Enzyme-catalyzed reactions of the carcinogen N-hydroxy-2-fluorenylacetamide with nucleic acid.

A protein fraction obtained by gel filtration of a 105,000g supernatant of rat liver catalyzes three reactions of the hepatocarcinogen N-hydroxy-2-fluorenylacetamide with nucleic acid. Cofactor requirements and isotopic studies suggest that the reactive intermediates involved may be N-2-fluorenylhydroxylamine, and phosphate and sulfate esters of N-hydroxy-2-fluorenylacetamide.

Activation of carcinogenic arylhydroxamic acids by human tissues.

Incubation of the carcinogenic arylhydroxamic acids N-hydroxy-N-2-fluorenylacetamide or N-hydroxy-N-4-biphenylacetamide and tRNA with 105,000 times g supernatants of homogenates of human small intestine, liver, or colon led to formation of arylamine-substituted nucleic acid adducts. These data indicated that enzymes of human tissues could activate arylhydroxamic acids by N leads to O acyl transfer. The unstable N-acetoxyarylamines formed by these enzymes reacted spontaneously with the tRNA to give covalently linked adducts with the nucleic acid.

Comparative effects of strain, species, and sex on the acyltransferase- and sulfotransferase-catalyzed activations of N-hydroxy-N-2-fluorenylacetamide.

The arylhydroxamic acid acyltransferase, an enzyme that promotes the introduction of arylamine groups into nucleic acids, is greater in the stomach, small intestine, colon, and lung of the Sprague-Dawley rat than in comparable tissues of Fischer animals. The enzyme is distributed relatively evenly from the glandular stomach to the distal portion of the colon. No consistent differences in acyltransferase activities of the liver, kidney, brain, or spleen of these two strains were noted. Acyltransferase activity was readily demonstrable in the livers of guinea pigs, hamsters, rabbits, and monkeys; in the kidneys of guinea pigs and hamsters; in the stomachs of guinea pigs and hamsters; in the small intestines of guinea pigs, hamsters, rabbits, and monkeys; in the colons of guinea pigs and hamsters; and in lungs of hamsters. Mouse, dog, and goat tissues were essentia-ly devoid of acyltransferase activity. The transformation of N-hydroxy-N-2-fluorenylacetamide into a reactive species by conjugation with sulfate was carried out with 105,000 times g supernatants of liver from Sprague-Dawley and Fischer rats and their Flhydrids. The abilities of liver extracts from the hybrids to carry out this activation were intermediate between those from animals of the same sex of the two parental strains.

Histological localization of messenger RNAs for rat acetyltransferases that acetylate serotonin and genotoxic arylamines.

Rat acetyltransferases (ATs) can acetylate the endogenous arylalkylamines tryptamine, 5-hydroxytryptamine (serotonin), and 5-methoxytryptamine, the immediate precursor of melatonin. The same enzymes also acetylate and activate exogenous, carcinogenic arylamines, thereby being immediately responsible for the generation of DNA adducts. Localization of AT transcripts in the pineal gland and in specific cells of the intestine, cerebral cortex, pituitary, and lung identifies cells that may be important to the neurotransmitter and hormonal roles of the tryptamine derivatives. Transcript localization i liver, mammary gland, Zymbal gland, kidney, forestomach, and bladder, as well as intestine and lung, identifies cells that may be at increased carcinogenic risk because they can convert N-hydroxylated arylamines to genotoxic metabolites. Highly specific expression is also observed in the reproductive organs of both the male and female, including the testes, epididymis, uterus, ovary, and fallopian tube. In addition to these diverse organs, which are consistent with possible roles of the enzyme in carcinogen metabolism, neurotransmission, or hormonal regulation, specific cells of the cornea, cilliary process of the eye, olfactory process, adrenal gland, exorbital lacrimal gland, and skin also exhibit highly specific expression of AT mRNAs for which one can only speculate as to their function. In virtually every case, the extent of labeling suggested that AT1 was expressed at levels that were orders of magnitude higher than those of AT2. Qualitative differences in the sites of mRNA of these two enzymes were seen only in the olfactory process, in which AT1 was expressed in both respiratory and olfactory epithelia as well as Bowman's cells, and AT2 was detected only in the latter cells. The available data support the conclusion that the ATs are likely to be involved both in the metabolic activation of exogenous carcinogenic amines as well as the metabolism of endogenous arylalkylamines that play important hormonal and neurotransmitter roles.

Irradiation-induced adduct formation of RNA with carcinogenic arylamine derivatives.

Radiolysis of N2O-saturated solutions of transfer RNA (tRNA) and the arylacethydroxamic acids, N-hydroxy-N-2-acetylaminofluorene and N-hydroxy-N-4-acetylaminobiphenyl; their corresponding acetamides, 2-acetylaminofluorene and 4-acetylaminofluorene; or the O-glucuronide of N-hydroxy-N-2-acetylaminofluorene resulted in adduct formation of the nucleic acid with these carcinogenic arylamine derivatives. The yield of adducts on irradiation of the arylacethdroxamic acids with tRNA was greater than that for their corresponding acetamides or the O-glucuronide. The fluorenylacethydroxamic acid and acetamide were also more reactive than the biphenyl analogs. Adduct formation resulting from radiolysis of tRNA and the arylacethydroxamic acids or the O-glucuronide proceeded with retention of both the aromatic nucleus and the N-acetyl group. The yields of adducts were much greater for irradiated mixtures than for irradiation of either component alone followed by mixing. Evaluation of the data shows that initial modification of the tRNA or the carcinogen can lead to adduct formation. In the case of primary radical attack of the nucleic acid, it has been shown that short-lived reactive RNA intermediates are responsible for a major fraction of the observed yield of adducts in the irradiated mixtures. Comparative studies showed that irradiation under conditions that favor reaction of oxidizing radicals enhanced formation of the adducts. Oxygen was shown to protect RNA from irradiation-induced binding of the arylacethydroxamine acids due to competition of O2 with the carcinogen for the reactive RNA intermediates.

Comparative adduct formation of 4-aminobiphenyl and 2-aminofluorene derivatives with macromolecules of isolated liver parenchymal cells.

Isolated parenchymal cells of rat liver have been used in a study of the metabolic activation of derivatives of the carcinogens 4-aminobiphenyl and 2-aminofluorene. The formation of adducts of these compounds with cellular RNA and protein has been taken as evidence of their transformation to metabolites that are capable of spontaneous reaction with tissue macromolecules. The hydroxamic acid N-hydroxy-N-4-acetylaminobiphenyl was bound to RNA to a greater extent than were the amino-, hydroxylamino-, nitroso-, nitro-, acetylamino-, or azoxybiphenyl derivatives. RNA adducts of the hydroxamic acid retained little of the acetyl group. The structural requirements for binding and the nature of the bound derivatives are consistent with the activation of N-hydroxy-N-4-acetylaminobiphenyl by N leads to O acyltransfer. Approximately equal quantities of 4-nitrosobiphenyl and the hydroxamic acid were bound to protein, but far less of the nitroso derivative was incorporated into RNA. Adduct formation of N-hydroxy-N-2-acetylaminofluorene with RNA occurred with retention of the acetyl group and was dependent on the concentration of sulfate in the media. Consequently, reaction of the fluorenyl derivative with RNA probably resulted from conjugation of the hydroxamic acid with sulfate.

Effects of partial hepatectomy and dietary phenobarbital on liver and mammary tumorigenesis by two N-hydroxy-N-acylaminobiphenyls in female CD rats.

The objective of this study was to investigate the induction of liver tumors by arylhydroxamic acids. The potential involvement of sulfate conjugation was minimized by the administration of a N-hydroxy-4-acylaminobiphenyl to female CD rats. This experimental design provided for the exposure of a target organ that has only a low capacity for activation of hydroxamic acids by sulfate conjugation, with a carcinogen that does not induce tumors in liver that possess a high sulfotransferase activity. A single dose of the N-formyl or N-acetyl derivatives of N-hydroxy-4-aminobiphenyl was given i.p. at 0.4 mmol/kg body weight to 34-day-old animals. In attempts to amplify the hepatocarcinogenic potential of the compounds, partial hepatectomy 24 hr before the chemical injection and subsequent long-term treatment with phenobarbital in the diet were carried out. For comparative purposes, other animals were subjected to three additional partial hepatectomies subsequent to the carcinogen administration instead of the phenobarbital treatment. The experiments were terminated 64 weeks after injection. Both the N-formyl and N-acetyl derivatives of N-hydroxy-4-aminobiphenyl, in conjunction with partial hepatectomy and subsequent treatment of dietary phenobarbital, induced a high incidence of neoplastic nodules and gamma-glutamyltranspeptidase-positive foci in the liver. Only one hepatocellular carcinoma was observed in each treatment group. Repeated partial hepatectomies enhanced the yield of gamma-glutamyltranspeptidase-positive foci but were ineffective in producing neoplastic nodules. In addition to the liver lesions, mammary tumors were also induced. Importantly, an inhibitory effect of the subsequent administration of phenobarbital was observed on mammary tumor formation, possibly because of alterations in hormone metabolism resulting from the induction of microsomal enzymes by phenobarbital, which resulted in a decreased promoting effect. There was no difference in the tumorigenicity of the formyl and acetyl derivatives in these experiments.

Metabolic activation of arylhydroxamic acids by N-O-acyltransferase of rat mammary gland.

The lactating mammary glands of rats contain an arylhydroxamic acid N,O-acyltransferase that catalyzes the formation of arylamine-substituted nucleic acid on incubation with N-hydroxy-N-2-acetylaminofluorene or N-hydroxy-N-4-acetylaminobiphenyl and transfer RNA. The acyltransferase activity migrates as a single component with a molecular weight of 28,000 on gel filtration on Sephadex G-100. Acyltransferase activities of the lactating mammary glands of Sprague-Dawley animals are approximately twice those of the less susceptible Fischer strain as determined by assay with either hydroxamic acid. The fluorene substrate was 15 times as efficient as the biphenyl compound in promoting adduct formation. Ribosomal RNA adducts formed in vivo after administration of N-hydroxy-N-2-acetylaminofluorene were consistent with an acyltransferase mechanism of activation in that the adducts did not retain the acetyl group.

Induction of rat mammary gland tumors by 1-nitropyrene, a recently recognized environmental mutagen.

1-Nitropyrene, a direct-acting mutagenic component of extracts of diesel exhaust and ambient air particulates, is one of the major nitroarenes found in products of incomplete combustion. Injection of this compound into newborn Sprague-Dawley-derived CD rats at 100 mumol/kg body weight s.c. once a week for 8 weeks induced 32 and 28% sarcomas, primarily malignant fibrous histiocytomas, at the site of injection in male and female animals, respectively. Mammary tumors, most of which were adenocarcinomas, developed distant from the site of injection in 47% of the females. Similar but lesser responses were observed in animals given one-half this dose. This carcinogenic response is consistent with the systemic effects of structurally related aromatic amines. 4-Nitrobiphenyl, given at equimolar doses, was not tumorigenic in this study.

Induction of repair synthesis of DNA in mammary and urinary bladder epithelial cells by N-hydroxy derivatives of carcinogenic arylamines.

Unscheduled DNA synthesis (UDS)-inducing activity was used as a parameter to estimate the abilities of rat mammary epithelial cells and urothelial cells from various species to activate carcinogenic aromatic amine derivatives. The N-hydroxy, N-hydroxy-N-acetyl, N-hydroxy-N-glucuronosyl derivatives of 2-aminofluorene (2-AF) and 4-aminobiphenyl (4-ABP) induced UDS in primary cultures of rat mammary epithelial cells, but 2-AF, the O-glucuronide of N-hydroxy-N-acetyl-2-AF (N-OH-AAF) and 4-ABP did not. Neither the activity of N-OH-AAF, N-hydroxy-N-formyl-2-AF, nor N-acetoxy-N-acetyl-2-AF was significantly altered by paraoxon, an inhibitor of microsomal N-deacetylase. Although N-hydroxy-3,2'-dimethyl-4-aminobiphenyl (N-OH-DMABP) also induced UDS, its N-acetyl derivative, which can not be activated by intramolecular, N,O-acetyltransfer, did not. Similarly, rat urothelial cells were responsive to the UDS-inducing activity of this hydroxylamine, but not the hydroxamic acid. In contrast, dog urothelial cells were responsive to both compounds. The UDS-inducing activity of N-OH-AAF was inhibited by paraoxon in the dog, but not in rat urothelial cells. N-Hydroxy-N,N'-diacetylbenzidine induced UDS in the urothelial cells of dog, rat, and rabbit, and a human urothelial cell line, HCV-29, whereas benzidine, N-acetylbenzidine, and N,N'-diacetylbenzidine did not. Co-treatment with 12-O-tetradecanoylphorbol-13-acetate did not enable benzidine to induce UDS in dog urothelial cells. Rat mammary epithelial cells activated N-OH-DMABP by acetyl coenzyme A-dependent O-acetylation and N-OH-AAF by N,O-acetyltransfer. They could not N-deacetylate N-OH-AAF. These results suggest that rat mammary and bladder epithelial cells are capable of activating N-arylhydroxylamine metabolites of these carcinogens, probably by N,O-acetyltransfer and O-acetylation, whereas dog urothelial cells are more likely to activate these metabolites by N-deacetylation and a reaction that has yet to be identified.

Comparative carcinogenicities of 1-, 2-, and 4-nitropyrene and structurally related compounds in the female CD rat.

The comparative carcinogenicities of N-hydroxy-N-acetyl-1-aminopyrene, N-acetyl-1-aminopyrene, and 1-, 2-, and 4-nitropyrene were determined following i.p. injection into weaning female CD rats (67 mumol/kg body weight in dimethyl sulfoxide; 3 times/week for 4 weeks). At sacrifice 61 weeks after the first injection the incidences of malignant mammary tumors were increased significantly to 45 and 24% in the 4-nitropyrene- and N-hydroxy-N-acetyl-2-aminofluorene-treated groups, respectively. Cellular altered foci in the liver were increased significantly in the N-acetyl-1-aminopyrene-, N-hydroxy-N-acetyl-1-aminopyrene-, and N-hydroxy-N-acetyl-2-aminofluorene- treated groups; the latter two compounds also led to significantly increased formation of hyperplastic nodules in this organ. Significant increases in leukemia induction were observed in animals treated with 2-nitropyrene or N-hydroxy-N-acetyl-2-aminofluorene. In an experiment designed to compare the influence of the route of administration on the carcinogenic potential of this agent, 1-nitropyrene was injected i.p. or s.c. into weanling female CD rats (100 mumol/kg body weight; once a week for 4 weeks). The animals were sacrificed at 87 to 90 weeks after the first treatment. The incidences of mammary gland tumors in animals receiving injections of 1-nitropyrene by either route (59%) were significantly higher than in solvent-injected controls (37%). The incidences of adenocarcinoma in the i.p. 1-nitropyrene group (28%) and fibroadenoma in the s.c. 1-nitropyrene group (52%) were significantly higher than in the control animals (7 and 27%, respectively). These data suggest that the demonstration of the weak carcinogenicity of 1-nitropyrene is probably more a function of the length of the observation period than of the routes of administration used here. A further exploration of the effect of the route of administration involved treatment of weanling female CD rats by direct injection of 1-, 2-, or 4-nitropyrene into the mammary fat pads. A total of 2.04 mumol of the nitrocompound in dimethyl sulfoxide was injected into the mammary glands under each of the 6 left nipples. The right mammary glands were treated with the solvent only. Injections of the thoracic nipple areas were carried out on day 1; inguinal areas were treated on day 2. The animals were sacrificed after 77 weeks. The number of mammary tumor-bearing animals (23 of 28), the number with fibroadenoma (15 of 28), and the number with adenocarcinoma (19 of 28) were significantly increased in the 4-nitropyrene-treated group as compared with animals treated with only dimethyl sulfoxide.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship of metabolic activation of N-hydroxy-N-acylarylamines to biological response in the liver and mammary gland of the female CD rat.

Intraperitoneal injection of the N-formyl, N-acetyl, or N-propionyl derivatives of N-hydroxy-4-aminobiphenyl, N-hydroxy-2-acetylaminofluorene, or N-(4-biphenyl)glycolamide disclosed that the ability of these compounds to induce mammary tumors in the female CD rat was greater if the compound was able to be metabolized to a reactive product by one of two soluble enzymes obtained from both the liver and mammary gland. A similar but weaker association between the formation of gamma-glutamyltranspeptidase-positive foci and cellular altered foci of the liver was also observed. The enzyme related to the tumorigenicity of these compounds was characterized by a highly specific capacity to form adducts from the acetyl and propionyl derivatives. The other enzyme exhibited greater activity with N-formyl substrates. The two enzyme activities were separable by ion-exchange chromatography on DEAE-cellulose and by gel filtration on Sephacryl. Liver microsomes also possessed the capacity to activate both the formyl and acetyl derivatives to reactive species; formyl substrates were 7 to 8 times more active than acetylated compounds. The microsomal activities and the formyl-preferring soluble enzyme were inhibited by diethyl-p-nitrophenylphosphate, a microsomal deacylase inhibitor. The cytosolic enzymes that are most active with the acetyl and propionyl substrates were little affected by this organophosphate compound. The microsomal activation was not due solely to deacylation of the hydroxamic acid, since formylated and acetylated substrates were hydrolyzed at approximately the same rates.

Activation of N-hydroxyphenacetin to mutagenic and nucleic acid-binding metabolites by acyltransfer, deacylation, and sulfate conjugation.

N-Hydroxyphenacetin was activated to a mutagen in the Salmonella-Ames test by rabbit liver acyltransferase, rat liver cytosol, and rat liver microsomes. N-[ring]3H]-Hydroxyphenacetin was bound to transfer RNA when activated by acyltransferase from rabbit or rat liver or rat liver microsomes. The acyltransferase-catalyzed binding was not inhibited by paraoxon, a deacetylase inhibitor. The use of N-hydroxyphenacetin radioactively labeled in the acetyl group, as well as the ring, indicated that deacetylation was involved in the microsome-catalyzed binding reaction. In addition, the microsome-catalyzed binding was inhibited 90% by paraoxon. p-Nitrosophenetole, a deacetylated derivative of N-hydroxyphenacetin, was synthesized and bound to transfer RNA without enzymatic activation. Activation of N-hydroxyphenacetin by sulfate conjugation was also found to lead to binding to transfer RNA. The data implicated acyl transfer, deacetylation, and sulfate conjugation as possible routes for the activation of N-hydroxyphenacetin.

Renal artery pseudoaneurysm following partial nephrectomy treated with stent-graft.

Haemorrhagic complications due to pseudoaneurysms of branch arteries can be treated by selective embolisation. Injuries to the main renal artery cannot be treated in this way without sacrificing the kidney. We report the successful percutaneous treatment of a main renal artery pseudoaneurysm with a stent-graft in a patient with a solitary kidney.

The endothelium partially obscures enhanced microvessel reactivity in DOCA hypertensive rats.

This study examined the contribution of the endothelium to pressor and depressor responses in the isolated, perfused mesentery of mineralocorticoid hypertensive rats. Following uninephrectomy, adult male rats were made hypertensive by subcutaneous implantation of deoxycorticosterone acetate (DOCA; 200 mg/kg); control rats were sham-treated. All rats received drinking water that contained 1.0% NaCl and 0.2% KCl. Following 4 to 6 weeks of treatment, the rats were anesthetized and the mesenteric vasculature was isolated and pump-perfused (constant flow with buffer) to evaluate changes in perfusion pressure. Vascular responses were determined before and after disruption of endothelial function by perfusion with oxygen free radicals generated in the buffer by electrical stimulation. Vasodilator responsiveness to acetylcholine and nitroprusside in the intact mesentery of hypertensive rats did not differ from that in the intact mesentery of normotensive rats, whereas pressor responses to norepinephrine in the intact mesentery of hypertensive rats were greater than normotensive values. Following disruption of endothelial function, depressor responses to acetylcholine were greatly attenuated whereas those to nitroprusside were unaltered or increased. Pressor responses to norepinephrine were potentiated in mesentery that had undergone endothelial disruption, and this potentiation was greater in hypertensive rats than in control rats. The slopes of pressure-flow curves in the presence of norepinephrine were less steep in mesentery with intact endothelium. The flow-modified component of these pressure-flow curves that was related to the endothelium was greater in mesenteric vascular beds of hypertensive rats. These results indicate that a factor released from the endothelium partially masks the enhanced vascular reactivity characteristic of this animal model of mineralocorticoid hypertension.

Calcium influx and vascular reactivity in systemic hypertension.

Numerous studies have focused on functional vascular changes that characterize the hypertensive state. Recent evidence that suggests that increased vascular reactivity in hypertension is due to changes in the delivery of activator Ca++ through channels in the cell membrane will be reviewed. The primary evidence supporting this hypothesis comes from studies that characterize the effects of Ca++-free solution and calcium channel blockers on contractile properties of isolated vascular smooth muscle. In the present study, experiments were performed to investigate the role of Ca++ influx in vascular contractions produced by interventions that cause membrane depolarization. Isometric tension development in helical strips of carotid arteries from stroke-prone spontaneously hypertensive rats in response to elevated K+ and tetraethylammonium chloride was greater than that in carotid arteries from Wistar-Kyoto normotensive rats. The rate of tension development to K+-free solution in carotid arteries from stroke-prone spontaneously hypertensive rats was faster than in Wistar-Kyoto normotensive rat arteries. Contractile responses to all 3 depolarizing interventions were reduced in arterial strips incubated in Ca++-free solution containing the chelator ethylene glycol bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid and in arterial strips treated with the Ca++ channel blocker verapamil. These results are consistent with the hypothesis that constrictor stimuli that produce membrane depolarization cause an opening of Ca++ channels in the plasma membrane that are sensitive to the organic channel blockers. Further, a change in Ca++ permeability or membrane depolarizing mechanisms contributes to increased contractile responsiveness in carotid arteries of stroke-prone spontaneously hypertensive rats.

Histological localization of acetyltransferases in human tissue.

The localization of human arylamine acetyltransferases (NAT) transcripts was performed by non-isotopic in situ hybridization, utilizing a combination of six NAT1 or NAT2 specific antisense oligonucleotide probes, in order to identify those tissues and organs that might be susceptible to the carcinogenic effects of aromatic amines. The intratissue differences in the level of NAT mRNA were observed: the most abundant NAT2 transcripts were found in hepatocytes, while NAT1 ones dominated in the urothelium and in the colon epithelial cells. The specific NAT1 and NAT2 mRNAs were present also in the epithelial lining of the lung bronchi, the mammary gland and the small intestine epithelial cells, the outer layer of placenta syncytiotrophoblast cells, the kidney tubules, and the pineal gland. Qualitative differences in the sites of mRNA of these two enzymes were seen only in the kidney specimens, in which NAT2 was expressed in both proximal and distal tubules, and the NAT1 was detected only in the former ones.

Vasopressin and oxytocin responses to acute and chronic osmotic stimuli in man.

The regulation of both arginine vasopressin (AVP) and oxytocin secretion was studied during rapid and prolonged osmotic stimuli in normal adult volunteers. In five subjects given an intravenous infusion of 0.85 mol NaCl at 0.05 ml/kg per min over 2 h there was a significant (P less than 0.05) rise only in plasma AVP, with no significant change in plasma levels of oxytocin. In six further subjects 5 days of restriction to 500 ml fluid daily resulted in significant increases of both plasma and 24-h urinary AVP, whereas there was no change in corresponding oxytocin levels. During another 5-day period in which the same subjects were given an additional 200 mmol sodium as well as having their fluid intake restricted to 1000 ml daily, there were again significant rises in plasma and 24 h urinary AVP with no change in corresponding oxytocin levels. We conclude that, in man, AVP is selectively secreted in response to both dehydration and high sodium intake, whilst even after the stimulus of rapidly increasing plasma osmolality during intravenous infusion of hypertonic saline the rise in oxytocin is not statistically significant. It therefore appears unlikely that oxytocin has a significant role in the physiological control of fluid balance in man.

Characterization of rat hepatic acetyltransferase.

Rat liver cytosol is capable of N-acetylation (NAT) of arylamines, O-acetylation (OAT) of arylhydroxylamines, and N,O-acetyltransfer (AHAT) of arylhydroxamic acids. Physical, enzymatic, and immunochemical techniques now support the conclusion that a single 32 kDa protein accounts for all of these activities. Of the five immunoglobulin (IgG1) mouse monoclonal antibodies (mAb) produced against this protein, each affected one or more of these acetylation activities. When mixed with rat hepatic cytosol and then chromatographed on a gel filtration column, mAbs 1F2 and 5F8 increased the apparent size of all enzymes capable of acetylation from 32 kDa to the exclusion volume. Each of the mAbs reacted with only a single 32 kDa protein on SDS-PAGE/Western blots, regardless of the state of purity of the enzyme. This enzyme is unstable in low salt solutions, as reflected by a relative loss in NAT versus AHAT activity, but it does not result in changes in either molecular weight or isoelectric point (pl). A second form of instability is shown by the formation of more basic peptides with pls as high as 6, again without change in molecular weight. Although NAT activity is retained in acetyltransferase (AT) that has a minimally modified pl, further increases in pl result in total loss of enzyme activity. The differential effects of the mAbs on AT suggest that the ratios of NAT, OAT, and AHAT may be highly dependent on the conformation of the enzyme and, consequently, provide insight as to why the abilities of ATs from different species exhibit such dissimilar potentials for the activation of aromatic amines by OAT and AHAT.

Acetylation, deacetylation and acyltransfer.

N-Substituted aromatic compounds can be metabolized in most species to N-acetylated derivatives that are themselves subject to further enzymatic transformations, including hydrolysis and N,O-acyltransfer. These proceses can either potentiate or ameliorate the biological responses to these N-substituted derivatives. Decreasing the levels of metabolites, such as arylhydroxylamines may, in some systems, reduce the probability of eliciting adverse biological effects. In others, arylhydroxamic acids produced by the acetylation of arylhydroxylamines may increase their potential for metabolic activation by N,O-acyltransfer. In the rabbit, rat and perhaps other species, the acetyl CoA-dependent N-acetyltransferase is also capable of activating arylhydroxamic acids by N-O-acyltransfer. These cytosolic organotriphosphate ester-resistant enzymes can utilize arylhydroxamic acid as a donor of the acetyl moiety in the acetyl transferase reaction and apparently are capable of activating arylhydroxamic acids because of their ability to O-acetylate the arylhydroxlamine. In mice, N-acetyltransferase and N,O-acetyltransferase seem not to exhibit this relationship. Enzymes from the microsomes of a number of species are also capable of activating arylhydroxamic acids. The particulate-bound enzymes are organotriphosphate ester-sensitive deacylases that are unable to form nucleic acid adducts on incubation with N-methoxy-N-acetylaminoarenes, substrates that are not capable of activation by N,O-acyltransfer. Thus, depending on the specificity of the enzymes involved, N-substituted aromatic compounds may be activated by N,O-acyltransfer during both the acetylation and deacylation process. The influence of this activation in the carcinogenic process is the object of continuing investigation.