Divergence of cofactor recognition across evolution: coenzyme A binding in a prokaryotic arylamine N-acetyltransferase

Comparative Investigation of Fifteen Xenobiotic Metabolizing N-Acetyltransferase (NAT) Homologs from Bacteria

Arylamines constitute a large group of industrial chemicals detoxified by certain bacteria through conjugation reactions catalyzed by N-acetyltransferase (NAT) enzymes. NAT homologs, mostly from pathogenic bacteria, have been the subject of individual studies that do not facilitate direct comparisons. By implementing a practicable pipeline, we present comparative investigation of fifteen NAT homologs from ten bacteria, mainly bacilli, streptomycetes, and one alphaproteobacterium. The new homologs were characterized for their sequence, phylogeny, predicted structural features, substrate specificity, thermal stability, and interaction with components of the enzymatic reaction. Bacillus NATs demonstrated the characteristics of xenobiotic metabolizing N-acetyltransferases, with the majority of homologs generating high activities. Non-pathogenic bacilli are thus proposed as suitable mediators of arylamine bioremediation. Of the Streptomyces homologs, the NAT2 isoenzyme of S. venezuelae efficiently transformed highly toxic arylamines, while the remaining homologs were inactive or generated low activities suggesting that xenobiotic metabolism may not be their primary role. The functional divergence of Streptomyces NATs was consistent with their observed sequence, phylogenetic, and structural variability. These and previous findings support classification of microbial NATs into three groups. The first includes xenobiotic metabolizing enzymes with dual acetyl-/propionyl-CoA selectivity. Homologs of the second group are more rarely encountered, acting as malonyltransferases mediating specialized ecological interactions. Homologs of the third group effectively lack acyltransferase activity and their study may represent an interesting research area. Comparative NAT enzyme screens from a broad microbial spectrum may guide rational selection of homologs likely to share similar biological functions, allowing their combined investigation and use in biotechnological applications. IMPORTANCE Arylamines are encountered as industrial chemicals or byproducts of agrochemicals that may constitute highly toxic contaminants of soils and groundwaters. Although such chemicals may be recalcitrant to biotransformation, they can be enzymatically converted into less toxic forms by some bacteria. Therefore, exploitation of the arylamine detoxification capabilities of microorganisms is investigated as an effective approach for bioremediation. Among microbial biotransformations of arylamines, enzymatic conjugation reactions have been reported, including NAT-mediated N-acetylation. Comparative investigations of NAT enzymes across a range of microorganisms can be laborious and expensive, so here we present a streamlined methodology for implementing such work. We compare fifteen NAT homologs from non-pathogenic, free-living bacteria of potential biotechnological utility, mainly Terrabacteria known for their rich secondary and xenobiotic metabolism. The analysis allowed insights into the evolutionary and functional divergence of bacterial NAT homologs, combined with assessment of their fundamental structural and enzymatic differences and similarities.

Biochemical Characterization of Arylamine N-acetyltransferases From Vibrio vulnificus

Vibrio vulnificus is a zoonotic bacterium that is capable of causing highly lethal diseases in humans; this pathogen is responsible for 95% of all seafood-related deaths in the United States. Arylamine N-acetyltransferases (NAT, E.C. 2.3.1.5) is a major family of xenobiotic-metabolizing enzymes that can biotransform aromatic amine chemicals. In this research, to evaluate the effect of NAT on acetyl group transformation in arylamine antibiotics, we first used sequence alignment to study the structure of V. vulnificus NAT [(VIBVN)NAT]. The nat gene encodes a protein of 260 amino acids, which has an approximate molecular mass of 30 kDa. Then we purified recombinant (VIBVN)NAT and determined the enzyme activity by PNPA and DTNB methods. The DTNB method indicates that this prokaryotic NAT has a particular substrate specificity towards aromatic substrates. However, (VIBVN)NAT lost most of its activity after treatment with high concentrations of urea and H₂O₂. In addition, we also explored the stability of the enzyme at different temperatures and pH values. In analyzing the influence of metal ions, the enzyme activity was significantly inhibited by Zn²⁺ and Cu²⁺. The kinetic parameters K_m and V_max were determined using hydralazine, isoniazid, 4-amino salicylic acid, and 4-chloro-3-methylaniline as substrates, and the T_m, T_agg and size distribution of (VIBVN)NAT were observed. In particular, a molecular docking study on the structure of (VIBVN)NAT was conducted to understand its biochemical traits. These results showed that (VIBVN)NAT could acetylate various aromatic amine substrates and contribute to arylamine antibiotic resistance in V. vulnificus.

3-(5-Nitrofuran-2-yl)prop-2-en-1-one Derivatives, with Potent Antituberculosis Activity, Inhibit A Novel Therapeutic Target, Arylamine N-acetyltransferase, in Mycobacteria

In this study, the inhibitory potential of 3-(5-nitrofuran-2-yl)prop-2-en-1-one derivatives was evaluated against a panel of bacteria, as well as mammalian cell lines to determine their therapeutic index. In addition, we investigated the mechanism of antibiotic action of the derivatives to identify their therapeutic target. We discovered compound 2 to be an extremely potent inhibitor of Mycobacterium tuberculosis H37Rv growth (MIC: 0.031 mg/L) in vitro, performing better than the currently used first-line antituberculosis drugs such as isoniazid, rifampicin, ethambutol, and pretomanid in vitro. Furthermore, compound 3 was equipotent to pretomanid against a multidrug-resistant M. tuberculosis clinical isolate. The derivatives were selective and bactericidal towards slow-growing mycobacteria. They showed low cytotoxicity towards murine RAW 264.7 and human THP-1 cell lines, with high selectivity indices. Compound 1 effectively eliminated the intracellular mycobacteria in a mycobacteria-infected macrophage model. The derivatives were assessed for their potential to inhibit mycobacterial arylamine N-acetyltransferase (NAT) and were identified as good inhibitors of recombinant mycobacterial NAT, a novel target essential for the intracellular survival of M. tuberculosis. This study provided hits for designing new potent and selective antituberculosis leads, having mycobacterial NAT inhibition as their possible endogenous mechanisms of action.

From transglutaminases (TGs) to arylamine N-acetyltransferases (NATs): Insight into the role of a spatially conserved aromatic amino acid position in the active site of these two families of enzymes

Transglutaminases (TG) and arylamine N-acetyltransferases (NAT) are important family of enzymes. Although they catalyze different reactions and have distinct structures, these two families of enzymes share a spatially conserved catalytic triad (Cys, His, Asp residues). In active TGs, a conserved Trp residue located close to the triad cysteine is crucial for catalysis through stabilization of transition states. Here, we show that in addition to sharing a similar catalytic triad with TGs, functional NAT enzymes also possess in their active site an aromatic residue (Phe, Tyr or Trp) occupying a structural position similar to the Trp residue of active TGs. More importantly, as observed in active TGs, our data indicates that in functional NAT enzymes this conserved aromatic residue is also involved in stabilization of transition states. These results thus indicate that in addition to the three triad residues, these two families of enzymes also share a spatially conserved aromatic amino acid position important for catalysis. Identification of residues involved in the stabilization of transition states is important to develop potent inhibitors. Interestingly, NAT enzymes have been shown as potential targets of clinical interest.

The actinobacterium Tsukamurella paurometabola has a functionally divergent arylamine N-acetyltransferase (NAT) homolog

Actinobacteria in the Tsukamurella genus are aerobic, high-GC, Gram-positive mycolata, considered as opportunistic pathogens and isolated from various environmental sources, including sites contaminated with oil, urban or industrial waste and pesticides. Although studies look into xenobiotic biotransformation by Tsukamurella isolates, the relevant enzymes remain uncharacterized. We investigated the arylamine N-acetyltransferase (NAT) enzyme family, known for its role in the xenobiotic metabolism of prokaryotes and eukaryotes. Xenobiotic sensitivity of Tsukamurella paurometabola type strain DSM 20162^T was assessed, followed by cloning, recombinant expression and functional characterization of its single NAT homolog (TSUPD)NAT1. The bacterium appeared quite robust against chloroanilines, but more sensitive to 4-anisidine and 2-aminophenol. However, metabolic activity was not evident towards those compounds, presumably due to mechanisms protecting cells from xenobiotic entry. Of the pharmaceutical arylhydrazines tested, hydralazine was toxic, but the bacterium was less sensitive to isoniazid, a drug targeting mycolic acid biosynthesis in mycobacteria. Although (TSUPD)NAT1 protein has an atypical Cys-His-Glu (instead of the expected Cys-His-Asp) catalytic triad, it is enzymatically active, suggesting that this deviation is likely due to evolutionary adaptation potentially serving a different function. The protein was indeed found to use malonyl-CoA, instead of the archetypal acetyl-CoA, as its preferred donor substrate. Malonyl-CoA is important for microbial biosynthesis of fatty acids (including mycolic acids) and polyketide chains, and the corresponding enzymatic systems have common evolutionary histories, also linked to xenobiotic metabolism. This study adds to accummulating evidence suggesting broad phylogenetic and functional divergence of microbial NAT enzymes that goes beyond xenobiotic metabolism and merits investigation.

A Chlamydia effector combining deubiquitination and acetylation activities induces Golgi fragmentation

Pathogenic bacteria are armed with potent effector proteins that subvert host signalling processes during infection¹. The activities of bacterial effectors and their associated roles within the host cell are often poorly understood, particularly for Chlamydia trachomatis², a World Health Organization designated neglected disease pathogen. We identify and explain remarkable dual Lys63-deubiquitinase (DUB) and Lys-acetyltransferase activities in the Chlamydia effector ChlaDUB1. Crystal structures capturing intermediate stages of each reaction reveal how the same catalytic centre of ChlaDUB1 can facilitate such distinct processes, and enable the generation of mutations that uncouple the two activities. Targeted Chlamydia mutant strains allow us to link the DUB activity of ChlaDUB1 and the related, dedicated DUB ChlaDUB2 to fragmentation of the host Golgi apparatus, a key process in Chlamydia infection for which effectors have remained elusive. Our work illustrates the incredible versatility of bacterial effector proteins, and provides important insights towards understanding Chlamydia pathogenesis.

Homology modeling and prediction of the amino acid residues participating in the transfer of acetyl-CoA to arylalkylamine by the N-acetyltransferase from Chryseobacterium sp

OBJECTIVES

To predict the amino acid residues playing important roles in acetyl-CoA and substrate binding and to study the acetyl group transfer mechanism of Chryseobacterium sp. 5-3B N-acetyltransferase (5-3B NatA).

RESULTS

A 3-dimensional homology model of 5-3B NatA was constructed to compare the theoretical structure of this compound with the structures of previously reported proteins belonging to the bacterial GCN5 N-acetyltransferase family. Homology modeling of the 5-3B NatA structure and a characterization of the enzyme's kinetic parameters identified the essential amino acid residues involved in binding and acetyl-group transfer. ¹²⁶Leu, ¹³²Leu, and ¹³⁵Lys were implicated in the binding of phosphopantothenic acid, and ¹⁰⁰Tyr and ¹³¹Lys in that of adenosyl biphosphate. The data supported the participation of ⁸³Glu and ¹³³Tyr in catalyzing acetyl-group transfer to L-2-phenylglycine.

CONCLUSIONS

5-3B NatA catalyzes the enantioselective N-acetylation of L-2-phenylglycine via a ternary complex comprising the enzyme, acetyl-CoA, and the substrate.

The Role of N-Acetyl Transferases on Isoniazid Resistance from Mycobacterium tuberculosis and Human: An In Silico Approach

Background

N-acetyl transferase (NAT) inactivates the pro-drug isoniazid (INH) to N-acetyl INH through a process of acetylation, and confers low-level resistance to INH in Mycobacterium tuberculosis (MTB). Similar to NAT of MTB, NAT2 in humans performs the same function of acetylation. Rapid acetylators, may not respond to INH treatment efficiently, and could be a potential risk factor, for the development of INH resistance in humans.

Methods

To understand the contribution of NAT of MTB and NAT2 of humans in developing INH resistance using in silico approaches, in this study, the wild type (WT) and mutant (MT)-NATs of MTB, and humans, were modeled and docked, with substrates and product (acetyl CoA, INH, and acetyl INH). The MT models were built, using templates 4BGF of MTB, and 2PFR of humans.

Results

On the basis of docking results of MTB-NAT, it can be suggested that in comparison to the WT, binding affinity of MT-G207R, was found to be lower with acetyl CoA, and higher with acetyl-INH and INH. In case of MT-NAT2 from humans, the pattern of score with respect to acetyl CoA and acetyl-INH, was similar to MT-NAT of MTB, but revealed a decrease in INH score.

Conclusion

In MTB, MT-NAT revealed high affinity towards acetyl-INH, which can be interpreted as increased formation of acetyl-INH, and therefore, may lead to INH resistance through inactivation of INH. Similarly, in MT-NAT2 (rapid acetylators), acetylation occurs rapidly, serving as a possible risk factor for developing INH resistance in humans.

Identification of the anti-mycobacterial functional properties of piperidinol derivatives

Background and Purpose

Tuberculosis (TB) remains a major global health threat and is now the leading cause of death from a single infectious agent worldwide. The current TB drug regimen is inadequate, and new anti‐tubercular agents are urgently required to be able to successfully combat the increasing prevalence of drug‐resistant TB. The purpose of this study was to investigate a piperidinol compound derivative that is highly active against the Mycobacterium tuberculosis bacillus.

Experimental Approach

The antibacterial properties of the piperidinol compound and its corresponding bis‐Mannich base analogue were evaluated against M. smegmatis and Gram‐negative organisms. Cytotoxicity studies were undertaken in order to determine the selectivity index for these compounds. Spontaneous resistant mutants of M. smegmatis were generated against the piperidinol and corresponding bis‐Mannich base lead derivatives and whole genome sequencing employed to determine the genetic modifications that lead to selection pressure in the presence of these compounds.

Key Results

The piperidinol and the bis‐Mannich base analogue were found to be selective for mycobacteria and rapidly kill this organism with a cytotoxicity selectivity index for mycobacteria of >30‐fold. Whole genome sequencing of M. smegmatis strains resistant to the lead compounds led to the identification of a number of single nucleotide polymorphisms indicating multiple targets.

Conclusion and Implications

Our results indicate that the piperidinol moiety represents an attractive compound class in the pursuit of novel anti‐tubercular agents.

Linked Articles

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro‐organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc

Cholesterol metabolism: a potential therapeutic target in Mycobacteria

Tuberculosis (TB), although a curable disease, is still one of the most difficult infections to treat. Mycobacterium tuberculosis infects 10 million people worldwide and kills 1.5 million people each year. Reactivation of a latent infection is the major cause of TB. Cholesterol is a critical carbon source during latent infection. Catabolism of cholesterol contributes to the pool of propionyl-CoA, a precursor that is incorporated into lipid virulence factors. The M. tuberculosis genome contains a large regulon of cholesterol catabolic genes suggesting that the microorganism can utilize host sterol for infection and persistence. The protein products of these genes present ideal targets for rational drug discovery programmes. This review summarizes the development of enzyme inhibitors targeting the cholesterol pathway in M. tuberculosis. This knowledge is essential for the discovery of novel agents to treat M. tuberculosis infection.

LINKED ARTICLES

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

Identification of cancer chemopreventive isothiocyanates as direct inhibitors of the arylamine N-acetyltransferase-dependent acetylation and bioactivation of aromatic amine carcinogens

Aromatic amines (AAs) are chemicals of industrial, pharmacological and environmental relevance. Certain AAs, such as 4-aminobiphenyl (4-ABP), are human carcinogens that require enzymatic metabolic activation to reactive chemicals to form genotoxic DNA adducts. Arylamine N-acetyltransferases (NAT) are xenobiotic metabolizing enzymes (XME) that play a major role in this carcinogenic bioactivation process. Isothiocyanates (ITCs), including benzyl-ITC (BITC) and phenethyl-ITC (PEITC), are phytochemicals known to have chemopreventive activity against several aromatic carcinogens. In particular, ITCs have been shown to modify the bioactivation and subsequent mutagenicity of carcinogenic AA chemicals such as 4-ABP. However, the molecular and biochemical mechanisms by which these phytochemicals may modulate AA carcinogens bioactivation and AA-DNA damage remains poorly understood.

This manuscript provides evidence indicating that ITCs can decrease the metabolic activation of carcinogenic AAs via the irreversible inhibition of NAT enzymes and subsequent alteration of the acetylation of AAs. We demonstrate that BITC and PEITC react with NAT1 and inhibit readily its acetyltransferase activity (k_i = 200 M⁻¹.s⁻¹ and 66 M⁻¹.s⁻¹ for BITC and PEITC, respectively). Chemical labeling, docking approaches and substrate protection assays indicated that inhibition of the acetylation of AAs by NAT1 was due to the chemical modification of the enzyme active site cysteine. Moreover, analyses of AAs acetylation and DNA adducts in cells showed that BITC was able to modulate the endogenous acetylation and bioactivation of 4-ABP. In conclusion, we show that direct inhibition of NAT enzymes may be an important mechanism by which ITCs exert their chemopreventive activity towards AA chemicals.

Xenobiotic-metabolizing enzymes in Bacillus anthracis: molecular and functional analysis of a truncated arylamine N-acetyltransferase isozyme

BACKGROUND AND PURPOSE

The arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes that play an important role in the detoxification and/or bioactivation of arylamine drugs and xenobiotics. In bacteria, NATs may contribute to the resistance against antibiotics such as isoniazid or sulfamides through their acetylation, which makes this enzyme family a possible drug target. Bacillus anthracis, a bacterial species of clinical significance, expresses three NAT isozymes with distinct structural and enzymatic properties, including an inactive isozyme ((BACAN)NAT3). (BACAN)NAT3 features both a non-canonical Glu residue in its catalytic triad and a truncated C-terminus domain. However, the role these unusual characteristics play in the lack of activity of the (BACAN)NAT3 isozyme remains unclear.

EXPERIMENTAL APPROACH

Protein engineering, recombinant expression, enzymatic analyses with aromatic amine substrates and phylogenetic analysis approaches were conducted.

KEY RESULTS

The deletion of guanine 580 (G580) in the nat3 gene was shown to be responsible for the expression of a truncated (BACAN)NAT3 isozyme. Artificial re-introduction of G580 in the nat3 gene led to a functional enzyme able to acetylate several arylamine drugs displaying structural characteristics comparable with its functional Bacillus cereus homologue ((BACCR)NAT3). Phylogenetic analysis of the nat3 gene in the B. cereus group further indicated that nat3 may constitute a pseudogene of the B. anthracis species.

CONCLUSION AND IMPLICATIONS

The existence of NATs with distinct properties and evolution in Bacillus species may account for their adaptation to their diverse chemical environments. A better understanding of these isozymes is of importance for their possible use as drug targets.

LINKED ARTICLES

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

The role of lysine(100) in the binding of acetylcoenzyme A to human arylamine N-acetyltransferase 1: implications for other acetyltransferases

The arylamine N-acetyltransferases (NATs) catalyze the acetylation of aromatic and heterocyclic amines as well as hydrazines. All proteins in this family of enzymes utilize acetyl coenzyme A (AcCoA) as an acetyl donor, which initially binds to the enzyme and transfers an acetyl group to an active site cysteine. Here, we have investigated the role of a highly conserved amino acid (Lys(100)) in the enzymatic activity of human NAT1. Mutation of Lys(100) to either a glutamine or a leucine significantly increased the Ka for AcCoA without changing the Kb for the acetyl acceptor p-aminobenzoic acid. In addition, substrate inhibition was more marked with the mutant enzymes. Steady state kinetic analyzes suggested that mutation of Lys(100) to either leucine or glutamine resulted in a less stable enzyme-cofactor complex, which was not seen with a positively charged arginine at this position. When p-nitrophenylacetate was used as acetyl donor, no differences were seen between the wild-type and mutant enzymes because p-nitrophenylacetate is too small to interact with Lys(100) when bound to the active site. Using 3'-dephospho-AcCoA as the acetyl donor, kinetic data confirmed that Ly(100) interacts with the 3'-phosphoanion to stabilize the enzyme-cofactor complex. Mutation of Lys(100) decreases the affinity of AcCoA for the protein and increases the rate of CoA release. Crystal structures of several other unrelated acetyltransferases show a lysine or arginine residue within 3Å of the 3'-phosphoanion of AcCoA, suggesting that this mechanism for stabilizing the complex by the formation of a salt bridge may be widely applicable in nature.

Insight into cofactor recognition in arylamine N-acetyltransferase enzymes: structure of Mesorhizobium loti arylamine N-acetyltransferase in complex with coenzyme A

Arylamine N-acetyltransferases (NATs) are xenobiotic metabolizing enzymes that catalyze the acetyl-CoA-dependent acetylation of arylamines. To better understand the mode of binding of the cofactor by this family of enzymes, the structure of Mesorhizobium loti NAT1 [(RHILO)NAT1] was determined in complex with CoA. The F42W mutant of (RHILO)NAT1 was used as it is well expressed in Escherichia coli and displays enzymatic properties similar to those of the wild type. The apo and holo structures of (RHILO)NAT1 F42W were solved at 1.8 and 2 Å resolution, respectively. As observed in the Mycobacterium marinum NAT1-CoA complex, in (RHILO)NAT1 CoA binding induces slight structural rearrangements that are mostly confined to certain residues of its `P-loop'. Importantly, it was found that the mode of binding of CoA is highly similar to that of M. marinum NAT1 but different from the modes reported for Bacillus anthracis NAT1 and Homo sapiens NAT2. Therefore, in contrast to previous data, this study shows that different orthologous NATs can bind their cofactors in a similar way, suggesting that the mode of binding CoA in this family of enzymes is less diverse than previously thought. Moreover, it supports the notion that the presence of the `mammalian/eukaryotic insertion loop' in certain NAT enzymes impacts the mode of binding CoA by imposing structural constraints.

Arylamine N-acetyltransferases: from drug metabolism and pharmacogenetics to drug discovery

Arylamine N-acetyltransferases (NATs) are polymorphic drug-metabolizing enzymes, acetylating arylamine carcinogens and drugs including hydralazine and sulphonamides. The slow NAT phenotype increases susceptibility to hydralazine and isoniazid toxicity and to occupational bladder cancer. The two polymorphic human NAT loci show linkage disequilibrium. All mammalian Nat genes have an intronless open reading frame and non-coding exons. The human gene products NAT1 and NAT2 have distinct substrate specificities: NAT2 acetylates hydralazine and human NAT1 acetylates p-aminosalicylate (p-AS) and the folate catabolite para-aminobenzoylglutamate (p-abaglu). Human NAT2 is mainly in liver and gut. Human NAT1 and its murine homologue are in many adult tissues and in early embryos. Human NAT1 is strongly expressed in oestrogen receptor-positive breast cancer and may contribute to folate and acetyl CoA homeostasis. NAT enzymes act through a catalytic triad of Cys, His and Asp with the architecture of the active site-modulating specificity. Polymorphisms may cause unfolded protein. The C-terminus helps bind acetyl CoA and differs among NATs including prokaryotic homologues. NAT in Salmonella typhimurium supports carcinogen activation and NAT in mycobacteria metabolizes isoniazid with polymorphism a minor factor in isoniazid resistance. Importantly, nat is in a gene cluster essential for Mycobacterium tuberculosis survival inside macrophages. NAT inhibitors are a starting point for novel anti-tuberculosis drugs. Human NAT1-specific inhibitors may act in biomarker detection in breast cancer and in cancer therapy. NAT inhibitors for co-administration with 5-aminosalicylate (5-AS) in inflammatory bowel disease has prompted ongoing investigations of azoreductases in gut bacteria which release 5-AS from prodrugs including balsalazide.

Differences between murine arylamine N-acetyltransferase type 1 and human arylamine N-acetyltransferase type 2 defined by substrate specificity and inhibitor binding

Background

The mouse has three arylamine N-acetyltransferase genes, (MOUSE)Nat1, (MOUSE)Nat2 and (MOUSE)Nat3. These are believed to correspond to (HUMAN)NAT1, (HUMAN)NAT2 and NATP in humans. (MOUSE)Nat3 encodes an enzyme with poor activity and human NATP is a pseudogene. (MOUSE)Nat2 is orthologous to (HUMAN)NAT1 and their corresponding proteins are functionally similar, but the relationship between (MOUSE)Nat1 and (HUMAN)NAT2 is less clear-cut.

Methods

To determine whether the (MOUSE)NAT1 and (HUMAN)NAT2 enzymes are functionally equivalent, we expressed and purified (MOUSE)NAT1*1 and analysed its substrate specificity using a panel of arylamines and hydrazines. To understand how specific residues contribute to substrate selectivity, three site-directed mutants of (MOUSE)NAT2*1 were prepared: these were (MOUSE)NAT2_F125S, (MOUSE)NAT2_R127G and (MOUSE)NAT2_R127L. All three exhibited diminished activity towards “(MOUSE)NAT2-specific” arylamines but were more active against hydrazines than (MOUSE)NAT1*1. The inhibitory and colorimetric properties of a selective naphthoquinone inhibitor of (HUMAN)NAT1 and (MOUSE)NAT2 were investigated.

Results

Comparing (MOUSE)NAT1*1 with other mammalian NAT enzymes demonstrated that the substrate profiles of (MOUSE)NAT1 and (HUMAN)NAT2 are less similar than previously believed. Three key residues (F125, R127 and Y129) in (HUMAN)NAT1*4 and (MOUSE)NAT2*1 were required for enzyme inhibition and the associated colour change on naphthoquinone binding. In silico modelling of selective ligands into the appropriate NAT active sites further implicated these residues in substrate and inhibitor specificity in mouse and human NAT isoenzymes.

Conclusions

Three non-catalytic residues within (HUMAN)NAT1*4 (F125, R127 and Y129) contribute both to substrate recognition and inhibitor binding by participating in distinctive intermolecular interactions and maintaining the steric conformation of the catalytic pocket. These active site residues contribute to the definition of substrate and inhibitor selectivity, an understanding of which is essential for facilitating the design of second generation (HUMAN)NAT1-selective inhibitors for diagnostic, prognostic and therapeutic purposes. In particular, since the expression of (HUMAN)NAT1 is related to the development and progression of oestrogen-receptor-positive breast cancer, these structure-based tools will facilitate the ongoing design of candidate compounds for use in (HUMAN)NAT1-positive breast tumours.

Electronic supplementary material

The online version of this article (doi:10.1186/2050-6511-15-68) contains supplementary material, which is available to authorized users.

Structural and functional characterization of an arylamineN-acetyltransferase from the pathogenMycobacterium abscessus: differences from other mycobacterial isoforms and implications for selective inhibition

Mycobacterium abscessusis the most pathogenic rapid-growing mycobacterium and is one of the most resistant organisms to chemotherapeutic agents. However, structural and functional studies ofM. abscessusproteins that could modify/inactivate antibiotics remain nonexistent. Here, the structural and functional characterization of an arylamineN-acetyltransferase (NAT) fromM. abscessus[(MYCAB)NAT1] are reported. This novel prokaryotic NAT displays significantN-acetyltransferase activity towards aromatic substrates, including antibiotics such as isoniazid andp-aminosalicylate. The enzyme is endogenously expressed and functional in both the rough and smoothM. abscessusmorphotypes. The crystal structure of (MYCAB)NAT1 at 1.8 Å resolution reveals that it is more closely related toNocardia farcinicaNAT than to mycobacterial isoforms. In particular, structural and physicochemical differences from other mycobacterial NATs were found in the active site. Peculiarities of (MYCAB)NAT1 were further supported by kinetic and docking studies showing that the enzyme was poorly inhibited by the piperidinol inhibitor of mycobacterial NATs. This study describes the first structure of an antibiotic-modifying enzyme fromM. abscessusand provides bases to better understand the substrate/inhibitor-binding specificities among mycobacterial NATs and to identify/optimize specific inhibitors. These data should also contribute to the understanding of the mechanisms that are responsible for the pathogenicity and extensive chemotherapeutic resistance ofM. abscessus.

Exploration of piperidinols as potential antitubercular agents

Novel drugs to treat tuberculosis are required and the identification of potential targets is important. Piperidinols have been identified as potential antimycobacterial agents (MIC < 5 μg/mL), which also inhibit mycobacterial arylamine N-acetyltransferase (NAT), an enzyme essential for mycobacterial survival inside macrophages. The NAT inhibition involves a prodrug-like mechanism in which activation leads to the formation of bioactive phenyl vinyl ketone (PVK). The PVK fragment selectively forms an adduct with the cysteine residue in the active site. Time dependent inhibition of the NAT enzyme from Mycobacterium marinum (M. marinum) demonstrates a covalent binding mechanism for all inhibitory piperidinol analogues. The structure activity relationship highlights the importance of halide substitution on the piperidinol benzene ring. The structures of the NAT enzymes from M. marinum and M. tuberculosis, although 74% identical, have different residues in their active site clefts and allow the effects of amino acid substitutions to be assessed in understanding inhibitory potency. In addition, we have used the piperidinol 3-dimensional shape and electrostatic properties to identify two additional distinct chemical scaffolds as inhibitors of NAT. While one of the scaffolds has anti-tubercular activity, both inhibit NAT but through a non-covalent mechanism.

Arylamine N-acetyltransferases: a structural perspective

Arylamine N-acetyltransferase (NAT) plays an important role in metabolism and detoxification of many compounds including drugs and environmental carcinogens through chemical modification of the amine group with an acetyl group. Recent studies have suggested that NATs are also involved in cancer cell growth and inhibition of the enzymes may be a potential target for cancer chemotherapy. Three-dimensional (3D) structures are available for NATs from both prokaryotes and eukaryotes. These structures provide valuable insights into the acetylation mechanism, features of the active site and the structural determinants that govern substrate/inhibitor-binding specificity. Such insights allow a more precise understanding of the structure-activity relationships for NAT substrates and inhibitors. Furthermore, the structural elucidation of NATs has generated powerful tools in the design of small molecule inhibitors that should alleviate cancer, based on the important role of the enzyme in cancer biology.

From arylamine N-acetyltransferase to folate-dependent acetyl CoA hydrolase: impact of folic acid on the activity of (HUMAN)NAT1 and its homologue (MOUSE)NAT2

Acetyl Coenzyme A-dependent N-, O- and N,O-acetylation of aromatic amines and hydrazines by arylamine N-acetyltransferases is well characterised. Here, we describe experiments demonstrating that human arylamine N-acetyltransferase Type 1 and its murine homologue (Type 2) can also catalyse the direct hydrolysis of acetyl Coenzyme A in the presence of folate. This folate-dependent activity is exclusive to these two isoforms; no acetyl Coenzyme A hydrolysis was found when murine arylamine N-acetyltransferase Type 1 or recombinant bacterial arylamine N-acetyltransferases were incubated with folate. Proton nuclear magnetic resonance spectroscopy allowed chemical modifications occurring during the catalytic reaction to be analysed in real time, revealing that the disappearance of acetyl CH₃ from acetyl Coenzyme A occurred concomitantly with the appearance of a CH₃ peak corresponding to that of free acetate and suggesting that folate is not acetylated during the reaction. We propose that folate is a cofactor for this reaction and suggest it as an endogenous function of this widespread enzyme. Furthermore, in silico docking of folate within the active site of human arylamine N-acetyltransferase Type 1 suggests that folate may bind at the enzyme’s active site, and facilitate acetyl Coenzyme A hydrolysis. The evidence presented in this paper adds to our growing understanding of the endogenous roles of human arylamine N-acetyltransferase Type 1 and its mouse homologue and expands the catalytic repertoire of these enzymes, demonstrating that they are by no means just xenobiotic metabolising enzymes but probably also play an important role in cellular metabolism. These data, together with the characterisation of a naphthoquinone inhibitor of folate-dependent acetyl Coenzyme A hydrolysis by human arylamine N-acetyltransferase Type 1/murine arylamine N-acetyltransferase Type 2, open up a range of future avenues of exploration, both for elucidating the developmental role of these enzymes and for improving chemotherapeutic approaches to pathological conditions including estrogen receptor-positive breast cancer.

Structural and biochemical characterization of an active arylamine N-acetyltransferase possessing a non-canonical Cys-His-Glu catalytic triad

Arylamine N-acetyltransferases (NATs), a class of xenobiotic-metabolizing enzymes, catalyze the acetylation of aromatic amine compounds through a strictly conserved Cys-His-Asp catalytic triad. Each residue is essential for catalysis in both prokaryotic and eukaryotic NATs. Indeed, in (HUMAN)NAT2 variants, mutation of the Asp residue to Asn, Gln, or Glu dramatically impairs enzyme activity. However, a putative atypical NAT harboring a catalytic triad Glu residue was recently identified in Bacillus cereus ((BACCR)NAT3) but has not yet been characterized. We report here the crystal structure and functional characterization of this atypical NAT. The overall fold of (BACCR)NAT3 and the geometry of its Cys-His-Glu catalytic triad are similar to those present in functional NATs. Importantly, the enzyme was found to be active and to acetylate prototypic arylamine NAT substrates. In contrast to (HUMAN) NAT2, the presence of a Glu or Asp in the triad of (BACCR)NAT3 did not significantly affect enzyme structure or function. Computational analysis identified differences in residue packing and steric constraints in the active site of (BACCR)NAT3 that allow it to accommodate a Cys-His-Glu triad. These findings overturn the conventional view, demonstrating that the catalytic triad of this family of acetyltransferases is plastic. Moreover, they highlight the need for further study of the evolutionary history of NATs and the functional significance of the predominant Cys-His-Asp triad in both prokaryotic and eukaryotic forms.

Design, synthesis and structure-activity relationships of 3,5-diaryl-1H-pyrazoles as inhibitors of arylamine N-acetyltransferase

The synthesis and inhibitory potencies of a novel series of 3,5-diaryl-1H-pyrazoles as specific inhibitors of prokaryotic arylamine N-acetyltransferase enzymes is described. The series is based on hit compound 1 3,5-diaryl-1H-pyrazole identified from a high-throughout screen that has been carried out previously and found to inhibit the growth of Mycobacterium tuberculosis.

Piperidinols that show anti-tubercular activity as inhibitors of arylamine N-acetyltransferase: an essential enzyme for mycobacterial survival inside macrophages

Latent M. tuberculosis infection presents one of the major obstacles in the global eradication of tuberculosis (TB). Cholesterol plays a critical role in the persistence of M. tuberculosis within the macrophage during latent infection. Catabolism of cholesterol contributes to the pool of propionyl-CoA, a precursor that is incorporated into cell-wall lipids. Arylamine N-acetyltransferase (NAT) is encoded within a gene cluster that is involved in the cholesterol sterol-ring degradation and is essential for intracellular survival. The ability of the NAT from M. tuberculosis (TBNAT) to utilise propionyl-CoA links it to the cholesterol-catabolism pathway. Deleting the nat gene or inhibiting the NAT enzyme prevents intracellular survival and results in depletion of cell-wall lipids. TBNAT has been investigated as a potential target for TB therapies. From a previous high-throughput screen, 3-benzoyl-4-phenyl-1-methylpiperidinol was identified as a selective inhibitor of prokaryotic NAT that exhibited antimycobacterial activity. The compound resulted in time-dependent irreversible inhibition of the NAT activity when tested against NAT from M. marinum (MMNAT). To further evaluate the antimycobacterial activity and the NAT inhibition of this compound, four piperidinol analogues were tested. All five compounds exert potent antimycobacterial activity against M. tuberculosis with MIC values of 2.3-16.9 µM. Treatment of the MMNAT enzyme with this set of inhibitors resulted in an irreversible time-dependent inhibition of NAT activity. Here we investigate the mechanism of NAT inhibition by studying protein-ligand interactions using mass spectrometry in combination with enzyme analysis and structure determination. We propose a covalent mechanism of NAT inhibition that involves the formation of a reactive intermediate and selective cysteine residue modification. These piperidinols present a unique class of antimycobacterial compounds that have a novel mode of action different from known anti-tubercular drugs.

The Bacillus anthracis arylamine N-acetyltransferase ((BACAN)NAT1) that inactivates sulfamethoxazole, reveals unusual structural features compared with the other NAT isoenzymes

Arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes that biotransform arylamine drugs. The Bacillus anthracis (BACAN)NAT1 enzyme affords increased resistance to the antibiotic sulfamethoxazole through its acetylation. We report the structure of (BACAN)NAT1. Unexpectedly, endogenous coenzymeA was present in the active site. The structure suggests that, contrary to the other prokaryotic NATs, (BACAN)NAT1 possesses a 14-residue insertion equivalent to the "mammalian insertion", a structural feature considered unique to mammalian NATs. Moreover, (BACAN)NAT1 structure shows marked differences in the mode of binding and location of coenzymeA when compared to the other NATs. This suggests that the mechanisms of cofactor recognition by NATs is more diverse than expected and supports the cofactor-binding site as being a unique subsite to target in drug design against bacterial NATs.

Sequence and structural characterization of tbnat gene in isoniazid-resistant Mycobacterium tuberculosis: identification of new mutations

The present study was carried out to investigate the presence of polymorphism in the N-acetyltransferase gene of 41 clinical isolates of Mycobacterium tuberculosis, that were resistant to isoniazid (INH) with no mutations in the hot spots of the genes previously described to be involved in INH resistance (katG, inhA and ahpC). We observed single nucleotide polymorphisms (SNPs) in ten of these, including the G619A SNP in five isolates and an additional four so far un-described mutations in another five isolates. Among the latter SNPs, two were synonymous (C276T, n=1 and C375G, n=3), while two more non-synonymous SNPs were composed of C373A (Leu→Met) and T503G (Met→Arg) were observed in respectively one and two isolates. Molecular modeling and structural analysis based in a constructed full length 3D models of wild type TBNAT (TBNAT_H37Rv) and the isoforms (TBNAT_L125M and TBNAT_M168R) were also performed. The refined models show that, just as observed in human NATs, the carboxyl terminus extends deep within the folded enzyme, into close proximity to the buried catalytic triad. Analysis of tbnat that present non-synonymous mutations indicates that both substitutions are plausible to affect enzyme specificity or acetyl-CoA binding capacity. The results contribute to a better understanding of structure-function relationships of NATs. However, further investigation including INH-sensitive strains as a control group is needed to get better understanding of the possible role of these new mutations on tuberculosis control.

Computational study of the three-dimensional structure of N-acetyltransferase 2-acetyl coenzyme a complex

N-Acetyltransferase 2 (NAT2) is one of the most important polymorphic drug-metabolizing enzymes and plays a significant role in individual differences of drug efficacies and/or side effects. Coenzyme A (CoA) is a cofactor in the experimentally determined crystal structure of NAT2, although the acetyl source of acetylation reactions catalyzed by NAT is not CoA, but rather acetyl CoA. In this study, the three-dimensional structure of NAT2, including acetyl CoA, was calculated using molecular dynamics simulation. By substituting acetyl CoA for CoA the amino acid residue Gly286, which is known to transform into a glutamate residue by NAT2*7A and NAT2*7B, comes close to the cofactor binding site. In addition, the binding pocket around the sulfur atom of acetyl CoA expanded in the NAT2-acetyl CoA complex.

Analysis of β-amino alcohols as inhibitors of the potential anti-tubercular target N-acetyltransferase

The synthesis and inhibitory potencies of a novel series of β-amino alcohols, based on the hit-compound 3-[3'-(4''-cyclopent-2'''-en-1'''-ylphenoxy)-2'-hydroxypropyl]-5,5 dimethylimidazolidine-2,4-dione as specific inhibitors of mycobacterial N-acetyltransferase (NAT) enzymes are reported. Effects of synthesised compounds on growth of Mycobacterium tuberculosis have been determined.

Probing the architecture of the Mycobacterium marinum arylamine N-acetyltransferase active site

Treatment of latent tuberculosis infection remains an important goal of global TB eradication. To this end, targets that are essential for intracellular survival of Mycobacterium tuberculosis are particularly attractive. Arylamine N-acetyltransferase (NAT) represents such a target as it is, along with the enzymes encoded by the associated gene cluster, essential for mycobacterial survival inside macrophages and involved in cholesterol degradation. Cholesterol is likely to be the fuel for M. tuberculosis inside macrophages. Deleting the nat gene and inhibiting the NAT enzyme prevents survival of the microorganism in macrophages and induces cell wall alterations, rendering the mycobacterium sensitive to antibiotics to which it is normally resistant. To date, NAT from M. marinum (MMNAT) is considered the best available model for NAT from M. tuberculosis (TBNAT). The enzyme catalyses the acetylation and propionylation of arylamines and hydrazines. Hydralazine is a good acetyl and propionyl acceptor for both MMNAT and TBNAT. The MMNAT structure has been solved to 2.1 Å resolution following crystallisation in the presence of hydralazine and is compared to available NAT structures. From the mode of ligand binding, features of the binding pocket can be identified, which point to a novel mechanism for the acetylation reaction that results in a 3-methyltriazolo[3,4-a]phthalazine ring compound as product.

Comparison of the Arylamine N-acetyltransferase from Mycobacterium marinum and Mycobacterium tuberculosis

Arylamine N-acetyltansferase (NAT) from Mycobacterium tuberculosis (TBNAT) is a potential drug target for anti-tubercular therapy. Recombinant TBNAT is much less soluble and is produced in lower yields than the closely related NAT from Mycobacterium marinum (MMNAT). In order to explore MMNAT as a model for TBNAT in drug discovery, we compare the two mycobacterial NAT enzymes. Two site-directed mutants of MMNAT have been prepared and characterised: MMNAT71, Tyr --> Phe and MMNAT209, Met --> Thr, in which residues within 6 A of the active-site cysteine have been replaced with the corresponding residue from TBNAT. Two chimeric proteins have also been produced in which the third domain of MMNAT has been replaced by the third domain of TBNAT and vice versa. The activity profile of the chimeric proteins suggests a role for the third domain in the evolutionary divergence of NAT between these closely related mycobacterial species.

Identification of arylamine N-acetyltransferase inhibitors as an approach towards novel anti-tuberculars

New anti-tubercular drugs and drug targets are urgently needed to reduce the time for treatment and also to identify agents that will be effective against Mycobacterium tuberculosis persisting intracellularly. Mycobacteria have a unique cell wall. Deletion of the gene for arylamine N-acetyltransferase (NAT) decreases mycobacterial cell wall lipids, particularly the distinctive mycolates, and also increases antibiotic susceptibility and killing within macrophage of Mycobacterium bovis BCG. The nat gene and its associated gene cluster are almost identical in sequence in M. bovis BCG and M. tuberculosis. The gene cluster is essential for intracellular survival of mycobacteria. We have therefore used pure NAT protein for high-throughput screening to identify several classes of small molecules that inhibit NAT activity. Here, we characterize one class of such molecules-triazoles-in relation to its effects on the target enzyme and on both M. bovis BCG and M. tuberculosis. The most potent triazole mimics the effects of deletion of the nat gene on growth, lipid disruption and intracellular survival. We also present the structure-activity relationship between NAT inhibition and effects on mycobacterial growth, and use ligand-protein analysis to give further insight into the structure-activity relationships. We conclude that screening a chemical library with NAT protein yields compounds that have high potential as anti-tubercular agents and that the inhibitors will allow further exploration of the biochemical pathway in which NAT is involved.

Update on the pharmacogenetics of NATs: structural considerations

The arylamine N-acetyltransferase (NAT) genes encode enzymes that catalyze the N-acetylation of aromatic amines and hydrazines and the O-acetylation of heterocyclic amines. These genes, which play a key role in cellular homeostasis as well as in gene-environment interactions, are subject to marked pharmacogenetic variation, and different combinations of SNPs in the human NAT genes lead to different acetylation phenotypes. Our understanding of the consequences of pharmacogenetic variability in NATs has recently been enhanced by structural studies showing that effects on protein folding, aggregation and turnover, as well as direct changes in active site topology, are involved. These developments pave the way for a better understanding of the role played by NATs in maintaining cellular homeostasis. In addition, the NATs represent a model for studying fundamental processes associated with protein folding and pharmacogenomic effects mediated by inheritance in human populations across a polymorphic region of the genome.

Temperature stability of proteins essential for the intracellular survival of Mycobacterium tuberculosis

In Mycobacterium tuberculosis, the genes hsaD (2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid hydrolase) and nat (arylamine N-acetyltransferase) are essential for survival inside of host macrophages. These genes act as an operon and have been suggested to be involved in cholesterol metabolism. However, the role of NAT in this catabolic pathway has not been determined. In an effort to better understand the function of these proteins, we have expressed, purified and characterized TBNAT (NAT from M. tuberculosis) and HsaD (2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid hydrolase) from M. tuberculosis. Both proteins demonstrated remarkable heat stability with TBNAT and HsaD retaining >95% of their activity after incubation at 60 degrees C for 30 min. The first and second domains of TBNAT were demonstrated to be very important to the heat stability of the protein, as the transfer of these domains caused a dramatic reduction in the heat stability. The specific activity of TBNAT was tested against a broad range of acyl-CoA cofactors using hydralazine as a substrate. TBNAT was found to be able to utilize not just acetyl-CoA, but also n-propionyl-CoA and acetoacetyl-CoA, although at a lower rate. As propionyl-CoA is a product of cholesterol catabolism, we propose that NAT could have a role in the utilization of this important cofactor.

Kinetic and chemical mechanism of arylamine N-acetyltransferase from Mycobacterium tuberculosis

Arylamine N-acetyltransferases (NATs) are cytosolic enzymes that catalyze the transfer of the acetyl group from acetyl coenzyme A (AcCoA) to the free amino group of arylamines and hydrazines. Previous studies have reported that overexpression of NAT from Mycobacterium smegmatis and Mycobacterium tuberculosis may be responsible for increased resistance to the front-line antitubercular drug, isoniazid, by acetylating and hence inactivating the prodrug. We report the kinetic characterization of M. tuberculosis NAT which reveals that substituted anilines are excellent substrates but that isoniazid is a very poor substrate for this enzyme. We propose that the expression of NAT from M. tuberculosis (TBNAT) is unlikely to be a significant cause of isoniazid resistance. The kinetic parameters for a variety of TBNAT substrates were examined, including 3-amino-4-hydroxybenzoic acid and AcCoA, revealing K m values of 0.32 +/- 0.03 and 0.14 +/- 0.02 mM, respectively. Steady-state kinetic analysis of TBNAT reveals that the enzyme catalyzes the reaction via a bi-bi ping-pong kinetic mechanism. The pH dependence of the kinetic parameters reveals that one enzyme group must be deprotonated for optimal catalytic activity and that two amino acid residues at the active site of the free enzyme are involved in binding and/or catalysis. Solvent kinetic isotope effects suggest that proton transfer steps are not rate-limiting in the overall reaction for substituted aniline substrates but become rate-limiting when poor hydrazide substrates are used.

Arylamine N-acetyltransferases in mycobacteria

Polymorphic Human arylamine N-acetyltransferase (NAT2) inactivates the anti-tubercular drug isoniazid by acetyltransfer from acetylCoA. There are active NAT proteins encoded by homologous genes in mycobacteria including M. tuberculosis, M. bovis BCG, M. smegmatis and M. marinum. Crystallographic structures of NATs from M. smegmatis and M. marinum, as native enzymes and with isoniazid bound share a similar fold with the first NAT structure, Salmonella typhimurium NAT. There are three approximately equal domains and an active site essential catalytic triad of cysteine, histidine and aspartate in the first two domains. An acetyl group from acetylCoA is transferred to cysteine and then to the acetyl acceptor e.g. isoniazid. M. marinum NAT binds CoA in a more open mode compared with CoA binding to human NAT2. The structure of mycobacterial NAT may promote its role in synthesis of cell wall lipids, identified through gene deletion studies. NAT protein is essential for survival of M. bovis BCG in macrophage as are the proteins encoded by other genes in the same gene cluster (hsaA-D). HsaA-D degrade cholesterol, essential for mycobacterial survival inside macrophage. Nat expression remains to be fully understood but is co-ordinated with hsaA-D and other stress response genes in mycobacteria. Amide synthase genes in the streptomyces are also nat homologues. The amide synthases are predicted to catalyse intramolecular amide bond formation and creation of cyclic molecules, e.g. geldanamycin. Lack of conservation of the CoA binding cleft residues of M. marinum NAT suggests the amide synthase reaction mechanism does not involve a soluble CoA intermediate during amide formation and ring closure.

Current trends in N-acetyltransferase research arising from the 2007 International NAT Workshop

Arylamine N-acetyltransferase (NAT) research has been influenced in recent years by the rapid progress in genomics, proteomics, structural genomics and other cutting-edge disciplines. To keep up with these advancements, the NAT scientific community has fostered collaboration and exchange of know-how between its members. As a specialized event bringing together experts from many different laboratories, the triennial International NAT Workshop has been instrumental in maintaining this culture over the past ten years. The 2007 Workshop took place in Alexandroupolis, Greece, and covered ongoing research on the structure and enzymatic function of human NATs, the prokaryotic and eukaryotic models for NAT, the mechanisms of NAT gene regulation and expression, the frequencies and effects of polymorphisms in the human NAT genes, and the involvement of NATs in multifactorial diseases, including cancer, allergic conditions, endometriosis and endemic nephropathies. Gene nomenclature issues were also addressed and the participants discussed current trends in the field.

Mouse N-acetyltransferase type 2, the homologue of human N-acetyltransferase type 1

There is increasing evidence that human arylamine N-acetyltransferase type 1 (NAT1, EC 2.3.1.5), although first identified as a homologue of a drug-metabolising enzyme, appears to be a marker in human oestrogen receptor positive breast cancer. Mouse Nat2 is the mouse equivalent of human NAT1. The development of mouse models of breast cancer is important, and it is essential to explore the biological role of mouse Nat2. We have therefore produced mouse Nat2 as a recombinant protein and have investigated its substrate specificity profile in comparison with human NAT1. In addition, we have tested the effects of inhibitors on mouse Nat2, including compounds which are endogenous and exogenous steroids. We show that tamoxifen, genistein and diethylstilbestrol inhibit mouse Nat2. The steroid analogue, bisphenol A, also inhibits mouse Nat2 enzymic activity and is shown by NMR spectroscopy, through shifts in proton peaks, to bind close to the active site. A three-dimensional structure for human NAT1 has recently been released, and we have used this crystal structure to generate a model of the mouse Nat2 structure. We propose that a conformational change in the structure is required in order for ligands to bind to the active site of the protein.