Specific Water-Soluble Binding Sites for Serum Insulin-Like Peptides in Human and Animal Tissues*

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Abstract
The cytosol fraction of rat liver and kidney and human placenta was shown to possess soluble binding proteins for an insulin-like somatomedin peptide (ILAs). A similar component was also found in the cytosol fraction of livers and kidneys from guinea pigs, rabbits, and dogs. Assessment of binding was performed by gel filtration on Sephadex in 25 mm Tris-HCl, pH 7.4, or by dextran-coated charcoal separation. Specific binding of [l25I]iodo-ILAs was shown, at 4 C in vitro, to reach equilibrium by 4 h. Specific binding was reduced and degradation of [l25I]iodo-ILAs was increased when incubations of rat liver and kidney cytosol preparations were carried out at 37 C. Cytosol binding of [l25I]iodo-ILAs was specifically inhibited by unlabeled ILAs but not by insulin, human GH, nerve growth factor, or epidermal growth factor even at concentrations of 1 μg/ml. Specific binding of [l25I]iodo-ILAs was shown to increase linearly as a function of cytosol concentration used and was consistently greatest in human placenta. No specific binding was observed for human [l25I]iodo-hGH, human [l25I]iodo-PRL, or [l25I]iodoinsulin in the rat tissues studied, whereas 5% or less of specific binding was observed for [l25I]iodoinsulin with human placental cytosol. The cytosol binding proteins were heat labile and trypsin sensitive. Rat liver and kidney cytosol binding proteins were excluded on Sephadex G-150 chromatography, suggesting a molecular size in excess of 150,000 daltons. Human placental cytosol proteins, however, eluted just before albumin, suggesting a molecular size near 60,000 daltons. Whether these differences represent species or tissue differences remains to be determined. The exact relationship of the specific cytosol binding proteins for ILAs to the cell surface membrane receptors, to plasma binding proteins, and to the recently reported intracellular binding sites of the Golgi apparatus is not clear at present. However, the demonstration of specific cytosol binding sites in various tissues for the somatomedin family of peptides could lead one to speculate that these binding sites may subserve an important function in the regulation of target cell function by these GHdependent peptides. (Endocrinology106: 113, 1980)