The purification of rat liver arylhydroxamic acid N,O-acyltransferase

Genotoxicity of potential metabolites of nitroscanate--an antischistosomal drug

The potential metabolites of nitroscanate(4-isothiocyanato-4'-nitrodiphenyl ether) such as 4-amino-4'-nitrodiphenyl ether (ANDE), 4-acetamido-4'-nitrodiphenyl ether (AcNDE), 4-acetamido-4'-nitrosodiphenyl ether (4-N = 0), 4-acetamido-4'-hydroxylaminodiphenyl ether (4-NHOH), 4-acetamido-4'-acetohydroxamicdiphenyl ether [4-N(OH)Ac], 4-acetamido-4'-formohydroxamicdiphenyl ether [4-N(OH)CHO] and 4-acetamido-4'-acetylaceto-hydroxamicdiphenyl ether [4-N(OAc)Ac] were synthesized and investigated in the standard Salmonella mutagenicity test using TA98, TA98NR, TA98/1,8-DNP6, TA100 and TA100NR as indicator strains, in the presence and absence of hepatic S9. The relative order of activity among nitro and its reduction products, 4-N = 0 and 4-NHOH in TA98 and TA100 was 4-N = 0 > 4-NHOH > AcNDE. In nitroreductase deficient strain TA98NR, AcNDE was inactive, but expressed a slight activity in TA100NR while 4-N = 0 and 4-NHOH showed a large increase in specific activity in both the strains. In O-acetyltransferase deficient strain TA98/1,8-DNP6, AcNDE was inactive, while 4-N = O and 4-NHOH showed a sharp fall in activity. The hydroxylamine derived products with an activity order 4-N(OAc)Ac > 4-N(OH)CHO > 4-N(OH)Ac in both TA98 and TA100, showed 3-6 times increase in the specific activity for the latter two compounds in the presence of S9 mix, which was inhibited in the presence of paraoxan, indicating N-deacylation as an important metabolic activation pathway. Except the 4-NO in TA100, the observed mutagenicity of nitroscante (NSC) was higher than those of potential metabolites and the nor-isothiocyanato derivative 4'-nitrodiphenyl ether, thereby showing that -NCS function has a potentiating effect on the mutagenicity of this drug.

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

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

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

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

An isozyme-selective affinity label for rat hepatic acetyltransferases

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

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

Enzymatic N,O-acyltransfer of carcinogenic N-arylhydroxamic acids such as N-hydroxy-2-acetylaminofluorene (N-OH-AAF) results in the production of reactive electrophiles that can bond covalently with nucleophiles and also can cause inactivation of acyltransferase activity in a mechanism-based manner. Incubation of partially purified rat hepatic N-acetyltransferases (NAT) with N-OH-AAF resulted in extensive inactivation of N-OH-AAF/4-aminoazobenzene (AAB) N,N-acetyltransferase and acetyl coenzyme A (AcCoA)/procainamide (PA) N-acetyltransferase activities, whereas AcCoA/p-aminobenzoic acid (PABA) N-acetyltransferase activity was inhibited only slightly. Affinity chromatography with Sepharose 6B 2-aminofluorene (2-AF) resulted in the separation of two NAT activities. NAT I primarily catalyzed the AcCoA-dependent acetylation of PABA; NAT II catalyzed, N,N-acetyltransfer (N-OH-AAF/AAB), AcCoA/PA N-acetyltransfer and N-OH-AAF N,O-acyltransfer (AHAT) activities. Most of the AcCoA/2-AF N-acetyltransferase activity eluted in the NAT II fraction. Results of inactivation experiments with N-OH-AAF and the NAT II fractions suggested that one NAT isozyme was responsible for catalyzing the N-OH-AAF/AAB, AcCoA/PA and N,O-acyltransfer reactions and that inactivation of NAT II correlated with the extent of covalent binding to protein. Further purification of the NAT II fractions by chromatofocusing resulted in a 1300-fold purification of the N-OH-AAF/AAB activity and the coelution of N-OH-AAF/AAB, AcCoA/PA and N,O-acyltransferase activities. These studies indicate that N,O-acyltransfer, arylhydroxamic acid-dependent N-acetylation of arylamines (N,N-acetyltransfer), and AcCoA-dependent N-acetylation of PA may be catalyzed by a common enzyme in rat liver, whereas a second enzyme is responsible for the AcCoA-dependent N-acetylation of PABA.

Acetylator genotype and arylamine-induced carcinogenesis

A diverse array of arylamine chemicals derived from industry, diet, cigarette smoke and other environmental sources are carcinogenic. These chemicals require metabolic activation by host enzymes to chemically reactive electrophiles to initiate the carcinogenic response. Genetic regulation of activation and/or deactivation pathways are thought to account in large measure for corresponding differences in tumor incidence from these chemicals between tissues, between species, or between individuals within a species. Various acetyltransfer reactions are involved in arylamine metabolism and much has been learned regarding their enzymology, genetic regulation, and toxicological significance. The small amount of human data are supported by systematic investigations carried out in animal models characterized with respect to the acetylation polymorphism. Enzymological and genetic investigations suggest that common enzymes encoded by the acetyltransferase gene carry out a diverse set of acetyltransferase reactions. Thus, the acetylation polymorphism can influence both activation and deactivation pathways in arylamine metabolism. Of particular significance recently have been reports documenting the O-acetylation of N-hydroxyarylamine carcinogens and its genetic coregulation with the well-characterized arylamine N-acetylation polymorphism. The toxicological consequences of this polymorphic pathway have yet to be fully explored. Epidemiological investigations show associations between acetylator phenotype and the incidence and/or severity of tumors in the urinary bladder, colon and larynx. Associations between acetylator phenotype and breast cancer are more equivocal and require further study. The divergent influence of acetylator phenotype on the incidence of tumors in different organ sites suggests an important role for extrahepatic acetyltransferases, and further characterization of them in human and animal tissues is needed. The advent of newer methodologies to monitor chemical exposures and to measure acetylator phenotype (rapid, intermediate and slow) using less invasive and more standardized protocols should soon result in a much more definitive understanding regarding the role of acetylator status in arylamine-induced carcinogenesis.

Irreversible inhibition of rat hepatic transacetylase activity by N-arylhydroxamic acids

Both N-hydroxy-2-acetamidofluorene (N-OH-AAF) and the heterocyclic analogue, 2-(N-hydroxyacetamido)carbazole (N-OH-AAC), were shown to be mechanism-based irreversible inhibitors (suicide inhibitors) of partially purified rat hepatic N-acetyltransferase (NAT) activity. Although N-OH-AAC exhibited an apparent first-order inactivation rate constant (ki) that was 7-fold lower than that of N-OH-AAF, their relative ki/KD values indicate that N-OH-AAC was the more potent and efficient inactivator of transacetylase activity. Inactivation of NAT activity by these N-arylhydroxamic acids appeared to involve contributions by electrophiles that react with the enzyme subsequent to release from the active site and by electrophiles that remain complexed with the active site. The irreversible nature of the enzyme inactivation was demonstrated by the failure to recover transacetylase activity upon either extensive dialysis or gel filtration of preparations that had been subjected to incubation with N-OH-AAF or N-OH-AAC. The use of the nucleophile N-acetylmethionine to trap the electrophilic reactants formed in the transacetylase-catalyzed bioactivation process resulted in a lower rate and extent of formation of methylthio adducts with N-OH-AAC than with N-OH-AAF. The results of this study indicate that N-OH-AAF and N-OH-AAC have potential for use as tools in the investigation of rat hepatic transacetylases.

Irreversible inhibition of the cytosolic metabolism of N-hydroxy-2-acetylaminofluorene by its glycolyl analog

The glycolyl hydroxamic acid derivative of 2-aminofluorene was found to be a potent inhibitor of its own metabolism and the metabolism of N-hydroxy-2-acetylaminofluorene by rat liver cytosol. The inhibition was irreversible, as well as time and concentration dependent, which indicates a suicide-inhibition type of metabolism. There was a direct correlation between the inhibition of N-hydroxy-2-acetylaminofluorene disappearance and 2-acetylaminofluorene formation. In contrast, both the glycolyl and acetyl hydroxamic acid derivatives were metabolized to a similar extent by enzymes in the microsomal fraction.

Metabolic activation of mutagenic heterocyclic aromatic amines from protein pyrolysates

Mutagenic heterocyclic amines are metabolized to mutagens which act directly on Salmonella typhimurium by P-448 forms of cytochrome P-450. These direct mutagens are N-hydroxylated heterocyclic amines, such as N-hydroxy-Trp-P-1, N-hydroxy-Trp-P-2, N-hydroxy-Glu-P-1, N-hydroxy-Glu-P-2, N-hydroxy-IQ, N-hydroxy-2-amino-alpha-carboline (N-hydroxy-A alpha C), and N-hydroxy-2-amino-3-methyl-alpha-carboline (N-hydroxy-MeA alpha C). The treatment of rats with polychlorinated biphenyl stimulated N-hydroxylation of heterocyclic amines about 10- to 260-fold depending on the substrates used. The N-hydroxylation activities of purified cytochrome P-448-H and P-448-L were markedly different. P-448-H, which had very low activity for benzo[a] pyrene metabolic activation, showed high N-hydroxylation activity. The activity ratio P-448-H:P-448-L was markedly different depending on the amines used. This ratio was 45, 22, 3, and 0.02, respectively, for Glu-P-1, IQ, Trp-P-2, and benzo[a] pyrene. On the other hand, N-acetylation of the heterocyclic amines was very low. Although marked species differences in the N-acetylation were observed, the activities of the heterocyclic amines were about 1/100 of that of 2-aminofluorene. N-Hydroxy-Trp-P-2 could react directly to DNA, but N-hydroxy-Glu-P-1 could not. Therefore we need to consider the presence of a further activating system in mammalian and bacterial cells. We observed that N-hydroxy-Trp-P-2 was activated by prolyl-t-RNA synthetase, but N-hydroxy-Glu-P-1 was not activated by the same system. In the bacterial cells, both N-hydroxy-Trp-P-2 and N-hydroxy-Glu-P-1 were not activated by prolyl-t-RNA synthetase. However, both hydroxylamines were activated by the acetyl-CoA-dependent mechanism in mammalian and bacterial cells. These results indicated that the O-acetylation is an important pathway for DNA damage by heterocyclic amines in chemical carcinogenesis.

Formation and persistence of arylamine DNA adducts in vivo

Aromatic amines are urinary bladder carcinogens in man and induce tumors at a number of sites in experimental animals including the liver, mammary gland, intestine, and bladder. In this review, the particular pathways involved in the metabolic activation of aromatic amines are considered as well as the specific DNA adducts formed in target and nontarget tissue. Particular emphasis is placed on the following compounds: 1-naphthylamine, 2-naphthylamine, 4-aminobiphenyl, 4-acetylaminobiphenyl, 4-acetylamino-4'-fluorobiphenyl, 3,2'-dimethyl-4-aminobiphenyl, 2-acetylaminofluorene, benzidine, N-methyl-4-aminoazobenzene, 4-aminoazobenzene, and 2-acetylaminophenanthrene.