A Specific Growth Hormone-Binding Protein in Human Plasma: Initial Characterization*

Abstract

Human (h) GH in plasma exists as a series of size isomers, which are in part explained by the presence of hGH oligomers. However, certain aspects of circulating large mol wt hGH, such as its high relative proportion compared to that in the pituitary, are poorly understood. To explore whether binding of hGH to plasma protein(s) could contribute to the phenomenon of large mol wt hGH, we incubated freshly prepared monomeric [125I]hGH or biosynthesized [3H]hGH with normal human plasma or serum at pH 7.4 for various time periods at 22 and 37 C. Plasma radioactive hGH patterns were then analyzed simultaneously with unincubated tracer hGH by Sephadex G-100 and G-200 chromatography. We found that part of the radioactivity was converted to a component with an apparent mol wt of 85,000, suggesting binding to a plasma protein(s). This phenomenon was inhibited in a dose-dependent fashion by unlabeled hGH. Saturation/Scatchard analysis indicated an association constant (Ka) of 2-3 X 10(8) M-1 and a maximum binding capacity of 20 ng hGH/ml plasma. Binding was rapid, reversible, and specific. A number of polypeptide hormones, including human placental lactogen, hPRL and rat GH, did not inhibit hGH binding. However, the 20K variant of hGH interacted weakly with the plasma binding component (Ka, 1.2 X 10(7) M-1; maximum binding capacity, 450 ng/ml). The binding component was heat labile and could be partially purified by gel permeation chromatography and affinity chromatography on a hGH-Sepharose column. Its estimated mol wt is 60,000-65,000, and it appears to bind one molecule of hGH to form a complex of 80,000-85,000 mol wt. The binding component is neither albumin nor an immunoglobulin. Under physiological conditions, a minimum of 15-18% of circulating hGH is presumably bound to this plasma component. We conclude that a specific, high affinity, low capacity binding protein for hGH with mol wt of 60,000-65,000 exists in normal and hypopituitary human plasma. hGH complexed with this protein forms part of big-big hGH. The biological significance of this binding protein remains to be investigated.

Identification and Modulation of a Growth Hormone-Binding Protein in Rainbow Trout (Oncorhynchus mykiss) Plasma during Seawater Adaptation

“…GH-BP is a short soluble form of the GH receptor. Serum GH-BP binds to circulating GH forming a high-affinity complex of 75-80 kDa (K a ϭ 3-9 ϫ 10 8 M Ϫ1 ) with limited capacity (20-150 ng/ml) (Barnard et al, 1985;Baumann et al, 1986;Baumann, 1991;Tar et al, 1990). The present study only detected hGH-BP complexes of high molecular weight 130-150 kDa.…”

Ligand-independent growth hormone receptor dimerization occurs in the endoplasmic reticulum and is required for ubiquitin system-dependent endocytosis

“…In this way, the extracellular domain of cell surface-localized GHRs is digested whereas the TMD and intracellular domain remain intact. These membrane-bound remnants resemble those that remain after shedding of the GHR extracellular domain, a process that occurs at the cell surface in the absence of GH (18,25). As seen in Fig.…”

“…Therefore, we speculate that the dimerization of the GHR is mediated by way of the TMD, which is in accordance with recent findings that the TMD of the EpoR (21), the EGF receptor (37), the MHC class II ␣ and ␤ subunits (32), and glycophorin A (38) are sufficient for (ligand-independent) dimerization. The proposed model also explains why in gel filtration experiments a complex of a monovalent GH antagonist and two extracellular GH-binding domains has never been identified, whereas such a complex was observed when normal, divalent GH was used (25,39). Although the TMD is required for dimerization, it may not be sufficient.…”

Soluble receptors in human disease

“…1C). This mode of action appears to be especially important when the site of ligand production is distant from its target tissue, as is the case of growth hormone [19,114,115]. Thus, the extracellular domain of the growth hormone receptor is the serum growth hormone binding protein that functions to keep growth hormone in the blood for a longer period of time [115,116].…”