Intestinal absorption of oestrone, oestrone glucuronide and oestrone sulphate in the rat in situ—I. Importance of hydrolytic enzymes on conjugate absorption

Abstract

The biliary excretion of steroid after administration of [ 3H]oestrone ([ 3H]E 1), [ 3H]oestrone glucuronide ([ 3H]E 1G) and [ 3H]oestrone sulphate ([ 3H]E 1S) into the hepatic portal vein of anaesthetized rats was very rapid with more than 70% of E 1S and greater than 80% of E 1 and E 1G excreted in the first 30 min. There was a lag period in the biliary excretion of E 1 S, this was less apparent with E 1 and absent with E 1G. Biliary excretion accurately reflects the amount of steroid in the portal circulation and was therefore used as an assessment of absorption from the gastrointestinal (GI) tract. Absorption (as judged by excretion in bile) was least after administration of each steroid into the stomach. The extent of absorption correlated well with the lipophilicity of the steroids as shown by their relative partition coefficients between n-octanol and pH 6.5 phosphate-buffered saline (E 1 ⩾ E 1S ⩾ E 1G) There was no significant difference in excretion profile when the steroids were given into the caecum (at 5 h, E 1, 46.3 ± 9.1%; E 1G, 42.2 ± 14.5%; E 1S, 39.9 ± 7.1%). The similarity, despite marked differences in physicochemical properties, suggested conjugate hydrolysis to the parent steroid. In contrast, after administration into the small intestine, excretion of E 1 was very rapid and was maximal at 1 h (72.5 ± 8.0%); E 1G showed a near-linear excretion rate (1 h, 14.4 ± 3.0%; 5 h, 80.0 ± 11.7), whereas in comparison E 1S excretion was low (1 h, 12.1 ± 2.4%; 5 h, 36.9 ± 2.7%). The involvement of hydrolytic enzymes in conjugate absorption was assessed. Ampicillin pretreatment (200 mg/kg/day for 2 days) reduced the absorption of E 1G from both the proximal and distal small intestine (by approximately 50%) but had no effect on the absorption of E 1S. There was, therefore, evidence that quantitative absorption of E 1G requires prior hydrolysis (by mammalian and/or microbial enzymes) but intact absorption of E 1S from this region of the tract was implicated. Ampicillin pretreatment reduced the absorption of both conjugates (greater with E 1S) from the caecum; hydrolysis clearly precedes absorption from the caecum. The above findings were supported by an in vitro study which showed that ampicillin pretreatment abolished the hydrolysis of E 1S by caecal contents but only partially reduced the hydrolysis of E 1G. The presence of mammalian glucuronidase enzyme may account for this difference.