Depentylation of the rat esophageal carcinogen, methyl-n-pentylnitrosamine, by microsomes from various human and rat tissues and by cytochrome P450 2A3

Methyl-n-pentylnitrosamine (MPN) is carcinogenic for the rat esophagus. To determine organ specificity for MPN activation by human tissues, microsomes isolated from human organs (snap-frozen <6 h after death or removed surgically) were incubated with [pentyl-³H]MPN, and [³H]pentaldehyde formation was measured by high-pressure liquid chromatography of its 2,4-dinitrophenylhydrazone using radioflow assay. With 100 μM MPN, mean depentylation rates were 6.6 (liver), 2.9 to 3.8 (kidney, stomach, small intestine, and colon), and 0.4 to 1.6 (esophagus, lung, and skin) pmol of pentaldehyde/mg of protein/min. Of 14 human esophagi, four showed relatively high depentylation rates of 3.3 to 4.1 pmol/mg/min. Apparent K_m was 80 to 160 μM (V_max, 3-15 pmol/mg/min) for three esophagi, 90 to 130 (2 livers), and 1330 (1 kidney) μM. Rat tissues showed mean depentylation rates for 100 μM MPN of 24.9 (liver), 14.5 (esophagus), 7.0 (lung), and 0.0 to 2.7 (5 other tissues) pmol/mg/min. MPN depentylation by rat cytochrome P450 2A3 showed an apparent K_m of 8 μM (V_max, 70 pmol/nmol of P450/min) and was competitively inhibited by the CYP2A inhibitor coumarin (apparent K_i, 4 μM). Coumarin (0.4 mM) inhibited microsomal depentylation of 100 μM MPN by 37 to 62% for human esophagus, liver, kidney, and colon and for rat esophagus but not for rat liver and lung. MPN depentylation by rat esophageal microsomes increased up to 90% on adding P450 reductase. The results indicate organ-specific MPN metabolism by rat but not human esophagus. Nevertheless, the relatively high activity of four human esophagi might indicate increased susceptibility of some individuals to carcinogenesis by unsymmetrical dialkylnitrosamines.