Somatomedin carrier proteins.

The somatomedins, such as the insulin-like growth factors I and II, somatomedins A and C, and multiplication-stimulating activity, are small, growth hormone dependent polypeptides which are the assumed mediators of growth hormone action on extraskeletal tissues. The somatomedins are unique among polypeptide hormones in that they exist in serum as two high molecular weight complexes of approximately 150 000 and 60 000 molecular weight. Little, if any, free hormone is present in plasma or serum. These circulating complexes appear to represent the reservoir for these hormones since none is stored in any other tissue and the circulating concentration (approx. 1 microgram/ml) exceeds the biologically active concentration of the free hormone by many fold. Many recent investigations regarding the subunit composition of the two serum carrier protein complexes and their effect on the biological activity of the somatomedins have revealed an elaborate system of intense physiological control ultimately aimed at delivering somatomedin to its specific receptor on the surface of the target cell.