Some new aspects of 17α-estradiol metabolism in man

Abstract

17α-Estradiol (1,3,5(10)-estratriene-3,17α-diol) together with a tracer dose of the tritium-labeled compound was administered orally and sublingually to male volunteers. The serum concentrations of 17α-estradiol (free and liberated by enzymatic hydrolysis) were quantified by GC/MS, and the serum total radioactivity and urinary radioactivity excretion were determined. After oral administration, 17α-estradiol was rapidly and intensively conjugated; only tiny quantities of the free steroid (<1% of total) appeared in serum. Sublingual administration resulted in temporary (up to 3 h p.a.) higher serum levels of the free compound. The metabolite patterns obtained by TLC of extracts from serum and urine demonstrated that 17α-estradiol is the subject of a poor phase I metabolism in man. A great discrepancy was found in the serum concentrations of 17α-estradiol (free+conjugated) determined by GC/MS and the serum radioactivity expressed in 17α-estradiol equivalents. By TLC analysis of the steroid conjugates extracted from serum, various 17α-estradiol conjugate peaks were found. By enzymatic hydrolysis with β-glucuronidase/aryl sulfatase from Helix pomatia they were only partially cleaved. Thus, the difference between the serum radioactivity and the 17α-estradiol levels determined by GC/MS had to be attributed to an incomplete conjugate hydrolysis. It has been shown with the synthesized 17α-estradiol sulfate conjugates that only the 3-sulfate is cleaved by enzymatic hydrolysis, whereas the 17-sulfate group resists enzymatic hydrolysis. The methanolysis procedure (acetyl chloride in MeOH) has proved to be an efficient method for cleaving both the 3-sulfate group and the 17-sulfate group. In contrast to the 17α-estradiol conjugates in serum, the urinary conjugates were intensively split by the enzyme preparation. From this, it has to be concluded that the serum conjugates were deconjugated and newly reconjugated before urinary excretion.