[Growth hormone specific binding sites : characteristics of lactogenic receptors induced by estrogens (author's...

In vitro binding of growth hormone was characterized in rat liver. Microsomal preparations were found more active than membranes purified with an aqueous two-phase polymer system. Binding conducted at 4 degrees C was found optimal after 72 hours of incubation in 5 mM Tris-Maleate buffer pH 6.4 with 25 mM CaCl2. Injecting estrone (25 microgram/100 g B.W.) for one week induced the formation of lactogenic receptors, and increased the specific binding of hGH from 2.8 +/- 1.9 to 22.3 +/- 7.1%. Prolactin and hGH, but not rGH or other pituitary hormones, could displace radioactive hGH. With incubations conducted at 37 degrees C, equilibrium was reached more rapidly but at the cost of a more extensive degradation. The presence of membrane receptors in the medium partly protected the hormone against aggregation and degradation. Scatchard plots were obtained from experiments conducted under optimal conditions and analyzed on computer using a program based on an iterative method. Data indicated that lactogenic receptors possessed a single specific binding site for hGH with a constant (Ka) of 2.18 +/- 0.22 x 10(9) M-1 and a binding capacity of 304 +/- 91 fm/mg proteins.