Tissue-specific metabolic activation of carcinogenic nitrosamines: N'-nitrosonornicotine, N-nitrosopiperidine, N-nitrosopyrrolidine, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

N'-Nitrosonomicotine (NNN), N-nitrosopiperidine (NPIP), and N-nitrosopyrrolidine (NPYR) are structurally related cyclic nitrosamines that induce tumors in laboratory animals and are likely causative agents in human cancers. Humans are exposed to these nitrosamines via tobacco and dietary sources. NNN is the most abundant of the carcinogenic nitrosamines in tobacco. NPIP and NPYR are present in tobacco and the diet. Additional exposure to these nitrosamines may come from endogenous formation. These nitrosamines have distinct tissue selectivity as carcinogens. NNN and NPIP are potent esophageal and nasal carcinogens in rats whereas NPYR primarily induces liver tumors, but does not induce esophageal tumors and is only weakly carcinogenic in the nasal cavity. The ability of these nitrosamines to cause tumor induction is likely due to their effective metabolic activation catalyzed by cytochrome P450 (P450) enzymes in the target tissue. Therefore, we hypothesized that differences in metabolic activation of these structurally similar nitrosamines mediate the differences in their carcinogenic activities in experimental animals.;In this thesis, nitrosamine bioactivation was investigated in vitro with rat microsomes and expressed cytochrome P450 (P450s) enzymes. It was demonstrated that rat esophageal and nasal microsomes (containing P450s) preferentially catalyzed NPIP bioactivation relative to NPYR, indicating that tissue-specific metabolic activation is a key factor in tissue-selective tumor formation. Striking differences in NNN, NPIP, and NPYR bioactivation were observed in a structure-activity study with closely related rat, mouse, and human P450s. Taken together, these data strongly support that P450-mediated bioactivation in the rodent and human target tissues is a crucial event mediating tissue-selective tumor formation by NNN, NPIP, NPYR, and other nitrosamines.;In additional studies, it was determined that human P450 2A13 is a functional protein in vivo. P450 2A13 is expressed in the lung and nasal cavity on an mRNA level and efficiently catalyzes the bioactivation of several carcinogenic nitrosamines, including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a likely causative agent for tobacco-related cancers in humans. Further, investigations were carried out to understand the low catalytic activities observed for NNK activation by human lung microsomes.