ROLE OF SOMATOMEDIN C/INSULIN-LIKE GROWTH FACTOR I BINDING PROTEINS IN REGULATION OF GROWTH

The somatomedin peptides are unique in being able to bind to proteins of larger molecular weight. There appear to be two classes of somatomedin binding proteins with different physiologic roles. One is a 150,000-dalton protein that is growth hormone-dependent and functions to transport somatomedins in plasma in an inactive state. The other binding protein is smaller, 33,000 to 43,000 daltons, depending on the species. The 150-Kd carrier protein has been thought to transport somatomedin to target tissues and possibly to regulate the level of free somatomedins. However, evidence is accumulating that somatomedins may function primarily as autocrines and paracrines, suggesting that the carrier protein may function to transport active somatomedins away from tissues. The smaller binding protein represents most, if not all, of the unsaturated plasma somatomedin binding capacity and may have a different physiological role. A possibly analogous protein produced by fibroblasts has been reported to potentiate the mitogenic action of somatomedin-C.