Initiation of hepatocarcinogenesis in infant male B6C3F1 mice by N-hydroxy-2-aminofluorene or N-hydroxy-2-acetylaminofluorene depends primarily on metabolism to N-sulfooxy-2-aminofluorene and formation of DNA-(deoxyguanosin-8-yl)-2-aminofluorene adducts

Strategy for genotoxicity testing—Metabolic considerations

The report from the 2002 International Workshop on Genotoxicity Tests (IWGT) Strategy Expert Group emphasized metabolic considerations as an important area to address in developing a common strategy for genotoxicity testing. A working group convened at the 2005 4th IWGT to discuss this area further and propose practical strategy recommendations. To propose a strategy, the working group reviewed: (1) the current status and deficiencies, including examples of carcinogens "missed" in genotoxicity testing, established shortcomings of the standard in vitro induced S9 activation system and drug metabolite case examples; (2) the current status of possible remedies, including alternative S9 sources, other external metabolism systems or genetically engineered test systems; (3) any existing positions or guidance. The working group established consensus principles to guide strategy development. Thus, a human metabolite of interest should be represented in genotoxicity and carcinogenicity testing, including evaluation of alternative genotoxicity in vitro metabolic activation or test systems, and the selection of a carcinogenicity test species showing appropriate biotransformation. Appropriate action triggers need to be defined based on the extent of human exposure, considering any structural knowledge of the metabolite, and when genotoxicity is observed upon in vitro testing in the presence of metabolic activation. These triggers also need to be considered in defining the timing of human pharmaceutical ADME assessments. The working group proposed two strategies to consider; a more proactive approach, which emphasizes early metabolism predictions to drive appropriate hazard assessment; and a retroactive approach to manage safety risks of a unique or "major" metabolite once identified and quantitated from human clinical ADME studies. In both strategies, the assessment of the genotoxic potential of a metabolite could include the use of an alternative or optimized in vitro metabolic activation system, or direct testing of an isolated or synthesized metabolite. The working group also identified specific areas where more data or experiences need to be gained to reach consensus. These included defining a discrete exposure action trigger for safety assessment and when direct testing of a metabolite of interest is warranted versus the use of an alternative in vitro activation system, a universal recommendation for the timing of human ADME studies for drug candidates and the positioning of metabolite structural knowledge (through in silico systems, literature, expert analysis) in supporting metabolite safety qualification. Lastly, the working group outlined future considerations for refining the initially proposed strategies. These included the need for further evaluation of the current in vitro genotoxicity testing protocols that can potentially perturb or reduce the level of metabolic activity (potential alterations in metabolism associated with both the use of some solvents to solubilize test chemicals and testing to the guidance limit dose), and proposing broader evaluations of alternative metabolic activation sources or engineered test systems to further challenge the suitability of (or replace) the current induced liver S9 activation source.

In Vivo and in Vitro Metabolism of Arylamine Procarcinogens in Acetyltransferase-Deficient Mice

Arylamine N-acetyltransferases (NATs) catalyze the biotransformation of a number of aromatic and heterocyclic amines, many of which are procarcinogenic agents. Interestingly, these enzymes are binary in nature, participating in both detoxification and activation reactions, and thus it is unclear what role NATs actually play in either preventing or enhancing toxic responses. The ultimate direction may be substrate-specific and dependent on its tissue-specific metabolism by competing, but genetically variable, drug-metabolizing enzymes. To investigate the effect of N-acetylation on the metabolism of some classical procarcinogenic arylamines, we have used our double knockout Nat1/2(-/-) mouse model to test both in vitro activity and the in vivo clearance of some of these agents. As expected, N-acetylation activity was undetectable in tissue cytosol preparations from Nat1/2(-/-) mice for 4-aminobiphenyl (ABP) and 2-aminofluorene (AF), whereas significant levels were measured in all wild-type tissue cytosols tested, indicating the widespread metabolism of these agents. Nat1/2(-/-) mice displayed a variable response with respect to in vivo pharmacokinetics. AF appeared to be most severely compromised, with a 3- to 4-fold increased area under the curve (AUC), whereas the clearance of ABP was found to be less dependent on N-acetylation, with no difference in ABP-AUC between wild-type and knockout animals. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine was neither N-acetylated nor was its clearance affected by NAT genotype, signifying a dependence on other drug-metabolizing enzymes. The elucidation of the role that N-acetylation plays in the clearance of procarcinogenic agents is the first step in attempting to correlate metabolism by NATs to toxic outcome prevention or augmentation.

Tissue Distribution and Ontogeny of Sulfotransferase Enzymes in Mice

Sulfotransferases (Sults) are phase-II conjugation enzymes that catalyze the transfer of a sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to target endo and xenobiotics. PAPS is formed from inorganic sulfate by the action of the enzyme PAPS synthase (PAPSs). In the present study, the tissue distribution and developmental changes in the mRNA expression of 11 Sult isozymes and 2 PAPSs isoforms in mice were quantified. Sult1a1, 1b1, 1c1, 1c2, 1d1, 1e1, 2a1/2, 2b1, 3a1, 4a1, 5a1, PAPSs1, and PAPSs2 mRNA expression was quantified in 14 tissues from male and female mice using the branched DNA signal amplification assay. Sult2a1/2 and 3a1 expression were highest in liver; Sult1b1, 2b1, and PAPSs2 in small intestine; Sult1a1 in large intestine; Sult1c2 in stomach; Sult1d1 in kidney; Sult1e1 in placenta; and Sult4a1 in brain. Sult1c1, 5a1, and PAPSs1 were ubiquitously expressed in most tissues. These enzymes demonstrated three different ontogenic expression patterns in liver. Sult1a1, 1c2, 1d1, 2a1/2, and PAPSs2 hepatic expression gradually increased from birth until about 3 weeks of age and then declined somewhat thereafter, Sult1c1 expression was highest before birth and declined after that, and Sult3a1 mRNA expression was very low in fetal livers and remained low until 30 days of age, when expression in females dramatically increased, whereas it never increased in males. The organ-specific distribution of Sults as well as the different expression of the Sults in young animals may affect the pharmacokinetic behavior and organ-specific toxicity of xenobiotics.

Pentachlorophenol inhibits micronuclei induction by 2-acetylaminofluorene but not by thioacetamide

Our study examined the capacity of pentachlorophenol (PCP) to inhibit the ability of 2-acetylaminofluorene (2-AAF) and thioacetamide (TAA) to induce micronuclei in mouse bone marrow cells in vivo. 2-AAF (5.6mg/kg) and TAA (60mg/kg) were administered intra-peritoneally (i.p.) to Mus musculus males (BALB/c), and the frequencies of polychromatic erythrocytes with micronuclei (PCE-MN) 24h after injection were analyzed. Treatment with 2-AAF or TAA resulted in high PCE-MN frequencies in comparison with untreated and negative controls (19.9 and 21.6‰, respectively, versus ≈3‰). Pretreatment with a single PCP dose (44mg/kg) 24h prior to the 2-AAF administration virtually eliminated micronuclei formation by 2-AAF, although it had no inhibitory effect on TAA-induced micronuclei. Animals receiving cyclophosphamide (CP) served as positive control. Since PCP is known to inhibit arylsulfotransferase (AST) activity, which is involved in 2-AAF activation, this mechanism most likely produced the results with PCP and 2-AAF. Our results also are consistent with a different pathway involved in TAA induction of micronuclei, one that is not inhibited by PCP.

Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Mutations in orthologous genes in human spondyloepimetaphyseal dysplasia and the brachymorphic mouse

The osteochondrodysplasias are a genetically heterogeneous group of disorders affecting skeletal development, linear growth and the maintenance of cartilage and bone. We have studied a large inbred Pakistani family with a distinct form of recessively inherited spondyloepimetaphyseal dysplasia (SEMD) and mapped a gene associated with this dwarfing condition to chromosome 10q23-24, a region syntenic with the locus for the brachymorphic mutation on mouse chromosome 19. We identified two orthologous genes, ATPSK2 and Atpsk2, encoding novel ATP sulfurylase/APS kinase orthologues in the respective regions of the human and mouse genomes. We characterized a nonsense mutation in ATPSK2 in the SEMD family and a missense mutation in the region of Atpsk2 encoding the APS kinase activity in the brachymorphic mouse. ATP sulfurylase/APS kinase catalyses the metabolic activation of inorganic sulfate to PAPS, the universal donor for post-translational protein sulfation in all cell types. The cartilage-specificity of the human and mouse phenotypes provides further evidence of the critical role of sulfate activation in the maturation of cartilage extracellular matrix molecules and the effect of defects in this process on the architecture of cartilage and skeletogenesis.

Lack of transcription-coupled repair of acetylaminofluorene DNA adducts in human fibroblasts contrasts their efficient inhibition of transcription

The N-(deoxyguanosine-8-yl)-2-acetylaminofluorene (dG-C8-AAF) lesion is among the most helix distorting DNA lesions. In normal fibroblasts dG-C8-AAF is repaired rapidly in transcriptionally active genes, but without strand specificity, indicating that repair of dG-C8-AAF by global genome repair (GGR) overrules transcription-coupled repair (TCR). Yet, dG-C8-AAF is a very potent inhibitor of transcription. The target size of inhibition (45 kilobases) suggests that transcription inhibition by dG-C8-AAF is caused by blockage of initiation rather than elongation. Cockayne's syndrome (CS) cells appear to be extremely sensitive to the cytotoxic effects of dG-C8-AAF and are unable to recover inhibited RNA synthesis. However, CS cells exhibit no detectable defect in repair of dG-C8-AAF in active genes, indicating that impaired TCR is not the cause of the enhanced sensitivity of CS cells. These and data reported previously suggest that the degree of DNA helix distortion determines the rate of GGR as well as the extent of inhibition of transcription initiation. An interchange of the transcription/repair factor TFIIH from promoter sites to sites of damage might underlie inhibition of transcription initiation. This process is likely to occur more rapidly and efficiently in the case of strongly DNA helix distorting lesions, resulting in a very efficient GGR, a poor contribution of TCR to repair of lesions in active genes, and an efficient inhibition of transcription.

Molecular cloning and expression of a cDNA encoding an olfactory-specific mouse phenol sulphotransferase

Previously we demonstrated the presence of phenol sulphotransferase (P-ST) in mouse nasal cytosols and identified its zonal location in mouse nasal cavity by staining with an antiserum raised against a rat liver P-ST isoenzyme, PSTg. In the present study a cDNA was isolated from a mouse olfactory cDNA library by immunological screening with the antiserum. The isolated cDNA consisted of 1347 bp with a 912 bp open reading frame encoding a 304-residue polypeptide. Both the nucleotide and deduced amino acid sequences of the cDNA were 94% identical with those of a rat liver P-ST isoenzyme, ST1C1. The expressed enzyme in Escherichia coli displayed high P-ST activity towards phenolic odorants such as eugenol and guaiacol, and it showed a high N-hydroxy-2-acetylaminofluorene sulphation activity in comparison with the rat ST1C1 enzyme. These results indicate that the olfactory P-ST encoded by the cDNA is a mouse orthologue of rat ST1C1; however, expression of the olfactory P-ST mRNA is specific for nasal tissues as revealed by reverse transcriptase-mediated PCR (RT-PCR).

Aryl sulfotransferase IV deficiency in rat liver carcinogenesis initiated with diethylnitrosamine and promoted with N-2-fluorenylacetamide or its C-9-oxidized metabolites

Down regulation of aryl sulfotransferase IV (AST IV) in promotion/progression of liver carcinogenesis by N-2-fluorenylacetamide (2-FAA) has been established. This study examined whether the C-9 oxidized metabolites of 2-FAA, which have recently been shown to promote diethylnitrosamine (DEN)-initiated liver carcinogenesis in male Sprague-Dawley rats, effect the above change. Hence, in DEN-initiated rats, the effects of promoting regimens of 9-OH-2-FAA or 9-oxo-2-FAA, 15 oral doses at 50 and 100 mumol/kg of body weight, were compared to those of 2-FAA at 50 mumol/kg of body weight and of the vehicle on the activity of N-hydroxy(OH)-2-FAA sulfotransferase (ST), an isozyme of AST IV and AST IV expression and distribution. Relative to the vehicle, treatment with the fluorenyl compounds led to decreased levels in hepatic N-OH-2-FAA ST activity and development of hepatic nodules and tumors which had still lower levels of the ST activity than the respective remnant livers. At approximately 8 months after treatment with the C-9-oxidized compounds at doses twice that of 2-FAA, the extents of decreases in the hepatic N-OH-2-FAA ST activity and cytosolic AST IV protein in tumors were comparable to those with 2-FAA. Immunocytochemical analysis showed close association of AST IV deficiency with neoplastic liver lesions. In comparison to N-OH-2-FAA, 9-OH-2-FAA had only low and 9-oxo-2-FAA lacked sulfate acceptor activity in the presence of male rat liver cytosol or AST IV. At 3.3-fold greater concentration than N-OH-2-FAA, 9-oxo-2-FAA inhibited (27%) the sulfate acceptor activity of N-OH-2-FAA in the presence of AST IV, which suggested interference by 9-oxo-2-FAA at the active site. Although the C-9-oxidized compounds do not appear to be substrates for N-OH-2-FAA ST, their ability to cause a decrease in N-OH-2-FAA ST activity and protein similar to that of 2-FAA supports their role in hepatocarcinogenesis. Whereas 9-OH-2-FAA had a 3.9-fold greater sulfate acceptor activity in the presence of female than male rat liver cytosol and inhibited dehydroepiandrosterone ST activity of female rat liver, N-OH-2-FAA and 9-oxo-2-FAA inhibited estrone ST activity of male rat liver, suggesting that the C-9-oxidized compounds as well as N-OH-2-FAA are substrates for STs other than AST IV.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Hypomethylation of the rat aryl sulfotransferase IV gene and amplification of a DNA sequence during multistage 2-acetylaminofluorene hepatocarcinogenesis

Rat hepatic aryl sulfotransferase IV (AST IV), which catalyses sulfuric acid esterification of N-hydroxy-2-acetylaminofluorene to its ultimate carcinogenic form, is differentially expressed during multistep 2-acetylaminofluorene (AAF) hepatocarcinogenesis. Two molecular mechanisms associated with this effect involve modulation of mRNA translational capacity at the early stages, and gene transcription at the late stages of the carcinogenic process. To characterize further the molecular mechanisms that may be involved in the transient regulation of the enzyme expression, an AST IV cDNA was used to assess the change in methylation profile and restriction fragment length polymorphism (RFLP) in the gene domain of genomic DNA derived from rats at different stages of carcinogenesis. The onset of hypomethylation of the AST IV gene domain and amplification of a 5.3-kb DNA sequence was found to correlate with the stage in AAF hepatocarcinogenesis, where rats begin to exhibit irreversible loss in hepatic enzyme expression and the liver becomes committed to hepatoma formation. This represents the first observation of both altered methylation status of AST IV gene domain and amplification of a DNA sequence whose expression may play a role in the genesis and/or progression of neoplastic transformation of initiated cells during AAF hepatocarcinogenesis.

Roles of electrophilic sulfuric acid ester metabolites in mutagenesis and carcinogenesis by some polynuclear aromatic hydrocarbons

Hydroxylation of meso-methyl groups with subsequent formation of reactive benzylic esters bearing a good leaving group (e.g. sulfate) was proposed as a possible biochemical mechanism of activation and tumorigenicity of methyl-substituted polycyclic aromatic hydrocarbons (PAHs). In support of this postulation, recent studies have demonstrated the formation by rodent hepatic sulfotransferase activity of electrophilic, mutagenic, and carcinogenic sulfuric acid esters of several hydroxymethyl aromatic hydrocarbons including hydroxymethyl derivatives of benz[a]anthracene, 6-hydroxymethylbenzo[a]pyrene, 5-hydroxymethylchrysene, 9-hydroxymethyl-10-methylanthracene, and 1-hydroxymethylpyrene. Besides these hydroxymethyl PAHs containing a primary benzylic alcoholic group, some aromatic hydrocarbons with secondary benzylic hydroxyl functional group(s) are also metabolically activated through sulfuric acid esterification.

Sulfonation in chemical carcinogenesis--history and present status

Many laboratories have characterized the electrophilic metabolites of chemical carcinogens and their covalently bound adducts with genomic DNA in vivo. Recent studies from our laboratory have shown that enzymatic sulfonation of members of several classes of proximate carcinogens containing C- or N-hydroxy groups converts them to electrophilic, mutagenic, and carcinogenic sulfuric acid ester metabolites in mouse liver. These compounds form the subject of this report.

Sulfation of carcinogenic aromatic hydroxylamines and hydroxamic acids by rat and human sulfotransferases: substrate specificity, developmental aspects and sex differences

Sulfation of the carcinogen N-hydroxy-2-acetylaminofluorene (N-OH-AAF) and structurally related hydroxamic acids by rat and human sulfotransferases was studied. There was a clear sex and age difference in the sulfation of N-OH-AAF and the other hydroxamic acids by rat liver cytosols; adult male rats had the highest sulfation activity. Experiments with purified aryl sulfotransferase IV (AST IV) indicated that the high expression of this enzyme in male rat liver may be responsible for these differences. No such sex or age difference was found for the sulfation of aromatic hydroxylamines. In cytosols of adult human livers, sulfation activity towards aromatic hydroxamic acids and hydroxylamines was clearly present, but activities were much lower than in rat liver cytosols. Sulfation activity towards these compounds was also found in fetal and neonatal liver and adrenals. These compounds probably are sulfated by several different sulfotransferases in humans.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

The human phenolsulphotransferase polymorphism is determined by the level of expression of the enzyme protein

We have examined the expression of platelet phenolsulphotransferase (PST) in 60 individuals. Using an antibody which recognizes both forms of PST present in man (P-PST and M-PST), we determined that the polymorphism of platelet P-PST activity is determined by the level of expression of the enzyme protein. The implications for susceptibility to adverse drug reactions and chemical carcinogenesis are discussed.

Induction of sister-chromatid exchanges by 2-aminofluorene in cultured human lymphocytes with and without erythrocytes

2-Aminofluorene (2-AF), an indirect mutagen reported to be metabolically activated by erythrocytes in the Salmonella mutagenicity test, was studied for the induction of sister-chromatid exchanges (SCEs) in human lymphocytes in vitro with (whole-blood cultures) and without erythrocytes (isolated lymphocyte cultures). 2-AF (0.025-0.8 mM) was present in the cultures for the last 48 h of 72-h cultures. In both types of culture, SCEs increased in a dose-dependent manner, with a statistically significant elevation already at the lowest concentration of 2-AF tested and maximum responses of 2.4-fold (whole blood) and 2.1-fold (isolated lymphocytes), in comparison with mean SCEs/cell in control cultures, at 0.4 and 0.2 mM concentrations (respectively). Thus, the induction of SCEs by 2-AF was not dependent on the presence of erythrocytes. Styrene (2 mM), a positive control chemical known to require erythrocytes for efficient SCE induction in vitro, was shown to produce a 4.9-fold increase in SCEs in whole-blood cultures, but only a slight (1.3-fold) effect in isolated lymphocyte cultures. The results suggest that leukocytes, but not erythrocytes, are important in the metabolic activation of 2-AF in the human lymphocyte SCE assay.

Mutagenic activity of arylnitrenium ions from arylazides--induction of sister chromatid exchange in mammalian (V79 Chinese hamster) cells

Photolysis of arylazides produces short-lived reactive species, very likely arylnitrenium ions which bind to nucleotides and DNA and produce mutations in Salmonella. The present report shows that arylazides can be photo-activated in mammalian (V79 Chinese hamster) cells and that sister chromatid exchange can thus be induced. Arylazides studied are (in order of decreasing SCE-inducing potency) azido-isoIQ, azido-MeIQ, azido-IQ, azido-MeIQx, azido-PhIP, 6-azido-chrysene, 2-azidofluorene, 4-azidofluorene, 2-azido-naphthalene, 4-azidobi-phenyl, 2-azidobiphenyl, 2,4,6-trimethylphenylazide, phenylazide (inactive). The structure-activity relationships emerging from the data are the same as those found previously in Salmonella. In line with this, a clearcut positive linear correlation was seen between the logarithm of the SCE-inducing potency in V79 cells and the logarithm of the mutagenic potency in Salmonella (r = 0.955). Therefore, the ultimate reactive species derived from IQ and related heterocyclic aromatic amines are extremely potent genotoxins, not only in a bacterial but also in a mammalian cell. Previous findings of only weak genotoxic activity of IQ and related food mutagens in certain cultured mammalian cells must therefore be reinterpreted as the result of an insufficient activation of these amines in the cells used, possibly because of insufficient acetylation competence.

Pentachlorophenol

Pentachlorophenol (PCP) is a substance whose widespread use, mainly in wood protection and pulp and paper mills, has led to a substantial environmental contamination. This in turn accounts for a significant exposure of the general human population, with rather high exposure levels being attained in occupational settings. Investigations on the genotoxic activity of PCP have given rise to divergent results which would seem to make an evaluation difficult. By grouping them into 3 categories a somewhat clearer picture, allowing finally an (admittedly tentative) assessment, can be obtained. PCP does seem to be at most a weak inducer of DNA damage: it produces neither DNA-strand breaks nor clear differential toxicity to bacteria in rec-assays in the absence of metabolic activation. Also in SCE induction no increase can be observed in vivo, while PCP is found marginally active in a single in vitro experiment. Metabolic activation, however, leads to prophage induction and to DNA strand breaks in human lymphocytes, presumably through the formation of oxygen radicals. A possible further exception in this area might be the positive results in the yeast recombination tests, although their inadequate reporting makes a full evaluation difficult. PCP does not seem to induce gene (point) mutations, as most bacterial assays, the Drosophila sex-linked recessive lethal test and in vitro assays with mammalian cells did not demonstrate any effects. Marginally positive results were obtained in the mammalian spot test in vivo and in one bacterial test; the positive result in the yeast assay for cycloheximide resistance is fraught somewhat with its questionable genetic basis. PCP does, however, induce chromosomal aberrations in mammalian cells in vitro and in lymphocytes of exposed persons in vivo. Those in vivo results that were unable to provide evidence of chromosomal damage are hampered either by methodological inadequacies or by too low exposure levels. The (rodent) metabolite tetrachlorohydroquinone might be a real genotoxic agent, capable of binding to DNA and producing DNA strand breaks; this activity is probably due to semiquinone radical formation and partly mediated through active oxygen species. Since this compound has not been tested in the common bacterial and mammalian mutagenicity assays, the few ancillary results on this substance cannot be used in a meaningful human risk assessment of PCP. Furthermore, this metabolite has only been produced by human liver microsomes in vitro, but has not been detected in exposed humans in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Sulfation of mono- and diaryl oximes by aryl sulfotransferase isozymes

Aryl sulfotransferases (3'-phosphoadenylsulfate:phenol sulfotransferase, EC 2.8.2.1) catalyze the sulfonation of a wide variety of hydroxyl-containing substrates, including numerous xenobiotics. The chemical diversity of aryl sulfotransferase substrates is in part attributable to the presence of multiple isozymes, each of which has broad substrate specificity. Of the aryl sulfotransferase isozymes in rat liver cytosol, two (designated isozymes I and II) have previously been shown to sulfonate phenolic compounds exclusively and, moreover, have very similar substrate specificity patterns. The recently reported unusually efficient, rapid isozyme I-catalyzed sulfonation of 9-fluorenone oxime (Mangold, J.B., Mangold, B.L.K. and Spina, A. (1986) Biochim. Biophys. Acta 874, 37-43) was therefore unexpected and suggested that aryl oximes may represent a useful class of model compounds to probe isozymic differences in substrate steric and electronic requirements. In the present study, several mono- and diaryl oximes have been prepared and tested as potential substrates for partially purified aryl sulfotransferases I and II from rat liver cytosol. The results indicate that steric factors, specifically planarity and hydroxyl group position, appear to be important requirements for enzyme-catalyzed sulfonation. In addition, although isozymes I and II had comparable activity with diaryl oximes, some striking differences in the ability of these two isozymes to sulfonate both substituted and unsubstituted monoaryl oximes were observed. This dissimilarity is consistent with distinct differences in the active sites of these isozymes.

Thiol-mediated incorporation of radiolabel from 1-[14C]-methyl-4-phenyl-5-nitrosoimidazole into DNA. A model for the biological activity of 5-nitroimidazoles

1-Methyl-4-phenyl-5-nitrosoimidazole (5NO), which has properties consistent with the biologically active form of a 5-nitroimidazole, was radiolabeled (1-[14C]-methyl) and shown to bind to DNA, but at a rate too slow to account for its bactericidal effect. In the presence of physiological intracellular concentrations of such thiols as glutathione, however, binding was enhanced by 2-3 orders of magnitude, which is quantitatively sufficient to account for the bactericidal effect of 5NO. That 5NO binding was greater for poly[d(G-C).d(G-C)] than for poly[d(A-T).d(A-T)] suggests that the reactive species binds to nucleophilic bases on DNA, a suggestion which is also supported by our finding of a thiol-dependent reaction to form an adduct between 5NO and aniline.

Reduction of intestinal carcinogen absorption by carcinogen-specific secretory immunity

A secretory immune response to the carcinogen 2-acetylaminofluorene (AAF) was elicited in rabbits by directly immunizing the small intestine with an AAF-cholera toxin conjugate. High-titer, high-affinity secretory immunoglobulin A (IgA) antibody to AAF was secreted into the intestinal lumen in response to this immunogen. Immune secretions reduced the transepithelial absorption of a 125I-labeled derivative of AAF by more than half. This reduction of absorption by hapten-specific IgA suggests that oral vaccines against carcinogens and toxicants could be developed for humans.

Variations of DNA-(cytosine-5-)-methyltransferase activities after administration of N-hydroxy-N-aminofluorene to Sprague-Dawley rats

DNA methylation in eukaryotic cells is a post-replicative process involving the transfer of methyl groups from S-adenosyl-L-methionine to the 5 position of cytosine residues through the action of DNA (cytosine-5-)-methyltransferase (DNA-methylase). There are two types of methylation within the cell: a maintenance methylation and a de novo methylation. Its major function is the maintenance methylation of hemimethylated sites after replication in order to preserve the pattern from one generation to the next. Nevertheless DNA-methylase is also able to transfer methyl groups to unmethylated sites in various substrates in a de novo reaction. Male Sprague-Dawley rats have a low specific activity of liver maintenance DNA-methylase and are sensitive to the toxic and carcinogenic effects of N-hydroxy-N-acetylaminofluorene (N-OH-AAF). Female Sprague-Dawley rats, on the contrary, have a 4-5 times higher maintenance DNA-methylase activity and are 6-7 times less sensitive to this carcinogenic effect. Their de novo DNA-methylase activity is the same. When female Sprague-Dawley rats are treated with N-OH-AAF their total DNA-methylase activity diminishes. On the contrary, the maintenance DNA-methylase activity of male Sprague-Dawley rats increases, whereas the de novo activity remains constant. In the spleen, which is not a target organ, the total DNA-methylase activity decreases after injection of N-OH-AAF. These variations of DNA-methylase activity are due to a variation of extractable nuclear DNA-methylase. When Swiss mice, which are not sensitive to the carcinogenic effect, are treated with N-OH-AAF, their total DNA-methylase activity decreases. A decrease of DNA-methylase activity in response to this carcinogen seems to be correlated to the resistance of the animals in developing a hepatocarcinoma.

A rapid and sensitive screening method for the detection of anti-2-acetylaminofluorene immunoglobulins

A method is described in which anti-2-acetylaminofluorene immunoglobulins may be detected using a simple and sensitive screening procedure. The method is based on immunoglobulin binding of an 125I derivatized 2-aminofluorene radiotracer. Tracer binding is not isotype specific, and thus the method is useful for the detection of either IgG or IgA. Competitive binding experiments with the radiotracer were used to determine the specificity of immunoglobulin response by measurement of cross-reactivity with related ligands. This method allows quantitation of the immune response to the carcinogen in serum and other biological fluids (i.e., intestinal secretions).