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.
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.
Frandsen
H
H
Institute of Food Safety and Toxicology, Morkhoj Bygade 19, DK-2860 Soborg, Denmark. hf@fdir.dk
Alexander
J
J
D000818:Animals; D001191:Arylamine N-Acetyltransferase / Q000378:metabolism*; D015415:Biomarkers / Q000032:analysis*; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D007093:Imidazoles / Q000378:metabolism* / Q000493:pharmacokinetics* / Q000494:pharmacology*; D008297:Male; D011725:Pyridines / Q000378:metabolism* / Q000493:pharmacokinetics*; D051381:Rats; D017208:Rats, Wistar
D015415:Biomarkers; D007093:Imidazoles; D011725:Pyridines; C060533:2-hydroxyamino-1-methyl-6-phenylimidazo(4,5-b)pyridine; C077584:2-amino-1-methyl-6-(4-hydroxyphenyl)imidazo(4,5-b)pyridine; C049584:2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine; D001191:Arylamine N-Acetyltransferase; D005978:Glutathione