Sulfuric acid esters as major ultimate electrophilic and hepatocarcinogenic metabolites of 4-aminoazobenzene and its N-methyl derivatives in infant male C57BL/6J x C3H/HeJ F1 (B6C3F1) mice

Azo dyes and human health: A review

Synthetic azo dyes are widely used in industries. Gerhardt Domagk discovered that the antimicrobial effect of red azo dye Prontosil was caused by the reductively cleaved (azo reduction) product sulfanilamide. The significance of azo reduction is thus revealed. Azo reduction can be accomplished by human intestinal microflora, skin microflora, environmental microorganisms, to a lesser extent by human liver azoreductase, and by nonbiological means. Some azo dyes can be carcinogenic without being cleaved into aromatic amines. However, the carcinogenicity of many azo dyes is due to their cleaved product such as benzidine. Benzidine induces various human and animal tumors. Another azo dye component, p-phenylenediamine, is a contact allergen. Many azo dyes and their reductively cleaved products as well as chemically related aromatic amines are reported to affect human health, causing allergies and other human maladies.

Suppression of sulfoconjugation reduces the protective effect of ortho-aminoazotoluene on hepatocarcinogenesis induced by diethylnitrosamine in mice

The effects of ortho-aminoazotoluene on carcinogenic activity of diethylnitrosamine were studied in CBA and ICR mice. Injection of ortho-aminoazotoluene before and after diethylnitrosamine led to a significant reduction of its anticarcinogenic effect, judging from significantly lower level of liver tumors. Pentachlorophenol, inhibitor of sulfotransferase (catalyzing the terminal stage of ortho-aminoazotoluene metabolic activity), stimulated its carcinogenic effect on mouse liver. On the other hand, pentachlorophenol reduced the protective effect of ortho-aminoazotoluene on diethylnitrosamine-induced hepatocarcinogenesis in mice. Presumably, the carcinogenic and anticarcinogenic effects of ortho-aminoazotoluene were realized by its initial form or intermediate (non-sulfated) metabolites.

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.

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).

Neonatal mouse assay for tumorigenicity: alternative to the chronic rodent bioassay

The chronic rodent bioassay for tumors has been utilized systematically for 25 years to identify chemicals with carcinogenic potential in man. In general, those chemicals exhibiting tumorigenicity at multiple sites in both mice and rats have been regarded as possessing strong carcinogenic potential in humans. In comparison, the value of data collected for those test chemicals exhibiting more sporadic tumorigenicity results (e.g., single species/single sex or dose-independent) has been questioned. As knowledge of the carcinogenic process has increased, several alternative test systems, usually faster and less expensive than the 2-year bioassay, have been suggested for identification of the strongly acting, transspecies carcinogens. The International Conference on Harmonization for Technical Requirements for the Registration of Pharmaceuticals for Human Use has proposed an international standard that allows for the use of one long-term rodent carcinogenicity study, plus one supplementary study to identify potential human pharmaceutical carcinogens. The neonatal mouse assay for tumorigenicity has been used since 1959; however, relative to other alternate tests, little has been written about this system. It is clear that this assay system successfully identifies transspecies carcinogens from numerous chemical classes, thus recommending itself as a strong candidate for a supplementary study to identify potential human carcinogens. In contrast, there are decidedly less data available from this assay in response to pharmaceuticals shown to exhibit weak and/or conflicting results in the 2-year bioassay, knowledge invaluable to the regulatory process. This paper reviews the historical development and our experience with the neonatal mouse assay and includes suggestions for a standardized protocol and strategies to document its response to "weak" and/or "nongenotoxic" carcinogens.

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.

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.

Azoreductase activity by purified rabbit liver aldehyde oxidase

Our laboratory has investigated the azoreduction of dimethylaminoazobenzene (DAB) and its analogs by hepatic microsomal cytochrome P450. We have extended these studies to the cytosolic fraction of the mammalian liver using the molybdoflavoenzyme, aldehyde oxidase. Purified rabbit liver aldehyde oxidase readily reduced azo dyes which are mainly water soluble and contain charged groups. Lipophilic azo dyes, although readily reduced by microsomal cytochrome P450, were either poor substrates or not reduced at all. Kinetic measurements revealed no relationship between Vmax and Km for all dyes. More extensive studies were conducted on four azo dyes, o-methyl, red, 2'-pyridyl-DAB, sulfonazo III and Orange II, with characteristic functional groups. With each of these substrates, azoreductase activity was greatest when 2-hydropyrimidine (2-OHP) was the electron donor compared to N1-methylnicotinamide (N-MN), propionaldehyde and butyraldehyde. With 2-OHP as the electron donor, o-methyl red and 2'-pyridyl DAB exhibited maximal activity at pH 5.0 while sulfonazo III and Orange II showed maximal activity at pH 9.5 and 7.0, respectively. Km values for o-methyl red and 2'-pyridyl DAB were lower at their pH optima whereas that for sulfonazo III was higher at its pH optimum. There was also no correlation between maximal activity and Km; apparently Km is not a primary determinant for activity. The degree of ionization of function groups depends on pH. Since highest activity is seen at that pH in which maximal ionization of the substrate occurs, it can be concluded that rate of reduction is at least partially dependent on the charged state of the substrate. Azoreduction was inhibited by menadione and SKF 525-A. Sensitivity to inhibition by menadione was greatest at the pH where 2-OHP exhibited considerably higher activity than N-MN, but no differential was seen at the pH where activities with the two-electron donors were similar. On the other hand, sensitivity of azoreductase activity to inhibition by SKF 525-A was the same irrespective of electron donor, indicating that the mechanisms for these two inhibitors were different.

Age- and sex-related differences in activation of the carcinogen 7-hydroxymethyl-12-methylbenz[a]anthracene to an electrophilic sulfuric acid ester metabolite in rats. Possible involvement of hydroxysteroid sulfotransferase activity

Metabolic activation of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA) and related hydroxymethyl polycyclic aromatic hydrocarbons to electrophilic and mutagenic sulfuric acid esters has been demonstrated previously (Watabe et al., In: Xenobiotic Metabolism and Disposition (Eds. Kato R, Estabrook RW and Cayen MN), pp. 393-400. Taylor & Francis, London, 1989). In the present study, the rat hepatic sulfotransferase activity catalyzing the formation of such reactive sulfuric acid esters was inhibited strongly by dehydroepiandrosterone, a typical substrate hydroxysteroid sulfotransferases (HSSTs). Pentachlorophenol, a potent phenol sulfotransferase inhibitor, had little effect in this regard. A marked sex difference was observed for the hepatic cytosolic sulfotransferase activity for HMBA in rats. This sex difference was age-related; no significant difference was observed in preweanling rats, whereas in adult rats female rat liver showed a much higher enzyme activity. These age- and sex-related differences in the sulfonation of HMBA reflect the regulation of HMBA sulfotransferase activity by gonadal hormones as previously demonstrated with HSSTs. Thus, pretreatment with estradiol benzoate significantly enhanced the sulfotransferase activity for HMBA in both male and female rats, (P less than 0.01 and P less than 0.05 respectively), whereas testosterone propionate pretreatment decreased this activity. Castration of male rats increased the HMBA sulfotransferase activity 2- to 3-fold compared with that in control animals. By contrast, ovariectomy reduced the enzyme activity 38% in females. These results imply that rat liver HSST activity is responsible for the sulfonation of HMBA. Intraperitoneal injection of HMBA (0.25 mumol/g body wt) into infant rats produced benzylic DNA adducts in the liver which were chromatographically identical with those obtained from incubations of HMBA with deoxyguanosine and deoxyadenosine in the presence of hepatic cytosolic sulfotransferase activity. Intraperitoneal administration of sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene resulted in much higher levels of these adducts and the deoxycytidine adduct in the liver DNA than did an equimolar amount of the parent hydroxymethyl hydrocarbon. The levels of hepatic benzylic DNA adducts formed from HMBA were reduced markedly by pretreatment of rats with dehydroepiandrosterone, a strong inhibitor of hepatic sulfotransferase activity for this hydrocarbon.

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)

Metabolism of azo dyes: implication for detoxication and activation

Azo dyes are consumed and otherwise utilized in varying quantities in many parts of the world. Such widely used chemicals are of great concern with regard to their potential toxicity and carcinogenic properties. Their metabolism has been studied extensively and is significant for detoxication and metabolic activation. Both oxidative and reductive pathways are involved in these processes. The majority of azo dyes undergo reduction catalyzed by enzymes of the intestinal microorganisms and/or hepatic enzymes including microsomal and soluble enzymes. The selectivity of substrate and enzyme may to a large extent be determined by the oxygen sensitivity of reduction since a normal liver is mainly aerobic in all areas, whereas the microorganisms of the lower bowel exist in an anaerobic environment. However, it should be pointed out that the pO2 of centrilobular cells within the liver is only a fraction that of air, where pO2 = 150 torr. Therefore, an azo dye reduction experiment performed aerobically may not be an accurate predictor of reductive metabolism in all areas of the liver. Many of the azo dyes in common use today have highly charged substituents such as sulfonate. These resist enzymic attack and for the most part are poorly absorbed from the intestinal tract, providing poor access to the liver, the major site of the mixed-function oxidase system. Lipophilic dyes, such as DAB, which are often carcinogenic, readily access oxidative enzymes and are activated by both mixed-function oxidase and conjugating systems. Reduction of the carcinogenic dyes usually leads to loss of carcinogenic activity. By contrast, most of the highly charged water-soluble dyes become mutagenic only after reduction. Even then, most of the fully reduced amines required oxidative metabolic activation. An outstanding example is the potent human bladder carcinogen benzidine, which derives from the reduction of several azo dyes. Many problems regarding mutagenic and carcinogenic activation remain to be solved. At the present time, it is apparent that both oxidative and reductive pathways yield toxic products. Toxicologic assessment of azo dyes must consider all pathways and particularly the oxygen sensitivity of azoreduction. This is critical in the treatment of waste from chemical plants where there is a great need for soil bacteria which catalyze reduction aerobically. Consideration of secondary pathways are also of great concern. For example, azoreduction of carcinogenic dyes such as DAB removes carcinogenic activity although oxidative metabolism of the primary amines yield mutagenic products. Such apparent dilemmas must be dealt with when considering metabolism/toxicity relationships for azo dyes.

Structural basis of the mutagenicity of phenylazoaniline dyes

The CASE (Computer-Automated Structure Evaluation) methodology was used to gain an understanding of the basis of mutagenicity of phenylazoanilines. It was found that the activity of these molecules is dependent upon an intact moiety that spans the azo linkage, i.e., the azo bond must remain intact for mutagenicity. The study also addressed the effect of sulfonation on the activity of these azo dyes. It was revealed that sulfonation at only certain sites resulted in loss of mutagenicity. Sulfonation of the structural moiety responsible for the activity of phenylazoaniline dyes did not necessarily result in complete elimination of activity as this substitution could generate new structural moieties which contribute to the activity of the molecules.

Relationships between structure and mutagenic activity of environmental chemicals

This review analyzes relationships between chemical structure and biological activity for several series of compounds. Its focus is on mutagenicity and carcinogenicity and the predictability of these properties on the basis of the chemical structure. Examples are selected from monocyclic aromatic amines, benzidine derivatives, aminoazobenzene derivatives, nitrofurans, aflatoxins, and sterigmatocystins. Results from mutagenicity tests in Salmonella typhimurium are summarized, and their correlation with carcinogenicity is discussed. The review is concluded with generalizations on the usefulness of studies on relationships between chemical structure and mutagenic activity.

A fluorometric assay for quantitating phenol sulfotransferase activities in homogenates of cells and tissues

A new, rapid, and sensitive method for assaying phenol sulfotransferase activity toward 2-naphthol is described. The product 2-naphthyl sulfate is quantitated fluorometrically. Optimal wavelengths for excitation and emission were determined by recording the three-dimensional fluorescence spectra of the substrate and the product. The new method is applicable to crude cell or tissue homogenates as well as to further purified preparations. A comparison to another widely used method is given to point out the advantages provided by the new procedure. In particular, sensitivity and accuracy of both methods are evaluated and the influence of interfering substances on both systems is compared. These results clearly indicate the superiority of the new method.

Hepatic DNA and RNA adduct formation from the carcinogen 7-hydroxymethyl-12-methylbenz[a]anthracene and its electrophilic sulfuric acid ester metabolite in preweanling rats and mice

DNA and RNA adducts that were chromatographically identical to those formed in vitro on reaction of 7-sulfooxymethyl-12-methyl-benz[a]anthracene with guanine and adenine nucleosides were formed in the livers of rats and mice given i.p. injections of 7-hydroxymethyl- or 7-sulfooxymethyl-12-methyl-benz[a]anthracene. Considerably higher levels of these hepatic adducts were obtained from the latter short-lived electrophilic ester than from the hydroxymethyl compound. These observations are consistent with the finding of rat liver cytosolic sulfotransferase activity for 7-hydroxymethyl-12-methylbenz[a]anthracene (Watabe et al., Science 215, 403, 1982). Formation of these hepatic adducts from 7-hydroxymethyl-12-methylbenz[a]anthracene was inhibited by prior administration to rats of dehydroepiandrosterone, an inhibitor of the sulfotransferase activity for this hydroxymethyl hydrocarbon.