Alpha-fetoprotein: Secondary estrogen binder in immature rat uterine cytosol

Alpha-fetoprotein (AFP), an estrogen(E)-binding glycoprotein in the serum of the immature rat, has been shown to be present in uterine cytosols and to migrate as 4 S in sucrose density gradients. Evidence is presented here which (1) indicates that AFP is not a component of the 8 S uterine E-binding complex in hypotonic cytosol (20 day old rat), and (2) confirms AFP as the E-binding moiety observed in the cytosol only at E concentrations which exceed that required to saturate the 8 S form. Thus, increased E (5–1000 nM) yields increased E binding in the 4 S rather than 8 S region. Binding in the 8 S region is eliminated by elevation of temperature; in contrast, AFP is thermostable. Removal of KCl from hypertonic cytosol containing [ 3H]-estradiol and [ 125I]-AFP followed by sucrose density gradient ultra-centrifugation of the resulting hypotonic cytosol did not effect incorporation of [ 125I]-AFP into the 8 S complex. Estradiol could be definitively associated with AFP in uterine cytosol only after steady-state polyacrylamide gel electrophoretic separation at 50 nM[ 3H]-estradiol. It is concluded that the thermolabile E-binding component of uterine cytosol which is saturated at 10nME and which is observed in the 8 S region in a hypotonic environment is not and does not contain AFP: rather. AFP is the thermostable, higher capacity, lower affinity E-binding component observed in the 4 S region.