IDENTIFICATION OF GROWTH-HORMONE BINDING-PROTEIN IN RAT SERUM

The present report describes the initial characterization of a specific, high-affinity growth hormone binding protein (GH-BP) in adult male rat serum. GH-BP activity was measured by incubation of rat serum with [125I]hGH and [125I]rGH and separation of bound from free GH by dextran-coated charcoal. [125I]hGH binding to rat serum was dependent on serum concentration and incubation time, equilibrium being reached within 10 min both at 4 and 37 °C. Binding was rapidly and completely reversible and specific for somatogenic (but not lactogenic) hormones. Scatchard analysis yielded a linear plot with an affinity (Ka) of 1.51 ± 0.63 × 108 M-1. Preliminary data obtained in various physiological conditions showed that GH-BP activity in adult male rats was 5.95 ± 0.20%/0.1 ml serum. Significantly higher values were obtained in sera of female (21.66 + 0.79%/0.1 ml serum) and pregnant rats (23.02 ± 1.15%/0.1 ml serum). A closer analysis of these binding values by Scatchard analysis revealed that the binding capacity in pregnant rats (50.5 ± 5.8 pmol/0.1 ml serum) was significantly higher than in adult female estrous rats (19.2 ± 6.5 pmol/0.1 ml serum), both being much higher than in adult male rats (2.5 ± 0.6 pmol/0.1 ml serum). The GH-BP activity of 10-day-old rats was only $ ̃63% of the adult male rat value. The presence of high-affinity GH-specific binding protein in rat serum suggests a probable action in regulation of GH activity. The detailed physiological role of rat serum GH-BP is currently being investigated. © 1990.