Metabolism of sesamin by CYPs and UGTs in human liver

Sesamin is a major lignan in sesame, and its biological effects such as antioxidant effect, anti-carcinogenic effects, and suppression of hypertension have been extensively studied by many researchers. However, its metabolic pathways and metabolic enzymes in human bodies have not been identified. Recently we demonstrated that CYP2C9 was the most important cytochrome P450 isoform in human liver. Next, we focused on metabolism of sesamin mono-catechol by cytochrome P450 or UDP-glucuronosyltransferase (UGT). Further catecholization of sesamin mono-catechol by cytochrome P450 enhances its anti-oxidant activity, whereas glucuronidation by UGT strongly reduces anti-oxidant activity. In human liver microsomes, glucuronidation activity toward sesamin mono-catechol was much higher than the di-catecholization activity. In contrast, both activities were similar in rat liver microsomes. These results suggest a large species-based difference between humans and rats in sesamin metabolism. In vitro studies using 10 individual human liver microsomes suggested that UGT2B7 was responsible for glucuronidation of sesamin mono-catechol in human liver. In addition, we observed a significant methylation activity toward sesamin mono-catechol by catechol O-methyl transferase (COMT) in human liver cytosol. Based on these results, we concluded that CYP2C9, UGT2B7, and COMT played essential roles in the metabolism of sesamin in human liver.