The role of peroxidases in the activation of chemical carcinogens

Bioactivation of 3-aminobenzanthrone, a human metabolite of the environmental pollutant 3-nitrobenzanthrone: evidence for DNA adduct formation mediated by cytochrome P450 enzymes and peroxidases

3-Nitrobenzanthrone (3-NBA) is a suspected human carcinogen found in diesel exhaust and ambient air pollution. The main metabolite of 3-NBA, 3-aminobenzanthrone (3-ABA), was detected in the urine of salt mining workers occupationally exposed to diesel emissions. We evaluated the role of hepatic cytochrome P450 (CYP) enzymes in the activation of 3-ABA in vivo by treating hepatic cytochrome P450 oxidoreductase (POR)-null mice and wild-type littermates intraperitoneally with 0.2 and 2mg/kg body weight of 3-ABA. Hepatic POR-null mice lack POR-mediated CYP enzyme activity in the liver. Using the (32)P-postlabelling method, multiple 3-ABA-derived DNA adducts were observed in liver DNA from wild-type mice, qualitatively similar to those formed in incubations using human hepatic microsomes. The adduct pattern was also similar to those formed by the nitroaromatic counterpart 3-NBA and which derive from reductive metabolites of 3-NBA bound to purine bases in DNA. DNA binding by 3-ABA in the livers of the null mice was undetectable at the lower dose and substantially reduced (by up to 80%), relative to wild-type mice, at the higher dose. These data indicate that POR-mediated CYP enzyme activities are important for the oxidative activation of 3-ABA in livers, confirming recent results indicating that CYP1A1 and -1A2 are mainly responsible for the metabolic activation of 3-ABA in human hepatic microsomes. No difference in DNA binding was found in kidney and bladder between null and wild-type mice, suggesting that cells in these extrahepatic organs have the metabolic capacity to oxidize 3-ABA to species forming the same 3-ABA-derived DNA adducts, independently from the CYP-mediated oxidation in the liver. We determined that different model peroxidases are able to catalyse DNA adduct formation by 3-ABA in vitro. Horseradish peroxidase (HRP), lactoperoxidase (LPO), myeloperoxidase (MPO), and prostaglandin H synthase (PHS) were all effective in activating 3-ABA in vitro, forming DNA adducts qualitatively similar to those formed in vivo in mice treated with 3-ABA and to those found in DNA reacted with N-hydroxy-3-aminobenzanthrone (N-OH-ABA). Collectively, these results suggest that both CYPs and peroxidases may play an important role in metabolizing 3-ABA to reactive DNA adduct forming species.

Heme Peroxidases: Structure, Function, Mechanism and Involvement in Activation of Carcinogens. A Review

Peroxidases are enzymes playing an important role in large and diverse numbers of physiological processes in organisms including human. We have attempted in this article to summarize and review the important structural and catalytic properties of principal classes of heme peroxidases as well as their biological functions. Major reactions catalyzed by these enzymes (a conventional peroxidase cycle, reactions using O2and halogenations) and their mechanism are reviewed, too. Moreover, the reaction mechanisms by which peroxidases are implicated in bioactivation of xenobiotic chemicals are presented. Numerous chemicals including protoxicants and procarcinogens are metabolized by equally numerous chemical reactions catalyzed by peroxidases. The unifying theme is the radical nature of the oxidations. The direct conventional peroxidase reaction forming reactive species is generally responsible for the activation of procarcinogenic substrates of peroxidases. The subsequent formation of a superoxide anion radical and peroxy radicals is necessary for activation of chemicals that are poor substrates for peroxidases. The significance of studies concerning the reactions catalyzed by peroxidases is underlined in the present review article. A review with 166 references.