Abstract

We have recently shown that, in human intestine, glucuronidation of androsterone and testosterone was on the nanomolar level and increased from proximal to distal intestine. In the present study, we have characterized estrogen UDP-glucuronosyltransferase activity in microsomes from intestine of seven human subjects. Intestinal microsomes from all segments of intestine from both males and females (except for one male) glucuronidated estrone (0.2–2.6 nmol/mg × min) and estradiol (0.5–3.1 nmol/mg × min) at levels 2 to 15 times higher than found with human liver microsomes (0.04–0.1 and 0.16–0.25 nmol/mg × min, for estrone and estradiol, respectively). Only with estriol were there significant hepatic glucuronidation (2.2–4.5 nmol/mg × min) and intestinal glucuronidation activities (0.2–2.2 nmol/mg × min) that were lower than those in liver. All-trans-retinoic acid was glucuronidated by all segments of intestine from both sexes at levels 50 to 80% of those found with human liver but quite low compared with estrogen glucuronidation. In the two subjects for whom stomach was available, there was no measurable activity in stomach microsomes toward any of the substrates. UGT2B RNA expression was examined in mucosa from stomach to colon from two subjects. There was significant expression of UGT2B7, but not of UGT2B4 or UGT2B15, in all segments of intestine. To our knowledge, this is the first direct demonstration of glucuronidation of estrogens by human intestinal microsomes. Thus, in humans, the intestine may be considered as part of the overall mechanism of detoxification via glucuronidation.