Initial Characterization and Sexual Dimorphism of Serum Growth Hormone-Binding Protein in Adult Rats*

The in vitro binding of 125I-bovine growth hormone (bGH) to adult rat serum was studied using Ultrogel AcA34 filtration. When analytical chromatography on a 1.6 x 100 cm column was performed, four peaks of radioactivity were revealed: the first two peaks with Mr +/- 220,000 and +/- 110,000 corresponded to bound 125I-bGH (abolished by excess of unlabeled bGH), the third corresponded to free 125I-bGH and the fourth to free Na125I (Vt). On a short (1 x 40 cm) column, bound 125I-bGH eluted as a single peak between the void volume (Vo) and the peak of free 125I-bGH. Serum 125I-bGH binding was specific, saturable, and time-dependent. Specific serum 125I-bGH binding (bound/total radioactivity x 100), calculated as the difference of binding in the absence and the presence of an excess unlabeled bGH, was higher in female than in male rats (26 +/- 2% vs. 11 +/- 1%, respectively; mean +/- SE; n = 6; P less than 0.01). Scatchard analysis revealed a binding affinity of 2 x 10(8) M-1 for both sexes, and a binding capacity of 6.4 x 10(-8) mol/liter for the female rats and 1.6 x 10(-8) mol/liter for the male rats (mean of three serum pools of three animals each). Specific binding of 125I-bGH to serum correlated significantly with 125I-bGH binding to liver homogenates (r = 0.83; n = 12; P less than 0.01). These results suggest the presence of a specific GH-binding protein in rat serum and provide further evidence for a close relationship between serum GH-binding protein and hepatic GH receptors.