Formation of the heterocyclic aromatic amine PhIP: identification of precursors and intermediates

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a heterocyclic aromatic amine belonging to a class of mutagens found in cooked food. Phenylacetaldehyde and 2-phenylethylamine, thermal degradation products of phenylalanine, were found to react with creatinine to form PhIP. To identify possible intermediates in the model system and in fried meat an aldol addition between phenylacetaldehyde and creatinine was made. The aldol addition product (2-amino-1-methyl-5-(1′-hydroxy-2′-phenylethyl)-imidazol-4-one) was then dehydrated to get the aldol condensation product (2-amino-1-methyl-5-(2′-phenylethenyl)-imidazol-4-one). These two chemically synthesised compounds were then used as standards. The aldol addition product could not be found in the model system and also not in fried meat. In contrast the aldol condensation product was identified, by liquid chromatography in combination with mass spectrometry and UV spectroscopy, in the model system and in fried meat. The data show that phenylacetaldehyde and the aldol condensation product of phenylacetaldehyde and creatinine are very important intermediates in the reaction mechanism to form PhIP.