GROWTH-HORMONE BINDING-PROTEINS - BIOCHEMICAL-CHARACTERIZATION AND ASSAYS

Circulating growth hormone binding proteins have recently been recognized in man and certain animals. In man, two specific growth hormone binding proteins have been described, one with high affinity and another with low affinity. The high affinity binding protein corresponds to the extracellular portion of the hepatic growth hormone receptor, whereas the low affinity binding protein appears unrelated to the receptor. Both binding proteins bind one molecule of growth hormone to form complexes of 85 kD (high affinity) and 125 kD (low affinity). The isoelectric points of the two binding proteins are about 5.0 and 7.1, respectively. Most of the complexed growth hormone in plasma is associated with the high affinity binding protein. The binding proteins can be assayed by their specific ability to bind human growth hormone. Generally, plasma is incubated with radiolabelled human growth hormone under standardized conditions, and bound growth hormone is then separated from free growth hormone by size exclusion or ion exchange chromatography, or by charcoal adsorption. Several technical modifications of these fundamental assay designs exist.