Isolation and NH₂-terminal amino acid sequences of rat serum carrier proteins for insulin-like growth factors

Three N-glycosylated carrier proteins (CP) for insulin-like growth factors (apparent molecular weights 30–32, 42 and 45 kDa) were isolated from adult rat serum. They share the same amino terminus (up to amino acid 31) and are constituents of the growth hormone-dependent native 150–200 kDa IGF carrier complex. Residues 12–31 display 60 and 50% sequence homology, respectively, to residues 2–21 of fetal rat and to residues 4–22 of a human amniotic fluid IGF carrier protein. No homology exists with the type I or II IGF receptors. Adult rat serum also contains a fourth IGF CP (24 kDa) whose 9 NH₂-terminal amino acids are identical to those of the fetal form. Our findings suggest that the three N-glycosylated components originate from the same IGF carrier protein (adult form) and that the 24 kDa protein is a separate (fetal) species.