The overall goal of this dissertation is to determine the role of CYP2A13, a human cytochrome P450 enzyme (P450), in the metabolism and toxicity of xenobiotics in the respiratory tract. The central hypothesis is that CYP2A13 plays a major role in the target tissue bioactivation of respiratory tract toxicants. The specific aims are (1) to demonstrate that CYP2A13 protein is detectable in human lung microsomes, where it is active in the bioactivation of NNK; (2) to identify the mechanisms underlying the reduced risk of lung adenocarcinoma associated with the CYP2A13*2 allele in human smokers; and (3) to investigate the role of CYP2A13 in the bioactivation of 3-methylindole (3MI), a pneumotoxicant that requires P450 bioactivation.

In the first aim (chapter 2), we demonstrated for the first time that CYP2A6 and CYP2A13 proteins can be detected in human lung microsomes. We found that the expression levels of both CYP2A enzymes were highly variable. Furthermore, we demonstrated that CYP2A13, but not CYP2A6, plays a major role in a-hydroxylation of NNK in human lung microsomes.

In the second aim (chapter 3), we characterized the CYP2A13*2 allele and the resulting variant protein, CYP2A13.2. We discovered that the CYP2A13.2 protein was less active than the CYP2A13.1 protein in the metabolism of several CYP2A13 substrates, including NNK. We also confirmed that the CYP2A13*2 allele had decreased gene expression compared to the wild-type CYP2A13*1 allele, and that decreased gene expression was at least partly due to the occurrence of a 26-bp deletion in the CYP2A13 promoter region. Our findings indicate that a combination of lowered expression and lowered activity of the CYP2A 13.2 protien might be the mechanism resulting in lowered risk of lung adenocarcinoma.

In the third aim (chapter 4), we found that CYP2A13 can metabolize 3-methylindole (3M1), a potent pneumotoxicant, forming multiple metabolites and reactive intermediates. Our finding that CYP2A13 can form toxic intermediates from 3MI indicates that CYP2A13 is capable of mediating 3MI toxicity in human lungs. However, we also observed that metabolism of 3MI can result in inactivation of CYP2A13, which may diminish the contributions of CYP2A13 to metabolism of xenobiotics in the respiratory tract.