The dietary mutagens 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are activated to genotoxins by rat and human liver cytochrome P450 (P450) 1A1- and 1A2-mediated N-oxidation. Immunoquantitation of 51 human liver samples revealed a wide range in P450 1A2 expression (10-250 pmol/mg of microsomal protein, median 71 pmol/mg), with 39% of the livers containing >100 pmol/mg of protein. There was no evidence for expression of P450 1A1 (<1 pmol/mg of protein). P450 1A2 levels were correlated to MeIQx and PhIP N-oxidation rates (r = 0.83, 0.73, respectively). In male Fischer-344 and Sprague-Dawley rats, hepatic P450 1A2 ranged from 5 to 35 pmol/mg of protein, while P450 1A1 was <1 pmol/mg. Animal pretreatment with 3-methylcholanthrene, beta-naphthoflavone, or polychlorinated biphenyls (PCB) resulted inasmuch as 340-fold and >1000-fold induction of P450 1A2 and 1A1, respectively, and a 220-fold increase in N-oxidation activity. Approximately 20% of the human samples were as active in N-oxidation and conversion of MeIQx to bacterial mutagens as microsomes of PCB-pretreated rats [3-4 nmol of NHOH-MeIQx formed min-1 (mg of protein)-1]. In contrast, microsomes from PCB-treated rats displayed higher rates of PhIP N-oxidation and activation to mutagens than the most active human liver microsomes [8-24 vs 2-4 nmol of HNOH-PhIP formed min-1 (mg of protein)-1]. Recombinant human P450 1A2 showed catalytic efficiencies of MeIQx and PhIP N-oxidation that were 10-19-fold higher than purified rat P450 1A2. Cytochrome P450 1A2 expression in rodent and human liver tissue varies greatly and there are considerable differences between the enzymes in the two species in the activation of some heterocyclic aromatic amines, which must be considered when assessing human health risk.

A novel method utilizing 14C palmitic acid for image analysis and liquid scintillation for quantifying oil uptake of French fries undergoing deep-fat frying is described. Radiolabeled computerized image analysis of the fried product furnished comprehensive visual information on the distribution and localization of oil uptake which was limited to the crust. A significant high linear correlation (R 2 = 0.977; P < 0.001) was found between oil uptake measured by liquid scintillation and differential scanning calorimetry, and between liquid scintillation and radiolabeled imaging (R 2 = 0.861; P < 0.001). The high specificity and sensitivity of the method allows quantifying uptake of very low oil concentrations ranging from several milligrams. Oil quantification via liquid scintillation was more sensitive than image analysis and requires up to 3 order of magnitude lower concentration of the radiolabeled marker. The low concentration and inherent stability of the radiolabeled chemical make this technique uniquely suitable for studying oil uptake mechanism during deep-fat frying of foodstuffs. Keywords: Deep-fat frying; radiolabeled; liquid scintillation; image analysis; oil uptake.