Glucuronidation and sulfation of the tea flavonoid (-)-epicatechin by the human and rat enzymes

(-)- Epicatechin (EC) is one of the flavonoids present in green tea, suggested to have chemopreventive properties in cancer. However, its bioavailability is not clearly understood. In the present study, we determined the metabolism of EC, focusing on its glucuronic acid and sulfate conjugation using human liver and intestinal microsomes and cytosol as well as recombinant UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) isoforms in comparison with that occurring in the rat. Surprisingly, EC was not glucuronidated by the human liver and small intestinal microsomes. There was also no evidence of glucuronidation by human colon microsomes or by recombinant UGT1A7, which is not present in the liver or intestine. Interestingly, in the rat liver microsomes EC was efficiently glucuronidated with the formation of two glucuronides. In contrast, the human liver cytosol efficiently sulfated EC mainly through the SULT1A1 isoform. For the intestine, both SULT1A1 and SULT1A3 contributed. Other SULT isoforms contributed little. High-performance liquid chromatography of the sulfate conjugates showed one major sulfatase- sensitive peak with all tissues. An additional minor sulfatase- resistant peak was formed by the liver and intestinal cytosol as well as with SULT1A1 but not by the Caco-2 cytosol and SULT1A3. In the rat, EC sulfation was considerably less efficient than in the human liver. These results indicate that sulfation is the major pathway in EC metabolism in the human liver and intestine with no glucuronidation occurring. There was also a large species difference both in glucuronidation and sulfation of EC between rats and humans.