Characterization of a Low Affinity Binding Protein for Growth Hormone in Rat Serum1

GH forms a high Mr complex in rat serum distinct from that with GH-binding protein (GHBP). The present study investigates the nature of this complex. When subjected to AcA44 filtration chromatography, 125I-labeled human GH (hGH) in rat serum eluted in four peaks. Peak 1 eluted at the void volume, whereas peaks 2, 3, and 4 corresponded to the GHBP complex, free hGH, and iodide, respectively. Stripping of GHBP in serum by immunoaffinity chromatography depleted peak 2 but did not affect peak 1. Peak 1 accounted for 11.4 ± 1.2% of the total radioactivity (mean± sem; n = 6) in stripped serum. Addition of unlabeled hGH (0.9–9 μm) demonstrated the binding of[ 125I]hGH to be specific, with Scatchard analysis revealing an affinity of 0.88 ± 0.03 × 105 m−1 (n = 3) and a capacity of 2.46± 0.14 μm. Sepharose CL-6B filtration chromatography showed the complex to be 260 kDa in size. The distribution of GH binding to GHBP and this high Mr serum factor was investigated by incubating [125I]hGH in sera containing a low (5 ...