Metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by human cytochrome P4501A1, P4501A2 and P4501B1

Abstract

While the metabolic activation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by N-hydroxylation has been well documented, the relative roles of the human cytochrome P450 (CYP) enzymes that catalyze this reaction have not been established. Previous studies indicated that the mutagenic activation product, 2-hydroxyamino-PhIP (N2-OH-PhIP), is produced primarily by CYP1A2, and to a lesser extent by CYP1A1. We recently reported that human CYP1B1 also produces N2-OH-PhIP (Carcinogenesis, 18, 1793-1798, 1997). In the present study, we examined PhIP metabolism by microsomes containing recombinant human CYP1A1, 1A2 or 1B1 expressed in Sf9 insect cells and compared the kinetic values for PhIP metabolite formation. PhIP metabolites were analyzed by high pressure liquid chromatography with fluorescence and absorbance detection. Vmax values for N2-OH-PhIP formation were 90, 16 and 0.2 nmol/min/nmol P450, and the apparent Km values were 79, 5.1 and 4.5 microM for human CYP1A2, 1A1 and 1B1, respectively. The non-mutagenic metabolite, 4'-hydroxy-PhIP, was also formed by all three CYP enzymes with Vmax values of 1.5, 7.8 and 0.3 nmol/ min/nmol P450 and apparent Km values of 43, 8.2 and 2.2 microM for human CYP1A2, 1A1 and 1B1, respectively. Although the Vmax for N2-OH-PhIP production was highest for CYP1A2, the catalytic efficiency (Vmax/Km) of CYP1A1 was greater than that of CYP1A2. These results suggest that, for humans, extrahepatic CYP1A1 may be more important than previously thought for the metabolic activation of the dietary carcinogen PhIP.

2‐Amino‐1‐methyl‐6‐phenylimidazo[4,5‐b]pyridine (PhIP)–induced mutagenesis in cultured Big Blue™ rat mammary epithelial and fibroblast cells

“…This is consistent with the finding that mammary gland microsomes from rats have almost no capacity to activate PhIP [Davis et al, 1994;Ghoshal et al, 1995]. Expression of cytochrome P450 forms 1A1 and 1B1 [Christou et al, 1995], both of which can catalyze PhIP N-hydroxylation to produce N-OH-PhIP [Crofts et al, 1998], has been reported in rat mammary epithelial and fibroblast cells. We attempted to measure P450 activity (EROD assay) in our mammary cell lines after treatment with the P450 inducers benzo[a]pyrene and TCDD, but no activity was detected (data not shown).…”

The new vertebrate CYP1C family: Cloning of new subfamily members and phylogenetic analysis

“…Mammalian CYP1A1 and CYP1A2 are active in the biotransformation of planar aromatic hydrocarbons and both aryl and heterocyclic amines, respectively [1][2][3], while CYP1B1 substrate specificity encompasses that of the two CYP1As [4][5][6]. CYP1B1 is the only known member of the CYP1B subfamily in humans and presumably other mammals, and exhibits the highest catalytic activities of all CYP1 enzymes for several of these substrates [4], which may potentially determine susceptibility to carcinogenesis by some planar aromatic hydrocarbons (PAHs) [7,8].…”

Mechanisms of acquired resistance to 2-(4-aminophenyl)benzothiazole (CJM 126, NSC 34445)

“…CYP1A1 catalyzes both detoxification and activation of xenobiotics and precarcinogens (Crofts et al, 1998). Recombinant CYP1A1 catalyzed C-6 oxidation of the benzothiazole nucleus (Chua et al, submitted) producing metabolites which: (i) like α-naphthoflavone and apigenin (Pastrakuljic et al, 1997), inhibit CYP1A1 activity and abrogate growth inhibition induced by 2-(4-aminophenyl)benzothiazoles; (ii) like apigenin, evoke a mitogenic response between concentrations of 300 nM and 5 µM in MCF-7 wt cells.…”