The circulating growth hormone (GH)-binding protein complex: a major constituent of plasma GH in man.

The recent discovery of a specific binding protein for human GH (hGH) in human plasma suggests that hGH circulates in part as a complex in association with the binding protein(s). However, the magnitude of the complexed fraction prevailing under physiological conditions is unknown because of 1) dissociation of the complex during analysis and 2) potential differences in the binding characteristics of radiolabeled and native hGH. We conducted experiments designed to minimize dissociation during analysis (gel filtration in prelabeled columns, frontal analysis, and batch molecular sieving) with both native and radioiodinated hGH. All three methods yielded similar estimates for the complexed fraction. In normal plasma the bound fraction for 22 K hGH averaged 50.1% (range, 39-59%), that for 20 K hGH averaged 28.5% (range, 26-31%). Above a hGH level of about 20 ng/ml the bound fraction declines in concentration-dependent manner due to saturation of the binding protein. We conclude that a substantial part of circulating hGH is complexed with carrier proteins. This concept has important implications for the metabolism, distribution, and biological activity of hGH.