significant factor in the development of SSc in patients with both subsets of this autoimmune disease, but further studies are required to confirm this conclusion.

Re-investigation of the concordance of human NAT2 phenotypes and genotypes

A comparative study of N-acetyltransferase 2 (NAT2) genotyping and phenotyping (caffeine test method) was performed on 211 persons to elucidate apparent discrepancies in the assignment of NAT2*12 and NAT2*13 alleles which occur in the literature. The study used the standard procedures of genotyping (two PCR runs and application of seven restriction enzymes) and phenotyping (determination of the two caffeine metabolites 5-acetylamino-6-formylamino-3-methyluracil (AFMU) and 1-methylxanthine (1X)), as documented in detail and validated by the Deutsche Forschungsgemeinschaft. The data were consistent with an AFMU/1X molar ratio of 0.85 as cut-off point (antimode) between phenotypically slow and rapid acetylators. Under this provision, several R/S allele combinations did not comply, either fully or partly, with their associated phenotypes. In particular, there was a wide phenotypic overlap of the alleged rapid allele combination groups (i) NAT2*12A/*5A; NAT2*12C/*5D; NAT2*4/*5B, (ii) NAT2*13/*6B; NAT2*4/*6A, and (iii) NAT2*13/*7A; NAT2*4/*7B. These groups obviously contained both phenotypically rapid and slow acetylators. If one assumes that the presence of one "wild type" allele NAT2*4 defines a rapid acetylator the assignment of the alleles NAT2*12A, NAT2*12C, and NAT*13 as determinants of a rapid acetylator phenotype must be questioned. This refers in particular to the nucleotide changes A803G (NAT2*12A, NAT2*12C) and C282T (NAT2*13). Based on discussions in the literature and the data presented here, there is accumulating evidence that current assignments of the NAT2*12 and NAT2*13 alleles as determinants of a rapid acetylator state should be reconsidered.

Genomic approaches in dissecting complex biological pathways

Advances in genomic research have provided many types of large-scale data that contain rich information on various biological pathways. Intensive efforts have been made to qualitatively or quantitatively model biological pathways using these genomic data. Some general network properties, such as the scale-free property and network motifs, have been discussed and various network models have been applied to reconstruct pathways. However, there is a lack of systematic integration of prior knowledge and different genomic data in these analyses. In this review, we discuss pathway reconstruction under the consideration of the complexity embedded in the biological system, and the global and local properties of biological pathways. We review major methodologies, including clustering methods, scale-free networks models, Bayesian networks models, Boolean networks models, systems of differential equations, and data integration methods. We focus on the difficulty of each methodology in modeling biological pathways, and emphasize that different models capture different aspects of biological pathways or genomic data. The 'noisy' large-scale genomic data require the mathematical models and computational methods to be both robust and identifiable. In addition, we believe that ideal models should have the capability of incorporating various data types and these models need to be assessed through rigorous comparisons with empirical data.

Comparative metabolic activation of benzidine and N-acetylbenzidine by prostaglandin H synthase

Benzidine and N-acetylbenzidine are activated to genotoxic metabolite(s) within the urothelial target tissue, with phase-I and phase-II enzymes being relevant. In principle, both benzidine and N-acetylbenzidine are activated by prostaglandin H synthase (PHS) to reactive intermediates. However, the relative impacts of benzidine and N-acetylbenzidine in this process remain unclear. Two experimental in vitro systems were used in the present comparative investigation: ram seminal vesicle microsomes rich in PHS and porcine urinary bladder epithelial cells (PUBEC) as a model system mimicking the general metabolic situation within the human urothelium. Benzidine, N-acetylbenzidine and N,N'-diacetylbenzidine were incubated with ram seminal vesicle microsomes and arachidonic acid and control incubations were performed with heat-inactivated microsomes. The metabolic disappearance of benzidine, N-acetylbenzidine or N,N'-diacetylbenzidine indicated a rapid turnover by PHS of benzidine and a slower turnover of N-acetylbenzidine. There was almost no PHS-associated metabolism of N,N'-diacetylbenzidine, suggesting that diacetylation of benzidine could represent a pathway of biological inactivation. Under similar conditions, incubations were performed with ram seminal vesicles and benzidine or N-acetylbenzidine upon addition of calf thymus DNA. After re-isolation of the DNA and 32P-postlabeling, with benzidine 2 distinct adducts were found of unknown nature, and with N-acetylbenzidine a single adduct appeared with co-migrated with the N'-(3'-monophosphodeoxyguanosin-8-yl)-N-acetylbenzidine. PUBEC cells were also incubated with benzidine or N-acetylbenzidine. No DNA adduct was found with benzidine, but a total of five adducts was produced from N-acetylbenzidine. The major adduct again co-migrated with N'-(3'-monophosphodeoxyguanosin-8-yl)-N-acetylbenzidine. When benzidine was incubated with PUBEC cells N-acetylbenzidine and, with some delay, N,N'-diacetylbenzidine were formed. Application of Lineweaver-Burk plots for the formation of N-acetylbenzidine from benzidine revealed a K(m) of 56.4 microM and a Vmax of 7.05 nmol/h per 10(6) PUBEC cells. The investigations generally support a key role of N-acetylbenzidine at the target site of the urothelium.

Relationship between intratumoral expression of genes coding for xenobiotic-metabolizing enzymes and benefit from adjuvant tamoxifen in estrogen receptor alpha-positive postmenopausal breast carcinoma

INTRODUCTION

Little is known of the function and clinical significance of intratumoral dysregulation of xenobiotic-metabolizing enzyme expression in breast cancer. One molecular mechanism proposed to explain tamoxifen resistance is altered tamoxifen metabolism and bioavailability.

METHODS

To test this hypothesis, we used real-time quantitative RT-PCR to quantify the mRNA expression of a large panel of genes coding for the major xenobiotic-metabolizing enzymes (12 phase I enzymes, 12 phase II enzymes and three members of the ABC transporter family) in a small series of normal breast (and liver) tissues, and in estrogen receptor alpha (ERalpha)-negative and ERalpha-positive breast tumors. Relevant genes were further investigated in a well-defined cohort of 97 ERalpha-positive postmenopausal breast cancer patients treated with primary surgery followed by adjuvant tamoxifen alone.

RESULTS

Seven of the 27 genes showed very weak or undetectable expression in both normal and tumoral breast tissues. Among the 20 remaining genes, seven genes (CYP2A6, CYP2B6, FMO5, NAT1, SULT2B1, GSTM3 and ABCC11) showed significantly higher mRNA levels in ERalpha-positive breast tumors than in normal breast tissue, or showed higher mRNA levels in ERalpha-positive breast tumors than in ERalpha-negative breast tumors. In the 97 ERalpha-positive breast tumor series, most alterations of these seven genes corresponded to upregulations as compared with normal breast tissue, with an incidence ranging from 25% (CYP2A6) to 79% (NAT1). Downregulation was rare. CYP2A6, CYP2B6, FMO5 and NAT1 emerged as new putative ERalpha-responsive genes in human breast cancer. Relapse-free survival was longer among patients with FMO5-overexpressing tumors or NAT1-overexpressing tumors (P = 0.0066 and P = 0.000052, respectively), but only NAT1 status retained prognostic significance in Cox multivariate regression analysis (P = 0.0013).

CONCLUSIONS

Taken together, these data point to a role of genes coding for xenobiotic-metabolizing enzymes in breast tumorigenesis, NAT1 being an attractive candidate molecular predictor of antiestrogen responsiveness.

Human Acetyl CoA:ArylamineN-Acetyltransferase Variants Generated by Random Mutagenesis

Acetyl CoA:arylamine N-acetyltransferase (NAT) enzymes catalyze the N-acetylation of aromatic amines and the O-acetylation of aryl hydroxylamines, reactions that govern the disposition and toxicity of many drugs and carcinogens. The human NAT genes and enzymes NAT1 and NAT2 are highly polymorphic and constitute one of the best studied examples of the genetic control of drug metabolism. Naturally occurring human NAT variants provide limited insight into the relationship between NAT amino acid sequence and enzyme activity. We have shown previously that the expression of recombinant NAT2 in bacterial tester strains results in greatly enhanced sensitivity to mutagenic nitroaromatic compounds (which are reduced to aryl hydroxylamines by bacterial enzymes). We hypothesized that random mutagenesis combined with rapid screening could be used to identify functionally significant amino acid residues in NAT enzymes. Pools of NAT2 variants were generated by polymerase chain reaction-mediated random mutagenesis of the complete coding sequence. Reversion induced by a NAT-dependent mutagen, 3-methyl-2-nitroimidazo[4,5-f]quinoline, was used as the basis for screening these pools to identify variants with altered enzyme activity. Eighteen variants were characterized by quantitative mutagenicity assays and enzyme kinetic measurements. This approach can provide new insight into the biochemistry of enzymes involved in the metabolic activation of mutagens.

Peroxynitrite irreversibly inactivates the human xenobiotic-metabolizing enzyme arylamine N-acetyltransferase 1 (NAT1) in human breast cancer cells: a cellular and mechanistic study

Arylamine N-acetyltransferases (NATs) play an important role in the detoxification and metabolic activation of a variety of aromatic xenobiotics, including numerous carcinogens. Both of the human isoforms, NAT1 and NAT2, display interindividual variations, and associations between NAT genotypes and cancer risk have been established. Contrary to NAT2, NAT1 has a ubiquitous tissue distribution and has been shown to be expressed in cancer cells. Given that the activity of NAT1 depends on a reactive cysteine that can be a target for oxidants, we studied whether peroxynitrite, a highly reactive nitrogen species involved in human carcinogenesis, could inhibit the activity of endogenous NAT1 in MCF7 breast cancer cells. We show here that exposure of MCF7 cells to physiological concentrations of peroxynitrite and to a peroxynitrite generator (3-morpholinosydnonimine N-ethylcarbamide, or SIN1) leads to the irreversible inactivation of NAT1 in cells. Further kinetic and mechanistic analyses using recombinant NAT1 showed that the enzyme is rapidly (k(inact) = 5 x 10(4) m(-1).s(-1)) and irreversibly inactivated by peroxynitrite. This inactivation is due to oxidative modification of the catalytic cysteine. We conclude that the reducing cellular environment of MCF7 cells does not sufficiently protect NAT1 from peroxynitrite-dependent inactivation and that only high concentrations of reduced glutathione could significantly protect NAT1. Thus, cellular generation of peroxynitrite may contribute to carcinogenesis and tumor progression by weakening key cellular defense enzymes such as NAT1.

Redox regulation of the human xenobiotic metabolizing enzyme arylamine N-acetyltransferase 1 (NAT1). Reversible inactivation by hydrogen peroxide

Oxidative stress is increasingly recognized as a key mechanism in the biotransformation and/or toxicity of many xenobiotics. Human arylamine N-acetyltransferase 1 (NAT1) is a polymorphic ubiquitous phase II xenobiotic metabolizing enzyme that catalyzes the biotransformation of primary aromatic amine or hydrazine drugs and carcinogens. Functional and structural studies have shown that NAT1 catalytic activity is based on a cysteine protease-like catalytic triad, containing a reactive cysteine residue. Reactive protein cysteine residues are highly susceptible to oxidation by hydrogen peroxide (H2O2) generated within the cell. We, therefore, investigated whether human NAT1 activity was regulated by this cellular oxidant. Using purified recombinant NAT1, we show here that NAT1 is rapidly (kinact = 420 m-1.min-1) inactivated by physiological concentrations of H2O2. Reducing agents, such as reduced glutathione (GSH), reverse the H2O2-dependent inactivation of NAT1. Kinetic analysis and protection experiments with acetyl-CoA, the physiological acetyl-donor substrate of the enzyme, suggested that the H2O2-dependent inactivation reaction targets the active-site cysteine residue. Finally, we show that the reversible inactivation of NAT1 by H2O2 is due to the formation of a stable sulfenic acid group at the active-site cysteine. Our results suggest that, in addition to known genetically controlled interindividual variations in NAT1 activity, oxidative stress and cellular redox status may also regulate NAT1 activity. This may have important consequences with regard to drug biotransformation and cancer risk.

Reversible inhibition of the human xenobiotic-metabolizing enzyme arylamine N-acetyltransferase 1 by S-nitrosothiols

Human arylamine N-acetyltransferase 1 (NAT1) is a polymorphic phase II xenobiotic-metabolizing enzyme which catalyzes the biotransformation of primary aromatic amines, hydrazine drugs, and carcinogens. Structural and functional studies have shown that the NAT1 and factor XIII transglutaminase catalytic pockets are structurally related with the existence of a conserved catalytic triad (Cys-His-Asp). In addition, it has been reported that factor XIII transglutaminase activity could be regulated by nitric oxide (NO), in particular S-nitrosothiols (RSNO). We thus tested whether NAT1 could be a target of S-nitrosothiols. We show here that human NAT1 is reversibly inactivated by S-nitrosothiols such as SNAP (S-nitroso-N-acetyl-DL-penicillamine). A second-order rate constant for the inactivation of NAT1 by SNAP was determined (k(inact)=270M(-1)min(-1)) and shown to be in the same range of values reported for other enzymes. The inhibition of NAT1 by S-nitrosothiols was reversed by dithiothreitol and reduced glutathione, but not by ascorbate. As reported for some reactive cysteine-containing enzymes, our results suggest that inactivation of NAT1 by S-nitrosothiols is due to direct attack of the highly reactive cysteine residue in the enzyme active site on the sulfur of S-nitrosothiols to form a mixed disulfide between these NO-derived oxidants and NAT1. Finally, our findings suggest that, in addition to the polymorphic-dependent variation of NAT1 activity, NO-derived oxidants, in particular S-nitrosothiols, could also regulate NAT1 activity.

Xenobiotic inducible regions of the human arylamine N-acetyltransferase 1 and 2 genes

Arylamine N-acetyltransferase (NAT) enzymes catalyze the addition of an acetyl group from acetyl-CoA to a terminal nitrogen on a suitable substrate such as environmentally relevant compounds and pharmaceuticals. In human, there are two highly polymorphic active allozymes, NAT1 and -2, and one inactive pseudogene, NATP. The expression of these enzymes is tissue-specific such that NAT1 is ubiquitously expressed and NAT2 is confined mainly to liver and colorectal tissues. We hypothesized that these genes would be tissue-specifically transcriptionally regulated, and so we isolated putative proximal control regions for both the NAT genes, which were inserted into luciferase vectors and transiently transfected into human liver and bladder cells. The transfected cells were dosed with 4-aminosalicylic acid, sulfamethazine or solvent and the resulting luciferase activity was measured. We found that both NAT1 and -2 regions were inducible in liver cells by both xenobiotics but only one of the NAT1 regions was inducible again by both xenobiotics in bladder cells. These results suggest that the NAT genes may be tissue-specifically transcriptionally regulated.

Structural investigation of mutant Mycobacterium smegmatis arylamine N-acetyltransferase: a model for a naturally occurring functional polymorphism in Mycobacterium tuberculosis arylamine N-acetyltransferase

Arylamine N-acetyltransferase (NAT) acetylates the front-line anti-tuberculosis drug isoniazid (INH) and has been identified in Mycobacterium tuberculosis. A naturally occurring single nucleotide polymorphism (SNP) was recently found in the NAT gene in clinical isolates of M. tuberculosis. The nucleotide change from G-->A (619) produces an amino acid change Gly(207) Arg, which appears to reduce the activity of the NAT from M. tuberculosis (TBNAT). It has not been possible to generate sufficient soluble recombinant TBNAT for 3D structural studies. Therefore, Mycobacterium smegmatis NAT (SMNAT), which has 60% identity to TBNAT and has Gly at 207, was used as a model to investigate the possible structural effects of the G-->A 619 SNP. The mutant form of SMnat (SM207Rnat) was constructed by in vitro site-directed mutagenesis and was heterologously expressed with an N-terminal His tag in Escherichia coli, for comparison with the SMNAT. Both recombinant SMNATs were purified using Ni affinity chromatography and treated with thrombin to cleave the tag. Both proteins were produced with average yields of over 10 mg/L and were active. Substrate specificity and thermal stability of SM207RNAT were assessed and compared with the wild type SMNAT using kinetic assays and circular dichroism spectroscopy. SM207RNAT was crystallised and a data set of 2.00 A resolution was obtained. The SM207RNAT had different substrate specificities to the wild type protein and the 3D structures revealed that the Gly(207) Arg mutation caused slight changes in the orientation of His(203) in SMNAT.