Metabolism of Estrogens in the Gastrointestinal Tract of Swine. III. Estradiol-17β-D-Glucuronide Instilled into Sections of Intestine3

Studies were conducted to determine the absorption and metabolic fate of 3H-estradiol-17 beta-glucuronide (3H-E2-G) in swine. The conjugate, 3H-E2-G (48.7 x 10(6) DPM, 45.5 Ci/mmol), was injected into ligated 15-cm sections of duodenum, proximal jejunum, distal jejunum, ileum and spiral colon of 10 kg female pigs. Blood from the jugular and portal veins and urine were collected at .5-h intervals for 5 h. Absorption from the colon was rapid and radioactivity peaked in both portal and jugular plasma by .5 h postinjection. In contrast, the highest plasma estrogen concentration from most other sections was reached at 5 h, the last sampling time. The urinary excretion patterns were nearly identical to those seen in plasma, with the radioactivity peaking early (1.5 h) after instillation of 3H-E2-G into the colon, but still rising at the end of the experiment after instillation into the duodenum, distal jejunum and ileum. The proximal jejunum, which produced low plasma estrogen concentrations, also produced low urine concentrations. The slower absorption of 3H-E2-G compared to 14C-estradiol-17 beta is consistent with the view that the limiting factor for the absorption of the conjugate is hydrolysis to a free estrogen. The predominant metabolites in portal venous plasma from all sections of the intestine at the end of the experiments were the monoglucuronides of estrone and estradiol. Because the administered 3H-E2-G was conjugated at C-17, the presence of estrone glucuronide in portal plasma indicates that, at least in the duodenum, ileum and colon, 3H-E2-G undergoes cleavage, followed by the oxidation of estradiol to estrone, which is subsequently reconjugated by the intestinal mucosa.