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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Massa et al. (1993) studied this question.