[Insulin-like growth factor proteins]

The insulin-like growth factors circulate in plasma complex with a family of binding proteins that extend the serum half-life of the IGF peptides, transport the insulin-like growth factors to target cells, and modulate the interaction of the insulin-like growth factors with surface membrane receptors. The binding proteins have insulin-like growth factor-independent actions too, possibly by interacting with their own receptors on the cell membrane. The insulin-like growth factor binding protein superfamily consists of six classical binding proteins (-1, -2, -3, -4, -5, -6), and insulin-like growth factor binding protein-related proteins (-1, -2, -3, -4). Specific radioimmunoassays have been developed for the six classical binding proteins, the measurement of insulin-like growth factor binding protein-3 appears to have the greatest clinical value because it is growth hormone dependent.