Insulin-like growth factor I (IGF-I, a.k.a. somatomedin C) is a 7.6 kDa, 70 amino acid residue peptide, which mediates the actions of growth hormone (GH).1 IGF-I is synthesized as a prohormone, a polypeptide consisting of A, C, B, D, and E domains.1, 2 After post-translational modification, the mature IGF-I consist of the A, C, B and D domains, and is structurally homologous to IGF-II and insulin. In vivo, IGF-I is secreted by the liver and several other tissues and is postulated to have mitogenic and metabolic actions at or near the sites of synthesis; i.e. paracrine effects.1 IGF-I also appears in the peripheral circulation where it circulates primarily in a high molecular weight tertiary complex with IGF-binding protein-3 (IGFBP-3) and acid-labile subunit (ALS).2,3 A smaller proportion of IGF-I circulates in association with other IGF-binding proteins.3

Recently, there has been research interest in the measurement of serum/plasma “unbound” IGF-I which, theoretically, is the biologically active fraction. Although the existence of a true unbound IGF serum/plasma compartment is controversial, pharmacokinetic studies indicate that a small percentage of plasma IGF-I is not associated with IGF-binding proteins.4,5 Unbound IGF-I has also been observed in saliva.6 In addition, it appears that IGF-I may exert a tonic hypoglycemic effect under normal conditions that is inhibited by exogenous IGFBP-1 administration.7

It is likely that the measured unbound IGF-I fraction is a combination of the true unbound and the fraction of IGF-I that can be readily dissociated from IGFBP’s under the specific assay conditions. In this respect, it has been shown that exogenously administered IGF-I almost immediately associates with low MW IGFBP’s, then quickly moves into the high MW tertiary complex.5,8,9 The tertiary complex does not appear to be easily dissociated and does not re-equilibrate with exogenously added IGF-I or IGFBP-3 to a significant degree.8 On the other hand, the low MW complexes have a rapid turnover, and may be the source for much of the measured unbound IGF-I.

Various methods have been used to estimate the unbound (or freely dissociated) IGF-I fraction.4,9,10 Size-exclusion chromatography and filtration methods.4,9 have the theoretical disadvantage of altering the sample matrix and the equilibrium between IGF-I and IGFBP’s. A direct detection unbound IGF-I assay using immobilized IGFBP-3 for capture and anti-IGF-I antibody for detection has been reported.11

The Ansh Labs Free Rat/Mouse IGF-1 kit uses a highly sensitive two-site antibody method which allows detection of unbound IGF-I.

References:
1. Daughaday E, Rotwein P: Insulin like growth factors I and II. Peptide, messenger ribonucleic acid and gene structures, serum and tissue concentrations. Endocrin Rev 10:68-91, 1989.
2. Baxter RC, Martin JL, Beniac VA: High molecular weight insulin-like growth factor binding protein complex. J Biol Chem 264:11843-11848, 1989.
3. Rechler M: Insulin-like growth factor binding proteins. Vit Horm 47:1-114, 1993.
4. Guler HP, Zapf J, Froesch ER: Short-term metabolic effects of recombinant human insulin-like growth factor-I in healthy adults. New Engl J Med 317:137-140, 1987.
5. Zapf J, Hauri C, Waldvogel M, Froesch ER: Acute metabolic effects and half-lives of intravenously administered insulinlike growth factors I and II in normal and hypophysectomized rats. J Clin Invest 77:1768-1775, 1986.
6. Costigan DC, Guyda HJ, Posner BI: Free insulin-like growth factor I (IGF-I) and IGF-II in human saliva. J Clin Endocrinol Metab 66:1014-1018, 1988.
7. Lewitt MS, Denyer GS, Cooney GJ, Baxter RC: Insulin-like growth factor-binding protein-1 modulates blood glucose levels. Endocrinology 129:2254-2256, 1991.
8. Lewitt MS, Saunders H, Baxter RC: Bioavailability of insulin-like growth factors (IGFs) in rats determined by the molecular distribution of human IGF-binding protein-3. Endocrinology 133:1797-1802, 1993.
9. Lieberman SA, Bukar J, Chen SA, Celniker AC, Compton PG, Cook J, Albu J, Perlman AJ, Hoffman AR: Effects of recombinant human insulin-like growth factor-I (rhIGF-I) on total and free IGF-I concentrations, IGF-binding proteins, and glycemic response in humans. J Clin Endocrinol Metab 75:30-36, 1992.
10. Lee PDK, Powell D, Baker B, Liu F, Mathew G, Levitsky I, Gutierrez OD, Hintz RL: Characterization of a direct, non-extraction immunoradiometric assay for free IGF-I. Presented at the 76th annual meeting of the Endocrine Society, Anaheim, 1994.
11. Mukku VR: A 96-well microtiter plate assay for free IGF-I in plasma using immobilized IGFBP-3. Presented at the 73rd annual meeting of The Endocrine Society, Washington, D.C., 1991. (abstract #462).
12. HHS Publication, 5th ed., 2007. Biosafety in Microbiological and Biomedical Laboratories. Available http://www.cdc.gov/biosafety/publications/bmbl5/BMBL5
13. DHHS (NIOSH) Publication No. 78–127, August 1976. Current Intelligence Bulletin 13 – Explosive Azide Hazard. Available http:// www.cdc.gov/niosh.
14. Kricka L. Interferences in immunoassays – still a threat. Clin Chem 2000; 46: 1037–1038.

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