N-Nitrosopiperidine (
NPIP) is a potent rat nasal
carcinogen whereas
N-nitrosopyrrolidine (NPYR), a hepatic
carcinogen, is weakly carcinogenic in the nose.
NPIP and NPYR may be causative agents in human
cancer. P450-catalyzed alpha-hydroxylation is the key activation pathway by which these
nitrosamines elicit their carcinogenic effects. We hypothesize that the differences in
NPIP and NPYR metabolic activation in the nasal cavity contribute to their differing carcinogenic activities. In this study, the kinetics of
tritium-labeled
NPIP or NPYR alpha-hydroxylation mediated by Sprague-Dawley rat nasal olfactory or respiratory microsomes were investigated. To compare alpha-hydroxylation rates of the two
nitrosamines, tritiated 2-hydroxytetrahydro-2H-pyran and 2-hydroxy-5-methyltetrahydrofuran, the major
NPIP alpha-hydroxylation products, and tritiated 2-hydroxytetrahydrofuran, the major NPYR alpha-hydroxylation product, were quantitated by HPLC with UV absorbance and radioflow detection. These microsomes catalyzed the alpha-hydroxylation of
NPIP more efficiently than that of NPYR. K(M) values for
NPIP were lower as compared to those for NPYR (13.9-34.7 vs 484-7660 muM). Furthermore, catalytic efficiencies (V(max)/K(M)) of
NPIP were 20-37-fold higher than those of NPYR. Previous studies showed that P450 2A3, present in the rat nose, also exhibited this difference in catalytic efficiency. For both types of nasal microsomes,
coumarin (100 muM), a P450 2A inhibitor, inhibited
NPIP and NPYR alpha-hydroxylation from 63.8 to 98.5%. Furthermore,
antibodies toward P450 2A6 inhibited
nitrosamine alpha-hydroxylation in these microsomes from 68.8 to 78.4% whereas
antibodies toward P450 2E1 did not inhibit these reactions. Further immunoinhibition studies suggest some role for P450 2G1 in
NPIP metabolism by olfactory microsomes. In conclusion, olfactory and respiratory microsomes from rat nasal mucosa preferentially activate
NPIP over NPYR with P450 2A3 likely playing a key role. These results are consistent with local metabolic activation of
nitrosamines as a contributing factor in their tissue-specific carcinogenicity.