Metabolic activation of a protein pyrolysate promutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline by rat liver microsomes and purified cytochrome P-450

Abstract

The enzymatic activation of a promutagenic pyrolysate, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), was studied using the Ames mutagenesis test system. The enzyme catalyzing the mutagenic activation of MeIQx is mainly localized in the microsomal fraction. A large number of revertants was observed in the presence of hepatic microsomes obtained from 3-methylcholanthrene (3-MC)- or polychlorinated biphenyl (PCB)-treated rats but only a minimal number with the hepatic microsomes from untreated or phenobarbital (PB)-treated rats. In addition, the microsomal activation was reduced efficiently by known inhibitors of cytochrome P-450-mediated reactions such as 7,8-benzoflavone, ellipticine and flavone. Among five forms of purified rat cytochrome P-450, the highest sp. act. (no. of revertants induced/nmol cytochrome P-450) for the activation of MeIQx was observed with a high-spin form of cytochrome P-450, P-448-H, followed by the low-spin form, P-448-L, and to a lesser extent by PB-inducible forms, P-450b and P-450e. P-450-male, which is a main constitutive form of cytochrome P-450 in male rat livers, showed considerable catalysis for the mutagenic activation of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and MeIQx. These results indicate that the metabolic activation of MeIQx is catalyzed mainly by two forms of cytochrome P-450, P-448-H and P-488-L, in the livers of PCB- or 3-MC-treated rats, but also that P-450-male may play an important role in the activation in livers of intact male rats.

Down‐regulation of 2‐Amino‐3,8‐dimethylimidazo[4,5‐f]quinoxaline (MeIQx)‐induced CYP1A2 Expression Is Associated with Bovine Lactoferrin Inhibition of MeIQx‐induced Liver and Colon Carcinogenesis in Rats

“…17,18) Levels of CYP1A2 protein induced by MeIQx were significantly decreased to almost normal levels by the concomitant administration of 2% bLF at weeks 3 and 8 in the liver, independently of initial DEN treatment (Fig. 3).…”

“…15,16) MeIQx is thought to be metabolically activated to a mutagenic/carcinogenic intermediate by initial N-oxidation in the liver, mediated primarily by cytochrome P450 (CYP) 1A2 to yield N-hydroxyMeIQx. 17,18) This is further activated through esterification reactions catalyzed by arylamine N-acetyltransferase (NAT) or phenol sulfotransferase (PST) to yield N-acetoxy-or N-sulfonyloxy-MeIQx, respectively, whose more reactive metabolites can covalently bind to DNA to form MeIQx-DNA adducts. 19,20) Previous studies have shown that at least two forms of NAT, NAT1 and NAT2, and three forms of PST, ST1A1, ST1B1 and ST1C1, are contained in rat livers.…”

Human cytochrome P-450PA (P-450IA2), the phenacetin O-deethylase, is primarily responsible for the hepatic 3-demethylation of caffeine and N-oxidation of carcinogenic arylamines.

“…8 and 9). ¶ Studies with purified rat and rabbit P-450s have shown high specificity for the N-oxidation of 2-acetylaminofluorene, 2-NA, ABP, and several heterocyclic amines to their proximate carcinogenic and/or mutagenic forms by the P450IA2 gene products in various species (8)(9)(10)(11)(12)(13)(14)(15). In humans, the orthologous P-450 (16, 17), termed P-45OPA (also HLd), appears to be primarily responsible for ABP N-oxidation (15) and for the mutagenic activation of several heterocyclic amines (18).…”

Enhanced liver tumor promotion but not liver initiation activity in rats subjected to combined administration of omeprazole and β-naphthoflavone

“…MeIQx is activated by CYP1A2, and the metabolites produced are highly reactive and bind covalently to DNA (Yamazoe et al, 1988). Previous reports have shown that CYP1A2 inducers such as fenbendazole and caffeine did not enhance the MeIQx-induced hepatocarcinogenesis in rats (Suzuki et al, 2002;Kuribayashi et al, 2006).…”