title: An investigation of sites that bind human somatotropin (growth hormone) in the liver of the pregnant rabbit.
abstract: The binding of 125I-labelled human somatotropin (growth hormone) to a crude membrane preparation from the liver of pregnant rabbit, and to receptors solubilized from this fraction by Triton X-100, was dependent on time, temperature and receptor concentration. At 4 degrees C a steady state was reached after 20 h, and maximum specific binding (as a percentage of total tracer added) was approx. 50% for both membrane-bound and solubilized receptors. Solubilization did not significantly affect the binding properties of the receptor at low concentrations of Triton X-100 (less than 0.05%, v/v, in the assay tube). However, at higher concentrations (approx. 0.1%, v/v), the detergent lowered the ability of some hormones, for example ovine prolactin, to displace 125I-labelled human somatotropin, but did not affect other hormones such as bovine somatotropin. Some somatogenic hormones, such as bovine somatotropin, and some lactogenic hormones, such as ovine prolactin, displaced 125I-labelled human somatotropin from membrane-bound and solubilized receptor preparations. Furthermore, 85% of 125I-labelled bovine somatotropin was displaced from membrane-bound receptors by ovine prolactin, and 125I-labelled ovine prolactin was almost completely displaced by bovine somatotropin. Scatchard analysis of the binding data for human somatotropin suggested a single class of binding sites in the membrane-bound receptor preparation, with an affinity (Ka) of 1.9 X 10(9) M-1 and a capacity of 1726 fmol/mg of protein; these values were slightly increased by solubilization (Ka = 3.2 X 10(9) M-1, capacity = 2103 fmol/mg of protein). Scatchard analysis of binding to membrane-bound receptors also indicated a single class of high-affinity binding sites for bovine somatotropin (Ka = 4.8 X 10(9) M-1, capacity = 769 fmol/mg) and for ovine prolactin (Ka = 6.1 X 10(9) M-1, capacity = 187 fmol/mg).
mesh_terms: D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D003902:Detergents / Q000494:pharmacology; D005260:Female; D013006:Growth Hormone / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D017830:Octoxynol; D011092:Polyethylene Glycols / Q000494:pharmacology; D011247:Pregnancy; D011270:Pregnancy, Animal; D011388:Prolactin / Q000378:metabolism; D011817:Rabbits; D051381:Rats; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D011986:Receptors, Somatotropin; D012756:Sheep; D013045:Species Specificity; D013552:Swine; D013696:Temperature
chemical_list: D003902:Detergents; D011956:Receptors, Cell Surface; D011986:Receptors, Somatotropin; D011092:Polyethylene Glycols; D011388:Prolactin; D013006:Growth Hormone; D017830:Octoxynol