Radioimmunoassay of the Somatomedins/Insulin-like Growth Factors

  • L. E. Underwood
  • M. G. Murphy
Part of the Handbook of Experimental Pharmacology book series (HEP, volume 82)


The somatomedins, also referred to as insulin-like growth factors, are a family of peptides whose serum concentrations are regulated principally by growth hormone and nutrient status. Evidence is now emerging that most or all of the biologic effects of the somatomedins can be attributed to two peptides, somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) and IGF-II (Van Wyk 1984). Sm-C/IGF-I has a molecular weight of 7649 and a pI of 8.0–8.7. It contains 70 amino acid residues in a single chain with 3 disulfide bridges. It is highly growth hormone dependent, and has potent growth-promoting activity in many in vitro systems (Zapf et al. 1984). IGF-II is also a single-chain peptide, which has a more nearly neutral pI and a molecular weight of 7471. IGF-II is less growth hormone dependent and appears to have less growth-promoting activity than Sm-C/IGF-I. Both Sm-C/IGF-I and IGF-II possess nearly 50% homology with proinsulin in regions of the molecule that correspond to the A and B chains of insulin. As in humans, two forms of somatomedin have been isolated from rat plasma. One of these, multiplication-stimulating activity (MSA) is the rat homolog of human IGF-II, differing from human IGF-II by only five amino acid residues (Marquardt et al. 1981). Although not certain, it is expected that other species will be found to have two somatomedins, similar to those in humans and rats. The term somatomedin-A has been applied to a substance that now appears to be a deamidated form of Sm-C/IGF-I, and the peptide referred to as somatomedin-B proved to be a fragment of a plasma-spreading factor contaminated with epidermal growth factor (EGF) (Heldin et al. 1981; Barnes et al. 1984).


Growth Hormone Radioreceptor Assay Heparinized Plasma Parallel Dose Supplementary Essential Amino Acid 
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© Springer-Verlag Berlin Heidelberg 1987

Authors and Affiliations

  • L. E. Underwood
  • M. G. Murphy

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