Metabolites of estradiol in serum, bile, intestine and feces of the domestic cat (Felis catus )

We wished to develop an efficient, noninvasive method for monitoring ovarian function in domestic and nondomestic Felidae. We hypothesized that the method could be based on measurement of one of the major excreted estrogen metabolites. To identify and characterize the major excreted metabolites, a bolus of (14)C-estradiol was administered into the femoral vein of adult female cats. We measured the amounts of total radioactivity per unit time contained in unconjugated and conjugated estradiol metabolites, in conjugated metabolites that were hydrolyzable, and in those not hydrolyzable by beta Glucuronidase / aryl sulfatase (the enzyme). Radionuclide levels were determined in voided feces and urine, in jugular vein plasma, bile, contents of the duodenum, and in the small intestine. Metabolites of (14)C-estradiol were voided preferentially in feces and in equal amounts either as unconjugated estradiol or as conjugates not hydrolyzable by the enzyme. In plasma, conjugated estrogens comprised an increasing proportion of the total radioactivity during the first 40 min after administration. Plasma pools of samples from 0.5 to 30 min and 40 to 360 min contained a monoconjugate and a diconjugate, respectively; both were hydrolyzable by the enzyme. Bile and intestinal samples were collected at 360 min after administration. In the bile, 99% of the total radioactivity was in conjugated compounds, only 20% of which were not hydrolysable by the enzyme. The proportion of unconjugated metabolites increased to 18% in the duodenum and to 45% in the small intestine. The major conjugates contained in voided feces not hydrolyzable by the enzyme were estradiol sulfate (m/z = 351.6836), distributed as the 3-sulfate (20%) and 17-sulfate (80%); of the latter, 70% were 17alpha- and 30% 17beta-estradiol sulfates. These data document the fate of estradiol in the circulation of the cat, they demonstrate that a large portion of the voided estradiol metabolites are not hydrolyzable by the enzyme, and account for those conjugates previously termed nonhydrolyzable.