Characterization of a Cyp2a(4/5)bgs‐null Mouse Model: Role of CYP2A and CYP2B in Nicotine Metabolism

Abstract

Nicotine metabolism is believed to affect not only nicotine's pharmacological effects, but also nicotine addiction. As a key step toward testing this hypothesis, we have produced knockout mouse models targeting P450 genes possibly involved in nicotine metabolism, including Cyp2a and Cyp2b. Our recent study of a Cyp2a5‐null mouse confirmed that, although CYP2A5 plays a major role in systemic clearance of nicotine and cotinine, other P450 enzymes may also have significant roles. The aim of this study is to determine, using a newly generated Cyp2a(4/5)bgs‐null mouse model, whether hepatic P450 enzymes (other than CYP2A5) encoded by the Cyp2a(4/5)bgs gene cluster, including CYP2A4 and the various CYP2B isoforms, contribute to nicotine metabolism in vivo. We found that, compared to the Cyp2a5‐null mice, the Cyp2a(4/5)bgs‐null mice showed ~60% decreases in hepatic microsomal nicotine oxidase activity in vitro, and in rates of systemic nicotine clearance in vivo. The decreases in nicotine metabolism in the Cyp2a(4/5)bgs‐null mice were accompanied by prolonged nicotine‐induced hypothermia and analgesia, compared to the behavioral responses in wild‐type mice. Thus, both CYP2A and CYP2B contribute to nicotine clearance in mice. The Cyp2a(4/5)bgs‐null and Cyp2a5‐null mouse models will be valuable for studies on the impact of genetic variations in nicotine metabolism, as occur in humans, on nicotine dependence.