Molecular mechanisms of polymorphism in acetylating enzymes for arylamines and N-hydroxyarylamines in hamster liver

Probing the catalytic potential of the hamster arylamine N-acetyltransferase 2 catalytic triad by site-directed mutagenesis of the proximal conserved residue, Tyr190

Arylamine N-acetyltransferases (NATs) play an important role in both the detoxification of arylamine and hydrazine drugs and the activation of arylamine carcinogens. Because the catalytic triad, Cys-His-Asp, of mammalian NATs has been shown to be essential for maintaining protein stability, rendering it impossible to assess alterations of the triad on catalysis, we explored the impact of the highly conserved proximal residue, Tyr190, which forms a direct hydrogen bond interaction with one of the triad residues, Asp122, as well as a potential pi-pi stacking interaction with the active site His107. The replacement of hamster NAT2 Tyr190 by either Phe, Ile or Ala was well tolerated and did not result in significant alterations in the overall fold of the protein. Nevertheless, stopped-flow and steady-state kinetic analysis revealed that Tyr190 was critical for maximizing the acetylation rate of NAT2 and the transacetylation rate of p-aminobenzoic acid when compared with the wild-type. Tyr190 was also shown to play an important role in determining the pK(a) of the active site Cys during acetylation, as well as the pH versus the rate profile for transacetylation. We hypothesized that the pH dependence was associated with global changes in the active site structure, which was revealed by the superposition of [(1)H, (15)N] heteronuclear single quantum coherence spectra for the wild-type and Y190A. These results suggest that NAT2 catalytic efficiency is partially governed by the ability of Tyr190 to mediate the collective impact of multiple side chains on the electrostatic potential and local conformation of the active site.

Over-expression, purification, and characterization of recombinant human arylamine N-acetyltransferase 1

Human arylamine N-acetyltransferase 1 (NAT1) has been overexpressed in E. coli as a mutant dihydrofolic acid reductase (DHFR) fusion protein with a thrombin sensitive linker. An initial DEAE anion-exchange chromatography resulted in partial purification of the fusion protein. The fusion protein was cleaved with thrombin, and human rNAT1 was purified with a second DEAE column. A total of 8 mg of human rNAT1 from 2 1 of cell culture was purified to homogeneity with this methodology. Arylamine substrate specificities were determined for human rNATI and hamster rNAT2. With both NATs, the second order rate constants (k(cat)/ Kmb) for p-aminobenzoic acid (PABA) and 2-aminofluorene (2-AF) were several thousand-fold higher than those for procainamide (PA), consistent with the expected substrate specificities of the enzymes. However, p-aminosalicylic acid (PAS), previously reported to be a human NAT1 and hamster NAT2 selective substrate, exhibits 20-fold higher specificity for hamster rNAT2 (k(cat)/Kmb 3410 microM(-1) s(-1)) than for human rNAT1 (k(cat)/Kmb 169.4 microM(-1) s(-1)). p-aminobenzoyl-glutamic acid (pABglu) was acetylated 10-fold more efficiently by human rNAT1 than by hamster rNAT2. Inhibition studies of human rNAT1 and hamster rNAT2 revealed that folic acid and methotrexate (MTX) are competitive inhibitors of both the unacetylated and acetylated forms of the enzymes, with K(I) values in 50 - 300 micro range. Dihydrofolic acid (DHF) was a much poorer inhibitor of human rNAT1 than of hamster rNAT2. The combined results demonstrate that human rNAT1 and hamster rNAT2 have similar but distinct kinetic properties with certain substrates, and suggest that folic acid, at least in the non-polyglutamate form, may not have an effect on human NAT1 activity in vivo.

Catalytic mechanism of hamster arylamine N-acetyltransferase 2

Arylamine N-acetyltransferases (NATs) catalyze an acetyl group transfer from AcCoA to primary arylamines, hydrazines, and hydrazides and play a very important role in the metabolism and bioactivation of drugs, carcinogens, and other xenobiotics. The reaction follows a ping-pong bi-bi mechanism. Structure analysis of bacterial NATs revealed a Cys-His-Asp catalytic triad that is strictly conserved in all known NATs. Previously, we have demonstrated by kinetic and isotope effect studies that acetylation of the hamster NAT2 is dependent on a thiolate-imidazolium ion pair (Cys-S(-)-His-ImH(+)) and not a general acid-base catalysis. In addition, we established that, after formation of the acetylated enzyme intermediate, the active-site imidazole, His-107, is likely deprotonated at physiological pH. In this paper, we report steady-state kinetic studies of NAT2 with two acetyl donors, acetyl coenzyme A (AcCoA) and p-nitrophenyl acetate (PNPA), and four arylamine substrates. The pH dependence of k(cat)/K(AcCoA) exhibited two inflection points at 5.32 +/- 0.13 and 8.48 +/- 0.24, respectively. The pK(a) at 5.32 is virtually identical with the previously reported pK(a) of 5.2 for enzyme acetylation, reaffirming that the first half of the reaction is catalyzed by a thiolate-imidazolium ion pair in the active site. The inflection point at 8.48 indicates that a pH-sensitive group on NAT2 is involved in AcCoA binding. A Brønsted plot constructed by the correlation of log k(4) and log k(H)2(O) with the pK(a) for each arylamine substrate and water displays a linear free-energy relationship in the pK(a) range from -1.7 (H(2)O) to 4.67 (PABA), with a slope of beta(nuc) = 0.80 +/- 0.1. However, a further increase of the pK(a) from 4.67 (PABA) to 5.32 (anisidine) resulted in a 2.5-fold decrease in the k(4) value. Analysis of the pH-k(cat)/K(PABA) profile revealed a pK(a) of 5.52 +/- 0.14 and a solvent kinetic isotope effect (SKIE) of 2.01 +/- 0.04 on k(cat)/K(PABA). Normal solvent isotope effects of 4.8 +/- 0.1, 3.1 +/- 0.1, and 3.2 +/- 0.1 on the k(cat)/K(b) for anisidine, pABglu, and PNA, respectively, were also determined. These observations are consistent with a deacetylation mechanism dominated by nucleophilic attack of the thiol ester for arylamines with pK(a) values <or=5.5 to deprotonation of a tetrahedral intermediate for arylamines with pK(a) values >or=5.5. The general base is likely His-107 because the His-107 to Gln and Asn mutants were found to be devoid of catalytic activity. In contrast, an increase in pH-dependent hydrolysis of the acetylated enzyme was not observed over a pH range of 5.2-7.5. On the basis of these observations, a catalytic mechanism for the acetylation of arylamines by NAT2 is proposed.

Irreversible inactivation of arylamine N-acetyltransferases in the presence of N-hydroxy-4-acetylaminobiphenyl: a comparison of human and hamster enzymes

Arylamine N-acetyltransferases (NATs) catalyze the N-acetylation of arylamines, the O-acetylation of N-arylhydroxylamines, and the conversion of N-(aryl)acetohydroxamic acids to N-acetoxyarylamines. NATs also undergo irreversible inactivation in the presence of N-(aryl)acetohydroxamic acids. We previously established that inactivation of hamster NAT1 by N-hydroxy-2-acetylaminofluorene is the result of sulfinamide adduct formation with Cys68. The purpose of this research was to determine the kinetics of inactivation of hamster NAT1, hamster NAT2, and human NAT1 by N-hydroxy-4-acetylaminobiphenyl (N-OH-4-AABP), to identify the amino acids that are modified upon NAT-catalyzed bioactivation of N-OH-4-AABP, to characterize the adducts and to identify factors that influence the propensity of NATs to undergo inactivation by N-arylhydroxamic acids. Mass spectrometric analysis of the NATs, after incubation with N-OH-4-AABP, revealed that the principal adduct of each protein was a (4-biphenyl)sulfinamide. Proteolysis of the adducted NATs caused hydrolysis of the sulfinamides to sulfinic acids. Tandem mass spectrometric analysis of the modified peptides revealed that each NAT isozyme contained a sulfinic acid on the Cys68 side chain. Minor adducts were identified as 4-aminobiphenyl conjugates of tyrosines. Hamster NAT1 was more rapidly inactivated by N-OH-4-AABP than either hamster NAT2 or human NAT1, and it was demonstrated that 4-nitrosoobiphenyl, which forms the sulfinamide adducts, accumulates during incubation of N-OH-4-AABP with hamster NAT2 and human NAT1 but not during incubations with hamster NAT1. Steady state kinetic analysis of the hydrolysis of acetylated NATs revealed that the half-lives of acetylated hamster NAT2 and human NAT1 are 7-8-fold greater than that of acetylated hamster NAT1. These results support the proposal that the mechanism of inactivation of NATs by N-OH-4-AABP involves initial deacetylation to produce N-OH-4-aminobiphenyl, which after oxidative conversion to 4-nitrosobiphenyl reacts with Cys68 to form a sulfinamide. The relatively short half-life of the acetylated form of hamster NAT1 contributes to its greater susceptibility to inactivation by N-OH-4-AABP.

Arylamine N-acetyltransferases: covalent modification and inactivation of hamster NAT1 by bromoacetamido derivatives of aniline and 2-aminofluorene

Kinetic analysis of the inactiviation of hamster NAT1 by 2-(bromoacetylamino)fluorene (Br-AAF) and bromoacetanilide revealed that Br-AAF is an active site directed affinity label whereas bromoacetanilide acts as a bimolecular alkylating agent. ESI MS analysis of NAT1 treated with Br-AAF showed that a single molecule of 2-acetylaminofluorene had been incorporated. Proteolysis with pepsin followed by sequencing of adducted peptides by ESI MS/MS identified the modified residue as the catalytically essential Cys-68. ESI Q-TOF MS analysis of NAT1 that had been treated with bromoacetanilide resulted in identification of a monoadducted protein as the primary product and a diadducted protein as a minor product. Pepsin digestion of bromoacetanilide-inactivated NAT1 and sequencing by ESI MS/MS identified Cys-68 as the primary site of adduct formation. Additional proteolysis of the bromoacetanilide-treated NAT1 led to the identification of a second modified peptide which was adducted at Cys-44. The data reveal substantial differences in the interactions of small hydrophobic alkylating reagents with hamster NAT1.