N-nitrosopiperidine (
NPIP) is a potent esophageal
carcinogen in rats whereas structurally similar
N-nitrosopyrrolidine (NPYR) induces liver, but not esophageal
tumors.
NPIP is a possible causative agent for human
esophageal cancer. Our goal is to explain mechanistically these differing carcinogenic activities in the esophagus. We hypothesize that differences in metabolic activation of these
nitrosamines could be one factor accounting for their differing carcinogenicity. alpha-Hydroxylation is the key metabolic activation pathway leading to
nitrosamine-induced
carcinogenesis. In this study, we examined the alpha-hydroxylation rates of [3,4-(3)H]
NPIP and [3,4-(3)H]NPYR by male F344 rat esophageal and liver microsomes. The major alpha-hydroxylation products of
NPIP and NPYR, 2-hydroxytetrahydro-2H-pyran (2-OH-THP) and 2-hydroxytetrahydrofuran (2-OH-THF), respectively, were monitored by high performance liquid chromatography with radioflow detection.
NPIP or NPYR (4 microM) was incubated with varying concentrations of esophageal microsomes and co-factors. Microsomes converted
NPIP to 2-OH-THP with a 40-fold higher velocity than NPYR to 2-OH-THF. Similar results were observed in studies with
NPIP and NPYR at substrate concentrations between 4 and 100 micro M. Kinetics of
NPIP alpha-hydroxylation were biphasic; K(M) values were 312 +/- 50 and 1600 +/- 312 microM. Expressed
cytochrome P450 2A3, found in low levels in rat esophagus, was a good catalyst of
NPIP alpha-hydroxylation (K(M) = 61.6 +/- 20.5 microM), but a poor catalyst of NPYR alpha-hydroxylation (K(m) = 1198 +/- 308 micro M).
Cytochrome P450 2A3 may play a role in the preferential activation of
NPIP observed in rat esophagus. Liver microsomes metabolized NPYR to 2-OH-THF (V(max)/K(M) = 3.23 pmol/min/mg/ microM) as efficiently as
NPIP to 2-OH-THP (V(max)/K(M) = 3.80-4.61 pmol/min/mg/ microM). We conclude that rat esophageal microsomes activate
NPIP but not NPYR whereas rat liver microsomes activate
NPIP and NPYR. These results are consistent with previous findings that tissue-specific activation of
nitrosamines contributes to tissue-specific
tumor formation.