Orientation of Circulating Human Estrogens by Albumin

¹ From the Chemical Laboratory, Santa Barbara Cottage Hospital Research Institute, Santa Barbara, California

Bioassays of last trimester human serum fractions, separated by the scheme of Veldhuis, indicate that the conjugated estrogen fraction (estrone sulfate, estriol glucuronide, etc.) contributes about one-fourth, free estrone less than one-twentieth, free estriol one-fifth, and free estradiol one-half of the total biologic activity. No protein conjugated estrogen was found on alkaline hydrolysis. By dialyzing between human serum and human albumin, it was shown that estriol and estradiol added to normal serum and the endogenous estrogens of last trimester serum migrate from serum to albumin at a rate and amount comparable to the migration from albumin to albumin. Since the data gave no indication of competition of the other serum constituents (globulins, lipoproteins, etc.) with albumin, it is concluded that albumin is the most important element in regulating the transport of estrogens in serum.