N-acetyltransferase-dependent activation of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine: formation of 2-amino-1-methyl-6-(5-hydroxy)phenylimidazo [4,5-b]pyridine, a possible biomarker for the reactive dose of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

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Abstract

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a mutagenic and carcinogenic heterocyclic amine formed during ordinary cooking. PhIP is metabolically activated to the ultimate mutagenic metabolite by CYP P450-mediated N-hydroxylation followed by phase II esterification, Incubation of N-hydroxy-PhIP (N-OH-PhIP) with cytosol, acetyl coenzyme A (AcCoA) and 2'-deoxyguanosine for 24 h resulted in the formation of three different adducts: N-2-(deoxyguanosin-8-yl)-PhIP, N-2-(guanosin-8-yl) -PhIP and PhIP-xanthine. One additional product, 5-hydroxy-PhIP (5-OH-PhIP), was also identified in the incubation mixtures. 5-hydroxy-PhIP is formed as a degradation product of conjugates formed from N-acetoxy-PhIP and protein, glutathione or buffer constituents, A similar spectrum of products was obtained using 3'-phosphoadenosine-5'-phosphosulfate (PAPS) instead of acetyl CoA, Addition of glutathione (3 mM) to the incubation mixture resulted in a 50% reduction in both adducts and 5-hydroxy-PhIP formation in liver cytosol, The main product detected was PhIP, suggesting glutathione-dependent reduction of the N-acetoxy-PhIP, Addition of glutathione to incubation mixtures from the other cytosolic preparations had less dramatic effects, In addition, increasing the amount of N-OH-PhIP in the incubation mixture resulted in proportional increased amounts of total adducts and 5-OH-PhIP, Incubation of rat and human S9 with PhIP resulted in the formation of only traces of 5-OH-PhIP, Fortification with AcCoA clearly increased the formation of 5-OH-PhIP, Addition of the CYP 450 1A2 inhibitor, furafylline, completely inhibited the formation of 5-OH-PhIP in incubations with human S9, These results indicate that both PhIP adducts and 5-OH-PhIP are formed by similar routes of activation of N-OH-PhIP. 5-OH-PhIP may therefore serve as a biomarker for the formation of the ultimate mutagenic metabolite of PhIP, A rat dosed orally with PhIP excreted 1% of the dose as 5-OH-PhIP in the urine at 24 h and 0.05 and 0.01% at 48 and 72 h, respectively. This shows that 5-OH-PhIP is also formed in vivo and indicates the possible use of 5-OH-PhIP as a urinary biomarker.
Original languageEnglish
JournalCarcinogenesis
Volume21
Issue number6
Pages (from-to)1197-1203
ISSN0143-3334
Publication statusPublished - 2000

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