Abstract
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 Km was 80 to 160 μM (Vmax, 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 apparentKm of 8 μM (Vmax, 70 pmol/nmol of P450/min) and was competitively inhibited by the CYP2A inhibitor coumarin (apparentKi, 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.
Footnotes
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↵1 Parts of this study were presented at two meetings (Chen and Mirvish, 1996; Chen et al., 1999b).
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This research was supported by Grant RO1-CA-35628 and core Grant P30-CA-36727 from the National Cancer Institute, Grant RO1-ES-07462 from the National Institute for Environmental Health Sciences, and Grant 97B-125 from the American Institute of Cancer Research.
- Abbreviations used are::
- MPN
- methyl-n-pentylnitrosamine
- MBZN
- methylbenzylnitrosamine
- NNN
- N′-nitrosonornicotine
- DMN
- dimethylnitrosamine
- PENT
- pentaldehyde
- P450
- cytochrome P450
- Received March 15, 2001.
- Accepted June 6, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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