Metabolism of flavonoids via enteric recycling: role of intestinal disposition.

The purpose of this study was to determine the importance of intestinal disposition in the first-pass metabolism of flavonoids. A four-site perfused rat intestinal model, rat liver and intestinal microsomes, Caco-2 cell microsomes, and the Caco-2 cell culture model were used. In the four-site model, approximately 28% of perfused aglycones are absorbed (approximately 450 nmol/30 min). Both absorption and subsequent excretion of metabolites were rapid and site-dependent (p < 0.05). Maximal amounts of intestinal conjugates excreted per 30 min were 61 and 150 nmol for genistein and apigenin, respectively. Maximal amounts of biliary conjugates excreted per 30 min were 50 and 30 nmol for genistein and apigenin, respectively. Microsomes, prepared from Caco-2 cells, rat intestine, and rat liver, always glucuronidated apigenin faster than genistein (p < 0.05). In addition, rat jejunal microsomes glucuronidated both flavonoids faster (p < 0.05) than rat intestinal microsomes prepared from other regions. When comparing glucuronidation in different organs, jejunal microsomes often but not always glucuronidated both flavonoids faster than liver microsomes. In the Caco-2 model, both flavonoids were rapidly absorbed and rapidly conjugated, and the conjugates were excreted apically and basolaterally. Similar to the four-site perfusion model, apigenin conjugates were excreted much faster than genistein conjugates (>2.5 times for glucuronic acid, >4.5 times for sulfate; p < 0.05). In conclusion, intestinal disposition may be more important than hepatic disposition in the first-pass metabolism of flavonoids such as apigenin. In conjunction with enterohepatic recycling, enteric recycling may be used to explain why flavonoids have poor systemic bioavailabilities.