Relationship between nucleic acid adduct formation and deacylation of arylhydroxamic acids

Structure-activity relationships in the deacetylation of O-glucosides of N-hydroxy-N-arylacylamides by mammalian liver microsomes

Structure-activity relationships in the deacylation of O-glucosides of N-hydroxy-N-aryl-acylamides were investigated to provide insights into the metabolic activation of carcinogenic/mutagenic O-glycosides of N-hydroxy-N-arylacylamides. In the subcellular fractions obtained from porcine liver, the deacetylation activity toward O-glucoside of N-hydroxyacetanilide (GAc) was mainly localized in the microsomes. Both the 2-chloro (2ClGAc) and 2-methyl (2MeGAc) derivatives of GAc were not deacetylated by the microsomes. Other compounds having either 3- or 4-substituent (chloro or methyl), however, were deacetylated and showed higher V(max)/K(m) values than that of GAc. 4-Methyl derivative (4MeGAc) was shown to competitively inhibit the deacetylation activity toward GAc, and the K(i) value of 4MeGAc was comparable with its K(m) value obtained in the microsome-catalyzed deacetylation. These apparent K(m) values were shown to correspond to their lipophilicities estimated from retention times on high-performance liquid chromatography (HPLC). As for the effect of acyl groups, the order of V(max)/K(m) values was N-propionyl derivative (GPr)>GAc>N-butyryl derivative (GBu). From the optimized structures and energy levels of the frontier orbitals of these compounds, calculated by the semi-empirical AM1 method, the effects of 2-substituents and acyl groups on the deacetylation activity is thought to be due to a steric factor. From the energy levels of the frontier orbitals of GAc and its 3- or 4-substituted derivatives, the compound having a lower level of LUMO was shown to be deacetylated effectively. There were marked species differences in the microsomal deacetylation activity toward GAc, and the highest activity was found in the rabbit, followed by the porcine, hamster, rat and then bovine liver microsomes.

Microsomal deacetylation of ring-hydroxylated 2-(acetylamino)fluorene isomers: effect of ring position and molecular mechanics considerations

Metabolism of arylamides such as 2-(acetylamino)fluorene to mutagenic products is catalyzed by various liver microsomal and cytosolic enzymes. Deacylation is believed to be a deactivating pathway, and the activity of the microsomal deacetylase toward N-hydroxy-2-AAF is exceedingly greater than toward the parent 2-AAF. Another deactivating pathway is cytochrome P450-catalyzed ring hydroxylation. We have studied the effect of ring hydroxyl substitution on the activity of the liver microsomal deacetylase from Aroclor 1254-treated rats in vitro. The deacetylase activity was generally decreased toward ring-hydroxylated derivatives in the order of 2AAF approximately 1-OH-AAF > 3-OH-AAF > 7-OH-AAF > 5-OH-AAF approximately 9-OH-AAF. The difference in activity between 2-AAF and 5-OH- and 9-OH-AAF was about eightfold. Molecular mechanics calculations reveal that structural and geometrical parameters are more important than the energies associated with the different isomers. We show that the greater the distance of the hydroxyl group on the fluorenyl ring structure from the acetylamino group, the slower the rate of deacetylation. The difference in reactivity between the 1-hydroxy-2-AAF and the other hydroxy-2-AAF isomers is due to the lack of planarity of the 1-hydroxy isomer as compared to the essentially planar configuration of the other isomers. The relative contribution of microsomal ring hydroxylation and deacetylation to detoxification of arylamides remains to be established.

Genotoxic effects of heterocyclic aromatic amines in human derived hepatoma (HepG2) cells

In order to study the mutagenic effects of heterocyclic aromatic amines (HAAs) in cells of human origin, five compounds, namely 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), 2-amino-3, 4-dimethyl-imidazo[4,5-f]quinoline (MeIQ), 2-amino-3, 8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx), the pyridoimidazo derivative 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), were tested in micronucleus (MN) assays with a human derived hepatoma (HepG2) cell line. All HAAs caused significant, dose-dependent effects. The activities of IQ, MeIQ, MeIQx and PhIP were similar (lowest effective concentrations 25-50 microM), whereas Trp-P-1 was effective at a dose of >/=2.1 microM. In addition, the HAAs were tested in MN assays with Chinese hamster ovary (CHO) cells and in Salmonella strain YG1024 using HepG2 cell homogenates as an activation mix. In the CHO experiments, positive results were obtained with Trp-P-1 and PhIP, whereas the other compounds were devoid of activity under all experimental conditions. The discrepancy in the responsivity of the two cell lines is probably due to differences in their acetylation capacity: enzyme measurements with 2-aminofluorene as a substrate revealed that the cytosolic acetyltransferase activity in the HepG2 cells is approximately 40-fold higher than that of the CHO cells. In the bacterial assays all five HAAs gave positive results but the ranking order was completely different from that seen in the HepG2/MN experiments (IQ > MeIQ > Trp-P-1 >/= MeIQx >> PhIP) and the mutagenic potencies of the various compounds varied over several orders of magnitude. The order obtained in bacterial tests with rat liver S9 mix was more or less identical to that seen in the tests with HepG2 cell homogenates but the concentrations of the amines required to give positive results were in general substantially lower (10(-5)-10(-1) microM). Overall, the results of the present study indicate that MN/HepG2 tests might reflect the mutagenic effects of HAAs more adequately than other in vitro mammalian cell systems due to the presence of enzymes involved in the metabolic conversion of the amines.

Carcinogenecity of the N-acyl derivatives of N-hydroxy-trans-4-aminostilbene in CD rats

Carcinogenicities of the N-formyl (N-OH-FAS), N-acetyl (N-OH-AAS) and N-propionyl (N-OH-PAS) derivatives of N-hydroxy-trans-4-aminostilbene (N-OH-AS) were investigated in male and female CD rats. They were injected, i.p. 10 mumol/kg body weight (bwt) twice a week for 6 weeks, and they were killed at the end of 62 weeks. The N-formyl, N-acetyl and N-propionyl derivatives of N-hydroxy-4-aminobiphenyl (N-OH-ABP) were similarly injected at a dose of 100 mumol/kg bwt for comparison in female CD rats. Tumors of the liver, mammary gland and ear duct were produced in the female rats by these N-OH-AS derivatives. N-OH-AAS and N-OH-PAS were more active in the induction of mammary and ear duct tumors than N-OH-FAS. These N-OH-AS derivatives produced more tumors than did the N-OH-ABP derivatives, even at 1/10 dose of the N-OH-ABP derivatives. In male CD rats, these N-OH-AS derivatives produced peritesticular mesothelioma and tumors of the pancreas and ear duct. N-OH-PAS also produced tumors of the small intestine and lung. The acetyl and propionyl derivatives were more carcinogenic than the formyl derivative of N-OH-AS for both male and female CD rats, suggesting that cytosolic acetyltransferases may be more important than the microsomal ones in activating these carcinogens.

Purification and characterization of guinea-pig liver microsomal deacetylase involved in the deacetylation of the O-glucoside of N-hydroxyacetanilide

A microsomal deacetylase that catalyses the deacetylation of the O-glucoside of N-hydroxyacetanilide (GHA) was purified from guinea-pig liver. The activity was located exclusively in the microsomes and not detected in the cytosol. The purified GHA deacetylase was a trimeric protein with a molecular mass of 160+/-10 (S.D.) kDa composed of subunits of 53+/-2 kDa; its pI was 4.7. The N-terminal amino acid sequence of GHA deacetylase was similar to those reported for guinea-pig and rat liver microsomal carboxylesterases. The GHA deacetylase showed a comparable hydrolytic activity towards p-nitrophenyl acetate (PNPA), although the activities towards N-hydroxyacetanilide, acetanilide and some endogenous acylated compounds were very low or not detectable. The deacetylase activity towards GHA was inhibited by organophosphates but not by p-chloromercuribenzoate, suggesting that GHA deacetylase can be classified as a B-esterase. The enzyme exhibited a positive homotropic co-operativity towards GHA. The values of the Hill coefficient, the half-saturating concentration ([S]0.5) for GHA, and Vmax were 1.59+/-0.03, 5.51+/-0.07 mM and 32.5+/-1.4 micromol/min per mg respectively, at the optimum pH of 8.5. The bell-shaped pH dependence of the Vmax/[S]0.5 profile indicated pKa values attributed to histidine and lysine residues. The study of stoichiometric inhibition by di-isopropyl fluorophosphate and kinetic analysis with the Monod-Wyman-Changeux model suggests that GHA deacetylase has six substrate binding sites and three catalytically essential serine residues per enzyme molecule.

Genotoxicity in the rodent urinary bladder

Elucidation of the mechanisms by which a chemical may induce urinary bladder tumours in rodents can be expected to provide insight into the relative risk from that agent. The methodologies for exploring whether tumour induction may be a response to direct genotoxic effect of the compound have been successfully applied to the bladders of both mice and rats. Thus, with experimental approaches that utilize adduct detection through the use of immunochemical and postlabelling techniques, unscheduled DNA synthesis and mutagenicity as studied with transgenic animals it is possible to obtain fundamental information on the genotoxic potential of carcinogens in the target bladder. Application of these experimental approaches to carcinogens for which the mechanisms of action are not known should permit assessment of the likelihood that genotoxic or non-genotoxic mechanisms are involved in the tumour induction process. Moreover, such studies may provide knowledge of the molecular pathways that are involved in the action of genotoxic agents, thus enabling judgements to be made as to whether humans are subject to tumour induction by the chemical.

Acecainide pharmacokinetics in normal subjects of known acetylator phenotype

The purpose of this study was to determine the pharmacokinetics of acecainide (formerly N-acetylprocainamide) in six normal subjects of known acetylator phenotype. Three subjects were fast acetylators and three slow acetylators by sulfapyridine phenotyping criteria. Each subject received a 20-min, 3 mg kg-1 intravenous acecainide infusion. Concentrations of acecainide, procainamide, and their deethylated metabolites were measured in serum and urine samples using HPLC. Acecainide renal clearance, nonrenal clearance, steady-state volume of distribution, and other pharmacokinetic parameters were estimated using standard approaches. Acecainide renal clearance and steady-state volume of distribution were (mean +/- SD) 13.6 +/- 1.581 h-1 and 135 +/- 20.31, respectively, and were not significantly different in fast and slow acetylators. Acecainide nonrenal clearance in the six subjects was 3.0 +/- 1.01 h-1; however, nonrenal clearance in slow acetylators was 1.8 times that in fast acetylators (3.9 vs 2.21 h-1, p = 0.012) with clear separation of the subjects into two groups when the data were grouped by acetylator phenotype. The nonrenal clearance of acecainide was inversely correlated with percentage sulfapyridine acetylation. Computer simulations were conducted to explore possible explanations for the observed difference in nonrenal clearance.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

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.

Mutagenicity of N-arylacetohydroxamic acids and their O-glucosides derived from chlorinated 4-nitrobiphenyl ethers

The mutagenic activity of N-arylacetohydroxamic acids, their O-acetates, their O-glucosides, and N-arylhydroxylamines, derived from chlorinated 4-nitrobiphenyl ethers (CNBs), was tested in the Salmonella reversion assay. N-Arylhydroxylamines were mutagenic by themselves; however, other compounds containing an N-acetyl group showed mutagenic activity in the presence of guinea pig liver S9. The mutagenic activation of the glucosides of N-arylacetohydroxamic acids was caused by Ms but not by S10.5, whereas their aglycones, N-arylacetohydroxamic acids, were activated to mutagens by both the fractions. The mutagenic activation of these compounds was inhibited by bis(p-nitrophenyl)phosphate, which indicates that enzymatic deacetylation is a crucial step in the mutagenic activation. Analysis of metabolites of the O-glucosides of N-arylacetohydroxamic acids by h.p.l.c. indicates that the corresponding deacetylated O-glucosides are primary metabolites, which decomposed to amino and azoxy (via hydroxylamine) derivatives, and that the deacetylating activity of S9 locates exclusively in Ms.