Comparison of binding proteins of glucocorticoids in mammary tissue and in blood sera from lactating cows.

Supernatant fractions (700 X g) isolated from homogenates of mammary tissue slices from three lactating Holstein cows (slices previously incubated with [hydrogen-3] cortisol for 1 h at 37 C) were electrophoresed on 7% polyacrylamide gels. The majority of radioactivity was in a protein(s) 2.5 to 3 cm from the origin whereas bovine serum incubated with [hydrogen-3] cortisol showed the majority of radioactivity in a protein 5 to 6 cm from the origin. N,N-diethylaminoethyl cellulose chromatography of 700 X g supernatant fluids from three lactating cows revealed that [hydrogen-3] cortisol was associated with a protein component that eluted with .3 M potassium phosphate, pH 8.0. In contrast, [hydrogen-3] cortisol bound to bovine sera eluted as two protein components with .05 and .1 M potassium phosphate, pH 8.0. Approximate molecular weights determined from gel filtration (four lactating cows) and sucrose density studies (two lactating cows) were estimated to be 2.5 X 10(5) to 3 X 10(6) for the 700 X g mammary receptor of cortisol and 6 X 10)4) to 8 X 10(4) for the primary protein which binds cortisol in serum. We conclude that lactating bovine mammary tissue contains a protein(s) capable of binding tritiated cortisol which is unique from the corticosteroid binding proteins of blood.