A new and convenient assay of growth hormone-binding protein activity in human serum

The recent discovery of a GH-binding protein (GH-BP) in human serum has important implications for the distribution, metabolism, and physiological activity of human GH (hGH). In this report we describe an accurate, simple, and rapid assay for measuring GH-BP in human serum, based on the dextran-coated charcoal technique for separating bound from free hGH. The present method reduces the degree of dissociation of the GH-BP complex during the separation procedure, thus increasing accuracy, allows routine use of small working volumes of sera, and can be practically used for running large scale assays. Binding of [125I]hGH to human serum GH-BP, identified and characterized by this procedure, was time, temperature, and dose dependent; the binding was highly specific for hGH and with a high affinity (Ka, 1.08 +/- 0.19 x 10(9) L/mol). Preliminary results, expressed as percent specifically bound [125I]hGH corrected for endogenous hGH, indicate that the mean GH-BP activity in 50 microL sera from normal adult men and women was 11.32 +/- 0.45%. Significantly lower values were obtained in sera of infants, 3 days after birth (1.65 +/- 0.15%), and in short-statured children with normal GH secretion (7.38 +/- 0.22%). In patients with liver cirrhosis GH-BP levels decreased to 5.93 +/- 1.14%. The availability of a simple and convenient procedure for large scale determination of GH-BP coupled with the suggestion that this protein can reflect the GH receptor provides us with an indirect means for assessing changes in GH receptor activity in various physiological and pathological conditions.