Identification and partial characterization of a growth hormone-binding protein in rat serum

A binding protein for growth hormone in serum from female rats has been identified and partially characterized. Serum was incubated with 125I-labelled human GH and fractionated on an agarose HPLC column. Complexes between the binding protein and 125I-hGH were detected as a peak eluted at a volume corresponding to a relative molecular weight of 159,000 +/- 11,000 (N = 8). The peak was not seen when the incubation was carried out in the presence of excess unlabelled hGH. The 125I-hGH bound with high affinity (Ka = 0.87 +/- 0.3 l/nmol; N = 3) and the binding was time- and dose-dependent. Bound 125I-hGH was displaced by rat GH and bovine GH, but not by rat prolactin. The protein was not detected in radioreceptor assay by the commonly used polyethylene glycol precipitation technique and was not recognized by a monoclonal antibody raised against lactogenic receptors from female rat liver. Covalent cross-linking of 125I-hGH to serum revealed in SDS electrophoresis two labelled complexes with molecular weights of 62,300 +/- 3900 and 77,600 +/- 4100, respectively (N = 10).