Prostaglandin H synthase-mediated reaction of carcinogenic arylamines with tRNA and homopolyribonucleotides

Benzidine mechanistic data and risk assessment: species- and organ-specific metabolic activation

The aromatic amine benzidine (BZ) has produced various tumors, including liver tumors, in mice, rats and hamsters. BZ forms DNA adducts in rodent liver, and it is positive in most genotoxicity tests. Only bladder tumors are produced in dogs and in humans who have been occupationally exposed, possibly related to the slow rate of liver detoxification by acetylation, allowing activation of BZ or its metabolites in urine. Despite these differences, risk assessment for humans, based on liver tumors in mice, was approximately predictive of the incidence of bladder tumors observed in industrially exposed humans.

Mutagenic response of mouse lymphoma cells after activation of benzidine and 2-aminofluorene with purified prostaglandin H synthase

The mutagenic response of L5178Y mouse lymphoma cells to the model aromatic amine carcinogens, benzidine (BNZ) and 2-aminofluorene (2-AF) in the presence of the mammalian peroxidase prostaglandin H synthase (PHS) was examined. Standard incubation conditions for mouse lymphoma cells and the PHS system were developed. The cells were exposed to BNZ and 2-AF with purified PHS in the presence or absence of a peroxide, 5-phenyl-4-pentenyl hydroperoxide (PPHP) which is required for PHS-dependent amine oxidation. Incubations were carried out in a medium consisting of Hanks' balanced salt solution with calcium and magnesium and 0.1% pluronic F-68. BNZ by itself or in the presence of PPHP induced a weak mutagenic response in mouse lymphoma cells, but the addition of PHS or PHS and its co-factor PPHP increased the mutagenic response approximately 5-fold over that observed in the absence of PHS. A maximal mutagenic response for BNZ was observed after incubation with the complete activating system, PHS and PPHP. These data are in agreement with the fact that BNZ is an excellent substrate for PHS. When 2-AF was incubated with mouse lymphoma cells, only a minimal mutagenic response was observed. Incubation of 2-AF with either PPHP or PPHP and PHS (complete peroxidase system produced a significant enhancement in mutagenic response. Thus, the mutagenic response of the mouse lymphoma cells to 2-AF was dependent on the peroxide, PPHP but not the enzyme PHS. These data suggest that 2-AF, which is a poor PHS substrate, is oxidized by a different catalyst than PHS. This work demonstrates that BNZ and 2-AF are converted by peroxide-dependent mechanisms to mutagens that can be detected in mammalian cells.

Activation of aromatic amines by prostaglandin H synthase

Prostaglandin H synthase catalyzes the first step in the synthesis of prostaglandins from arachidonic acid. The peroxidase activity of this enzyme can support the oxidation of xenobiotics, particularly aromatic amines. This pathway of metabolism may contribute to the activation of carcinogenic aromatic amines in target tissues such as the skin, lung, and bladder. In this review, recent work on this subject is summarized. I emphasize the elucidation of the structures of aromatic amine oxidation products, and their interactions with biological macromolecules. Prostaglandin H synthase supports the activation of benzidine to a mutagenic species in the Ames (Salmonella typhimurium) test, and our studies of the mechanism of this activation are described.

The formation of 2-aminofluorene-DNA adducts in vivo: evidence for peroxidase-mediated activation

Formation of DNA adducts in various tissues of dogs fed a single dose of the carcinogen 2-aminofluorene was investigated. Adduct analysis was performed using a technique that allows measurement of both N-(deoxyguanosin-8-yl)-2-amino-2-aminofluorene-DNA adduct formed by reaction of N-hydroxy-2-aminofluorene with DNA, as well as the polar 2-aminofluorene-DNA adducts formed when 2-aminofluorene is activated by prostaglandin H synthase-peroxidase in vitro. Two male beagle (A and B) dogs were examined and a different DNA adduct profile was observed with each dog. For the dog A, N-(deoxyguanosin-8-yl)-2-aminofluorene was the major adduct found in hepatic DNA; no peroxidase-derived adducts were detected in this tissue. In contrast, adducts eluting similarly to peroxidase-derived adducts were found in urinary tract tissues of this dog with the relative abundance of these adducts in the order urothelium greater than renal medulla greater than renal cortex, which correlates with the respective tissues' prostaglandin H synthase activity. N-(Deoxyguanosin-8-yl)-2-aminofluorene was detected in the renal tissues, but not in urothelium. For dog B, only the N-(deoxyguanosin-8-yl)-2-aminofluorene adduct was observed in all tissues examined, including the urothelium. However, total binding to liver, kidney, and bladder were two-, two-, and four-fold lower, respectively, than dog A. These data indicate that both prostaglandin H synthase-mediated activation and N-hydroxylation of 2-aminofluorene occur in vivo and may be subjected to pharmacodynamic considerations. Furthermore, the tissue distribution of the peroxidase-mediated 2-aminofluorene adducts suggests this process may also be of importance in the bladder-specific carcinogenicity of aromatic amines.

Co-oxidation of xenobiotics: lipid peroxyl derivatives as mediators of metabolism

Systems which carry out peroxyl-dependent oxidations can serve as activation systems for carcinogenic compounds. Some function via classical peroxidase reactions in which an enzyme-derived oxidant performs the electron abstraction from or oxygen donation to the oxidizable substrate. This mechanism applies to the peroxidative activation of aromatic amines and of the phenolic compound diethylstilbestrol. These classical peroxidase reactions may be initiated by hydrogen peroxide or by organic peroxides, including lipid hydroperoxides. A different mechanism is involved in the oxygenation of polycyclic aromatic hydrocarbons and of aflatoxin B1. In these cases the oxidant is a peroxyl radical, and the reaction occurs by the direct, non-enzymatic interaction of the peroxyl radical and the oxidizable substrate. Most peroxyl radicals in biological systems are lipid-derived. The key reaction which distinguishes the peroxyl radical-dependent oxidations from the classical peroxidase reactions is the ability of the former to epoxidize activated carbon-carbon double bonds. The epoxidation of benzo[a]pyrene derivatives has been studied extensively in subcellular and whole cell and tissue systems, and is discussed as a model for this class of reaction. Determining the generality of this activation path and its role in vivo present the major questions to be answered in regard to the importance of these reactions in chemical carcinogenesis.

Cytochemical expression of epidermal peroxidase and cytochrome oxidase activities in pathological skin conditions of man

The cytochemical expression of epidermal peroxidase and cytochrome oxidase activity was recently well documented in normal human skin. We report here its expression in basal and squamous cell carcinomas, actinic keratoses, psoriasis, allergic contact dermatitis, seborrheic keratoses, and autosomal dominant ichthyosis vulgaris. The two enzyme activities were evaluated using the diaminobenzidine method. If present, the two enzymes were always localized in the same organelles as in normal epidermis: endogenous peroxidase in the nuclear envelope and endoplasmic reticulum, and cytochrome oxidase in mitochondria. In basal and squamous carcinomas, actinic keratoses and psoriasis, the keratinocytes lost their peroxidase activity, but maintained their cytochrome oxidase activity. In seborrheic keratoses, allergic contact dermatitis and ichthyosis vulgaris, the cytochrome oxidase activity was greatly reduced or abolished in keratinocytes, Langerhans' cells, and melanocytes, whereas the peroxidase activity was present as in normal epidermis. These results indicate that the two peroxidatic enzymes studied are not interrelated and alternatively suppressed by different cellular dysfunctions.

Induction of sister chromatid exchanges by benzidine in rat and human hepatoma cell lines and inhibition by indomethacin

The genotoxic activity of benzidine was studied in two cell lines derived from rat (H4) and human (HepG2) hepatomas which have been shown to be capable of activating certain promutagens. The responses were compared to results in two lung-derived fibroblast lines (IMR-90 and V79) which appear to have little or no metabolizing capability. Benzidine was found to induce sister chromatid exchanges in the two liver-derived cell lines in a dose-dependent fashion but failed to induce sister chromatid exchanges in the fibroblast lines. Since one proposed pathway for benzidine activation involves prostaglandin-mediated metabolism, we tested the effect of pretreatment with indomethacin, an inhibitor of this metabolic pathway. Indomethacin was highly effective in inhibiting benzidine-induced sister chromatid exchanges in both H4 and HepG2 cells. These results suggest that some DNA damage may occur in the livers of fast acetylating species such as the rat without prior N-acetylation and that some amount of DNA damage may occur in the livers of slow acetylating species, even when the liver is not the target organ for carcinogenesis.

Formation and persistence of arylamine DNA adducts in vivo

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

Peroxidase-catalyzed benzidine binding to DNA and other macromolecules

[14C]Benzidine is rapidly oxidized by a peroxidase/H2O2 system to products which bind irreversibly to DNA. The presence of exogenous DNA also prevented benzidine polymerization to 'benzidine brown' and azobenzidine. Two molar equivalents of H2O2 were required to oxidize the benzidine and achieve maximal DNA binding. Furthermore, 95% of the benzidine was trapped and 36 nmol benzidine was bound per mg DNA. Polyriboguanylic acid was as effective as DNA in binding benzidine, but polyriboadenylic acid, polyribouridylic acid and polyribocytidylic acid were much less effective. Binding of [14C]benzidine correlated well with the absorbance at 295 nm and 390 nm of the modified DNA or various synthetic homopolymers of ribonucleotides isolated from the reaction mixture. The peroxidase/H2O2 system also catalyzed the binding of dichlorobenzidine, o-tolidine and o-dianisidine to DNA but 3,5,3',5'-tetramethylbenzidine, a non-carcinogen, did not bind. The binding could be prevented by various biological hydrogen donors, thiols, or phenolic antioxidants. The mechanisms for DNA protection were investigated; the oxidized benzidine species involved in binding can be reduced with ascorbate, NADPH, or thiols, and trapped by thiols or phenolic antioxidants to form conjugates or adducts.

Peroxidase-mediated irreversible binding of arylamine carcinogens to DNA in intact polymorphonuclear leukocytes activated by a tumor promoter

Addition of the tumor promoter phorbol myristate acetate to polymorphonuclear leukocytes results in the oxidation of the arylamine carcinogens; [14C]benzidine, N-[14C]methylaminoazobenzene and [14C]aminofluorene to reactive intermediate(s) that bind irreversibly to the leukocyte DNA. The binding was dependent on oxygen and was decreased by sulfhydryl inhibitors and phenolic antioxidants that inhibit the respiratory burst triggered by the phorbol myristate. Both the binding and the respiratory burst were increased by azide, presumably as a result of intracellular catalase inhibition. However higher concentrations of azide and cyanide prevented binding without affecting the respiratory burst indicating that myeloperoxidase is a catalyst for the binding. Granules isolated from the activated leukocytes and H2O2 catalyzed a cyanide sensitive benzidine binding to calf thymus DNA. Myeloperoxidase and H2O2 also catalysed extensive binding of these arylamines to calf thymus DNA. The leukocytes appear to be a useful model cell for studying one electron oxidation-catalyzed carcinogen activation.

Reactive products formed by peroxidase catalyzed oxidation of p-phenetidine

The nature of the reactive metabolites formed during HRP/H2O2 catalyzed oxidation of p-phenetidine was investigated. Interaction with DNA measured as the induction of DNA single strand breaks and DNA binding resulted in a time-dependent decrease in the interaction and could be related to the primary oxidation of p-phenetidine. Oxygen uptake observed during p-phenetidine metabolism in the presence of GSH also exhibited such a correlation. GSH-conjugate formation and protein binding on the other hand exhibited an initial increase and did not appear to be directly related to primary p-phenetidine oxidation since maximal interaction was obtained when p-phenetidine had been completely metabolized. The GSH-conjugate and protein binding ratio of ring labelled to ethyl labelled p-phenetidine of approx. 2:1 indicated that these reactive metabolites(s) may be dimer(s) whose formation presumably involved loss of one ethoxy group of p-phenetidine. Accordingly formation of ethanol, indicative of ethoxy group elimination, could be observed during p-phenetidine metabolism. Only one metabolite generated from p-phenetidine oxidation exhibited a concentration dependent binding to protein. This metabolite also reacted with GSH to form water-soluble conjugates. Prior reduction of the metabolite by ascorbic acid prevented this conjugate formation. The mass spectral fragmentation pattern of the reactive protein- and GSH-binding metabolite was compatible with the structure N(4-ethoxyphenyl)-p-benzoquinoneimine.