Homologous somatotropin radioreceptor assay utilizing recombinant bovine growth hormone

A homologous radioreceptor assay using recombinant bovine growth hormone and bovine liver membranes is described. The total specific binding of ¹²⁵I-labeled recombinant bovine growth hormone to the 100000 × g pellet was 48% in 24 h at 25 °C. Hormone binding was partially reversible, with 40% of the radiolabeled hormone being irreversibly bound. The amount of specific binding varied with assay pH, with the optimum occurring at pH 7.8. Specific binding was temperature-dependent, with greater specific binding occurring at 25°C than at 5°C or 37°C during a 24 h period. Recombinant bovine growth hormone, human growth hormone, ovine growth hormone and recombinant porcine growth hormone competed effectively with ¹²⁵I-labeled recombinant bovine growth hormone for binding sites, while bovine prolactin and ovine prolactin were needed in amounts 10⁶-fold the concentration of recombinant bovine growth hormone to displace the radiolabeled hormone. Surprisingly, human placental lactogen did not displace the radiolabeled hormone.