Regulation of growth hormone binding protein in man: comparison of gel chromatography and immunoprecipitation methods

GH circulates in part bound to a high affinity binding protein (GHBP). Gel chromatography is the established method for measuring GH binding activity in plasma, but is slow and tedious. The separation of bound from free GH by immunoprecipitation using a monoclonal antibody to the GH receptor may be a more practical alternative. We have examined the effects of GH and estrogen status on GHBP measured in 24-h pool samples and compared results obtained from gel filtration and immunoprecipitation. GHBP activity (percent specific binding of [125I]GH) was measured in normal, GH-deficient, and acromegalic subjects; and in two groups of postmenopausal women before and after oral (ethinyl estradiol 20 micrograms daily) or transdermal (17 beta-estradiol 100 micrograms daily) estrogen therapy. GHBP activity was not significantly different between normal, GH-deficient, and acromegalic subjects matched for age and sex. Neither GH administration to GH-deficient and normal subjects, nor octreotide treatment of patients with acromegaly, significantly altered GHBP activity. Oral estrogen treatment significantly increased GHBP activity. This change was associated with an increase in binding capacity (P = 0.001) but not affinity, as revealed by Scatchard analysis. GHBP activity did not change significantly with transdermal estrogen therapy. GHBP activity obtained by the two methods was significantly correlated (n = 70, r = 0.92, P = 0.001) although values for the antibody method were 25% higher. Similarly, capacity estimates were highly correlated (r = 0.90, P = 0.0001), with values by immunoprecipitation being 40% higher. We conclude that GH secretory status is not a determinant of GHBP. Oral estrogens increase GHBP activity through an increase in capacity and not affinity. The estrogen effect is route dependent and the oral response is likely to reflect a first pass hepatic effect. The higher binding activity and capacity estimated from immunoprecipitation are likely to reflect a more rapid separation process thereby minimizing dissociation of GHBP complex. Immunoprecipitation offers the advantages of simplicity, convenience, and accuracy for the measurement of GH binding activity and GHBP concentrations in serum.