The Growth Hormone Dependence of a Somatomedin-Binding Protein in Human Serum*

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Abstract
We previously reported that a serum somatome-din (SM)-binding protein is GH dependent in the rat. We now show that a similarly sized SM-binding protein in humans is also GH dependent. Multiplication-stimulating activity (MSA), purified from medium conditioned by the BRL-3A rat liver cell line, was radioiodinated and used as a human SM analog for these studies. A normal human serum pool and sera obtained from nine GH-deficient patients before and after GH treatment were gel filtered on Sephadex G-200 in 0.05 M NH4HCO3. Specifi c [l25I]MSA-binding activity was measured in each column fraction. When pre-GH sera were gel filtered over Sephadex G-200, specific [I25I]MSA-binding activity was low and eluted in the γ-globulin region (peak II), in the albumin region (peak III), or in both depending upon the patient. After GH treatment, specific [125I]MSA binding increased dramatically and was always found primarily in peak II, resembling the pattern for normal human serum. For three patients, Sephadex G-200 column pools were chromatographed on Sephadex G-50-1 M acetic acid to dissociate and separate binding protein from SM activity. SM activity was measured by a [3H]thymidine incorporation assay in chick embryo fibroblasts and by a competitive protein-binding assay using rat serum binding protein. SM activity coeluted with specific [l25I]MSA-binding activity on Sephadex G-200 gel filtration of normal, pre-GH, and post-GH sera and was higher in column fractions from post-GH and normal sera than from pre-GH sera. To determine whether [l25I]MSA-binding activity measurements on Sephadex G-200 column fractions reflected the distribution of binding capacity, Scatchard analysis of MSA binding to stripped binding protein was performed on Sephadex G-200 pools that had been gel filtered on Sephadex G-50-1 M acetic acid. The maximal binding capacity was found in the Sephadex G-200 column fractions identified by the highest [125I]MSA binding activity. Binding capacities of peak II binding protein increased several-fold after GH treatment. We conclude that the level of a γ-globulin-sized SM-binding protein is GH dependent in humans.