High Affinity Testosterone, Corticosterone and Progesterone Binding Activities in Pregnant Hamster Serum

Abstract

Binding of testosterone, corticosterone, progesterone and 17.beta.-estradiol by sera from pregnant, control adult and fetal hamsters was studied by gel equilibration and electrophoretic techniques. Significant increases of binding activities for testosterone, corticosterone and progesterone were demonstrated in pregnant hamster serum compared to the serum of adults of both sexes; fetal serum exhibited only weak steroid-protein interactions. Association constants, concentration of binding sites and relative displacement abilities were evaluated for the interactions of the 3 active ligands with whole serum from pregnant hamsters. For corticosterone and testosterone, at least 2 classes of high affinity, saturable, binding sites were demonstrated; 1 class of sites showed particularly elevated association constants (Ka = 1 .times. 109 M-1 at 25.degree. C) for either corticosterone or testosterone. Electrophoretic evidence favors the existence, in these pregnancy sera, of at least 2 steroid binding entities; both are able to bind corticosterone, testosterone and progesterone. One of these proteins migrates with the .alpha.1-globulins, and is also detected in control serum, whereas the 2nd protein, located in the prealbumin zone, appears specific to pregnancy.