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Comparison between the human serum growth hormone-binding protein and the water-soluble growth hormone-binding site
G Massa1, M Ilondo, M Vanderschueren-Lodeweyckx
1Department of Paediatrics, University of Leuven, Belgium.
Insights
Human serum growth hormone-binding protein (GHBP) characteristics match those of a soluble binding site from IM-9 cells. This supports GHBP being released from cellular growth hormone receptors.
Area of Science:
- Endocrinology
- Molecular Biology
- Protein Chemistry
Background:
- Growth hormone-binding protein (GHBP) in human serum plays a crucial role in regulating growth hormone (GH) bioavailability.
- The origin and precise relationship of serum GHBP to the cellular GH receptor remain areas of investigation.
Purpose of the Study:
- To compare the biochemical and binding characteristics of human serum GHBP with a soluble GH-binding site derived from IM-9 lymphocytes.
- To investigate the hypothesis that serum GHBP is generated from the cellular GH receptor.
Main Methods:
- Preparation of a water-soluble GH-binding site from cultured IM-9 lymphocytes using iodoacetamide.
- High-performance liquid chromatography (HPLC) gel filtration to analyze the molecular size of GH-binding complexes.
- Binding studies to assess specificity and affinity using 125I-labeled human GH (125I-hGH) and different GH variants.
Main Results:
- The water-soluble GH-binding site from IM-9 cells exhibited similar molecular weight and elution profiles when complexed with 125I-hGH as serum GHBP.
- Binding specificities for GH were identical across serum GHBP, the soluble IM-9 binding site, and the cellular GH receptor.
- Serum GHBP and the soluble IM-9 binding site showed similar, but lower, binding affinities for GH compared to the cellular GH receptor.
Conclusions:
- The characteristics of serum GHBP are highly comparable to the soluble GH-binding site released from IM-9 cells.
- These findings provide strong support for the hypothesis that human serum GHBP is produced via proteolytic cleavage of the cellular GH receptor.
Abstract:
The characteristics of the human serum growth hormone-binding protein (GHBP) were compared with those of a water-soluble GH-binding site prepared by incubating cultured IM-9 lymphocytes in assay buffer with 25 mmol/l iodoacetamide. High-performance liquid chromatography gel filtration of the water-soluble GH-binding site incubated with 125I-labeled human GH ([125I]hGH) revealed a large peak of bound [125I]hGH eluting at the same position as the peak of [125I]hGH bound to the GHBP in serum. The estimated M(r) of the peak was 120,000, presumably representing one [125I]hGH bound to two binding sites. The binding specificities of the serum GHBP, the water-soluble GH-binding site and the GH receptor on IM-9 lymphocytes were identical. The binding affinities for 22,000 hGH and for 20,000 hGH of the serum GHBP were similar to the binding affinity of the water-soluble GH-binding site but lower than those of the cellular GH receptor. These findings show that the characteristics of the serum GHBP are comparable to those of the water-soluble GH-binding site released from IM-9 cells and support the hypothesis that in man the serum GHBP is produced by proteolytic cleavage of the cellular GH receptor.
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