title: Purification of the 22000- and 20000-mol.wt. forms of human somatotropin and characterization of their binding to liver and mammary binding sites.
abstract: Quantitative data concerning the binding of 22000-mol.wt. human somatotropin and its 20000-mol.wt. variant are described using pregnant-rabbit liver and mammary-gland receptors. The purification and the complete chemical characterization of both human somatotropin and its 20000-mol.wt. variant is also presented. Contamination of the 20000-mol.wt.-variant preparation by 22000-mol.wt. hormone was found to be 0.5% by weight as measured in radioimmunoassay using monoclonal antibody. Labelling of human somatotropin and its 20000-mol.wt. variant using the Iodogen method is described as well as the characterization of the binding to pregnant-rabbit liver and mammary-gland receptor preparations. The maximum binding capacity of the 125I-labelled human somatotropin was between 50 and 60% to liver particulate receptor, whereas that of the 20000-mol.wt. variant was 30%. The specificity of binding of both forms to rabbit hepatic and mammary-gland receptor was found to be similar for both proteins in the same system. The affinity constants and capacity were respectively 0.7 X 10(10)M-1 and 815 fmol/mg of protein for human somatotropin and 0.6 X 10(10)M-1 and 1.250 fmol/mg of protein for the 20000-mol.wt. variant. These data suggest that both proteins behave as partial agonists to the receptors studied.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D013006:Growth Hormone / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008321:Mammary Glands, Animal / Q000378:metabolism*; D008970:Molecular Weight; D011817:Rabbits; D011863:Radioimmunoassay; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011986:Receptors, Somatotropin
chemical_list: D011956:Receptors, Cell Surface; D011986:Receptors, Somatotropin; D013006:Growth Hormone