Human liver growth hormone receptor and plasma binding protein: characterization and partial purification

The human liver GH receptor has been further characterized using several biochemical approaches. Crosslinking of [125 I]human GH (hGH) to microsomal receptors and to particulate and solubilized plasma membrane receptors, followed by gel electrophoresis and autoradiography, revealed two predominant receptor-hormone complexes with a mol wt of 124,000 and 75,000, respectively. As previously shown, the 70-80 k band appears to be generated from the 124 k band in the presence of beta-mercaptoethanol, suggesting intersubunit disulfide linkages. A minor complex of mol wt 150,000 was sometimes found. By immunoblot, using a polyclonal antibody raised against a synthetic peptide (residues 391-405 of the mature human GH receptor), a single band of 100 k was detected. Human liver GH receptor and plasma GH-binding protein (BP) were purified 1,000- and 4,000-fold, respectively. The partially purified membrane receptor, analyzed by ligand-blot, showed two major bands of 55 and 32 k and minor bands of 68 and 47 k. Crosslinking of the purified GH-BP or purified receptor with [125I]hGH revealed a 75 k receptor-hormone complex. Polyclonal antibodies raised against the hepatic GH receptor inhibited the binding of hGH to the human receptor and were able to immunoprecipitate the GH receptor and also the GH-BP complex. Our findings demonstrate the existence of multiple forms of the GH receptor in human liver (major components of 100 and 50-55 k, minor component of 130 k); they lend more support to the possible subunit structure of the GH receptor; and finally, they also suggest a close relationship, with common antigenic properties, of the membrane receptor and the plasma GH-BP.