Metabolism of exogenous 4- and 2-hydroxyestradiol in the human male

The metabolic fate of the isomeric catecholestrogens 4-hydroxyestradiol (4-OHE₂) and 2-hydroxyestradiol (2-OHE₂) was studied to elucidate possible differences in their metabolism as an explanation for their different bioactivities. Healthy young men (n = 3 each) were infused (90 min) with 4-OHE₂ (60 μg/h) or 2-OHE₂ (100 μg/h). The main metabolites were determined in plasma and urine before, during and after infusion. Unconjugated and conjugated steroids, the latter after hot acid hydrolysis, were subjected to chromatography on LH-20 columns and measured by specific RIAs. During the infusion 4-OHE₂ reached significant plasma concentrations whereas 2-OHE₂ was so rapidly metabolised that its plasma levels remained virtually undetectable in spite of a higher infusion rate. The metabolism of 4-OHE₂ was dominated by direct conjugation, that of 2-OHE₂ by methyl ether formation. These findings were corroborated by the urinary excretion rates: during the infusion and the first hours afterwards, 4-OHE₂ was mainly excreted as 4-OHE₂ and 4-hydroxyestrone, while 2-OHE₂ was predominantly excreted as 2-hydroxyestradiol 2-methyl ether and 2-hydroxyestrone 2-methyl ether.