Identification of the mutagens in cooked beef.

The purification of cooking mutagens depends on the extraordinary sensitivity of the Ames/Salmonella mutagenicity test and its usefulness for tracking the mutagens during the purification steps. Following aqueous/acid (pH 2) extraction of fried ground beef (cooked at 200, 250, or 300 degrees C), XAD-2 column adsorption and elution with acetone, and acidic and basic liquid/liquid extractions, the samples are separated into six distinct peaks with preparative reverse-phase HPLC. A total of nine distinct mutagens can be separated after two additional HPLC steps. These compounds fall into a class of compounds called aminoimidazoazaarenes (AIAs). The majority of the mutagenic activity is made up of MeIQx1 (m/z 213, C11H11N5), DiMeIQx (m/z 227, C12H13N5), trimethylimidazopyridine (TMIP) (m/z 176, C9H12N4) and phenylimidazopyridine (PhIP) (m/z 224, C13H12N4). Smaller contributions are from IQ (m/z 198, C11H10N4), MeIQ (m/z 213, C12H12N4), a nonpolar peak containing oxygen and two unidentified trace polar mutagens. Mass estimates (per kilogram uncooked beef) include: 15 micrograms for PhIP, 1.0 micrograms for MeIQx, 0.5 microgram for DiMeIQx, and 0.02 microgram for IQ. Because of the uncoupling of mutagenic and carcinogenic potencies of these aromatic amines, the PhIP, which contributes the highest mass content to the cooked meat, but has the lowest mutagenic potency, might ultimately make a significant contribution to the carcinogenicity.