Metabolism of nicotine in rat lung microvascular endothelial cells

The aim of this study was to examine whether cultured rat lung microvascular endothelial cells (LMECs), which constitute the gas-blood barrier, have the ability to metabolize nicotine. Nicotine was biotransformed to cotinine and nicotine N'-oxide by cytochrome 450 (CYP) and flavin-containing monooxyganase (FMO), respectively, in rat LMECs. The intrinsic clearance (Vmax1/Km1) for the cotinine formation was about 20 times as high as that for the trans-nicotine N'-oxide formation in the low-Km phase, indicating that oxidation by CYP was much higher than that by FMO. On the other hand, as shown in Eadie-Hofstee plots, the formation of cis-nicotine N'-oxide was monophasic, whereas the plot for the trans-nicotine N'-oxide formation was clearly biphasic. These results suggest that nicotine N'-oxide was stereoselectively metabolized to cis and trans forms. However, in the high-Km phase there was no significant difference in N'-oxidation between the cis and trans forms. Moreover, we suggest that CYP2C11 and CYP3A2 are key players in the metabolism to cotinine of nicotine in rat LMECs using the respective enzyme inhibitors (tranylcypromine and troleandomycine). On the other hand, methimazole (5 microM) caused 73 and 45% decreases in the formation of N'-oxides of cis- and trans- enantiomers, respectively, demonstrating the presence of FMO in rat LMECs. These results suggest that rat LMEC enzymes can convert substrates of exogenous origin such as nicotine for detoxication, indicating LMECs are an important barrier for metabolic products, besides hepatic cells.