Corticosteroid-binding proteins in human colostrum and milk and rat milk.
A corticosteroid-binding protein was detected in the whey of human colostrum and milk which resembles serum corticosteroid-binding globulin in certain respects: the equilibrium association constants for cortisol and progesterone binding and the apparent molecular size, as determined by Sephadex G-200 chromatography, were similar, and cortisol andd progesterone competed strongly for binding to the same site in each instance. Dexamethasone-binding activity could not be detected. The concentration of corticosteroid-binding protein in the colostrum obtained before parturition is about 0.1 muM; the concentration declines rapidly after parturition to about 0.01 muM. A corticosteroid-binding protein was found, also, in the whey of mature rat milk at levels of about 0.3 muM. This protein resembles rat serum corticosteroid-binding globulin: the equilibrium association constants for cortisol, corticosterone, and progesterone binding, and the apparent molecular size, as determined by Sephadex G-200 chromatography, were similar; the elution behavior of the respective proteins on anion exchange chromatography with DEAE-Sephadex A-50 was similar, also. Identity of the corticosteroid-binding proteins in whey with corticosteroid-binding globulin in serum is not presumed, however. Rat and human whey exhibited very little testosterone- or 17 beta-estradiol-binding activity. It is suggested the corticosteroid-binding proteins may play a significant physiological role in regulating the concentration of the bound and unbound forms of progesterone and cortisol in the fluids bathing the epithelial cells lining the mammary ducts and acini.