alpha-Fetoprotein is not a component of the estradiol receptor of the rat uterus.

Abstract

In high-salt medium, cytosol from immature rat uteri displays 2 main high-affinity estradiol-binding peaks after ultracentrifugation in a sucrose gradient. The 2 components are the estradiol receptor which has a sedimentation coefficient of 5.5 S, and the .alpha.-fetoprotein which sediments at 4.5 S. The dissociation rate constants (k-1) of plasma .alpha.-fetoprotein-estradiol complexes measured at 0.degree. C in the absence or presence of 0.4 M KCl were found to be 7 .times. 10-5 and 8 .times. 10-5 s-1, respectively. The half-time of dissociation of these hormone-plasma protein complexes is 100-200 times more rapid than that of the estradiol-receptor complexes. These data led to the use of 2 differential dissociation methods for the measurement of the hormone-binding protein complexes. In a high-salt cytosol, the charcoal technique measured selectively the receptor binding sites; the hydroxylapatite technique measured the sum of the .alpha.-fetoprotein plus receptor binding sites. Under these conditions, binding specificity studies provided evidence that .alpha.-fetoprotein is not a subunit of the receptor. This was confirmed by binding specificity studies in high-salt medium of the receptor separated from .alpha.-fetoprotein by ultracentrifugation.