Activation of Heterocyclic Aromatic Amines by Rat and Human Liver Microsomes and by Purified Rat and Human Cytochrome P450 1A2

Abstract

The dietary mutagens 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are activated to genotoxins by rat and human liver cytochrome P450 (P450) 1A1- and 1A2-mediated N-oxidation. Immunoquantitation of 51 human liver samples revealed a wide range in P450 1A2 expression (10-250 pmol/mg of microsomal protein, median 71 pmol/mg), with 39% of the livers containing >100 pmol/mg of protein. There was no evidence for expression of P450 1A1 (<1 pmol/mg of protein). P450 1A2 levels were correlated to MeIQx and PhIP N-oxidation rates (r = 0.83, 0.73, respectively). In male Fischer-344 and Sprague-Dawley rats, hepatic P450 1A2 ranged from 5 to 35 pmol/mg of protein, while P450 1A1 was <1 pmol/mg. Animal pretreatment with 3-methylcholanthrene, beta-naphthoflavone, or polychlorinated biphenyls (PCB) resulted inasmuch as 340-fold and >1000-fold induction of P450 1A2 and 1A1, respectively, and a 220-fold increase in N-oxidation activity. Approximately 20% of the human samples were as active in N-oxidation and conversion of MeIQx to bacterial mutagens as microsomes of PCB-pretreated rats [3-4 nmol of NHOH-MeIQx formed min-1 (mg of protein)-1]. In contrast, microsomes from PCB-treated rats displayed higher rates of PhIP N-oxidation and activation to mutagens than the most active human liver microsomes [8-24 vs 2-4 nmol of HNOH-PhIP formed min-1 (mg of protein)-1]. Recombinant human P450 1A2 showed catalytic efficiencies of MeIQx and PhIP N-oxidation that were 10-19-fold higher than purified rat P450 1A2. Cytochrome P450 1A2 expression in rodent and human liver tissue varies greatly and there are considerable differences between the enzymes in the two species in the activation of some heterocyclic aromatic amines, which must be considered when assessing human health risk.

Metabolism of a Heterocyclic Amine Colon Carcinogen in Young and Old Rats

“…IQ has been shown to be metabolized by human CYP1A1 and 1A2, although to a much greater extent by CYP1A2 (McManus et al, 1990). However, of relevance to these studies, BNF increases CYP1A1 activity to a greater degree than 1A2 activity in rodents (Turesky et al, 1998). Preliminary studies with liver slices from young rats have demonstrated almost complete blockage of 5-O-glucuronide and 5-sulfate formation by ellipticine, a CYP1A1 inhibitor (data not shown).…”

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).…”

Regulation and function of family 1 and family 2 UDP-glucuronosyltransferase genes (UGT1A, UGT2B) in human oesophagus

“…PhIP undergoes N-hydroxylation by cytochromes P4501A1 and P4501A2 at the two-amino position [42] to generate the mutagenic metabolite [43]. Since P4501A1 is not expressed abundantly in liver [44] and P4501A2 is limited to expression in hepatic tissue, it has been proposed [45] that N-hydroxy-PhIP is transported from the liver in the circulation to target tissues where it forms DNA adducts. Thus, understanding which of the UGTs participates in the metabolism of PhIP is important.…”