Insulin-like growth factors I and II (IGF-I and IGF-II) are single chain peptides with a molecular weight of 7649 and 7471 respectively. They show a high degree of sequence homology between each other and to proinsulin (Rinderknecht & Humbel, 1978a&b). IGF-I is identical to somatomedin C (Klapper, Svoboda & Van Wyk, 1983) and somatomedin A (Enberg et al., 1984). These designations should not be used any more according to a generally accepted recommendation on nomenclature (Daughaday et al., 1987).
IGF-I and IGF-II in the blood and in other compartments are non-covalently bound to specific carrier proteins (IGF-binding proteins – IGFBP) with high affinity (dissociation constants in the order of magnitude of 10-11 mol/L). Six classes of IGFBPs are known, IGFBP-1 through -6, with a high degree of sequence homology (Ballard et al., 1989; Drop, 1992). Some biochemical properties are given in Table 2.1. In 1998, other homologous proteins that are structurally related to IGFBPs were described. They have a much lower affinity to IGFs and, therefore, were termed IGFBP-related proteins (Yang et al., 1998).
Cells need IGF-I to feel comfortable
IGF-I plays a central role in the regulation of the activity of many cell types with only a few exceptions such as hepatocytes (Figure 2.1). In general, IGF-I stimulates proliferation, differentiation and specific cellular functions such as steroidogenesis by gonadal cells, synthesis of glycosaminoglycanes by chondrocytes or even apparently remote actions as for instance natural killer cell activity. A prerequisite, however, is that cells are ready to perform these actions.