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Endocrinology, Vol 123, 373-381, Copyright © 1988 by Endocrine Society

ARTICLES

Serum half-life and biological activity of mutants of human insulin- like growth factor I which do not bind to serum binding proteins

MA Cascieri, R Saperstein, NS Hayes, BG Green, GG Chicchi, J Applebaum and ML Bayne
Department of Biochemical Endocrinology, Merck, Sharp, and Dohme Research Laboratories, Rahway, New Jersey 07065.

We have characterized the biological properties of two mutants of human insulin-like growth factor I (IGF-I) which, as we have shown previously, have normal affinity for the type I IGF receptor, but drastically reduced affinity for the acid-stable components of human serum binding proteins. [Phe-1,Val1,Asn2,Gln3,His4,Ser8,His9,Glu12 ,Tyr15,Leu16]IGF I (B-chain mutant) and [Gln3,Ala4,Tyr15,Leu16]IGF I have 1000 and 500 times lower affinity than IGF-I for the native 150K binding protein in adult rat serum. Like IGF-I, these two peptides migrate as monomers during size exclusion chromatography on TSK 125. [125I]IGF-I, [125I]B-chain mutant, and [125I] [Gln3,Ala4,Tyr15,Leu16]IGF-I have in vivo serum half-lives of 100, 27.5, and 26.9 min, respectively, after iv injection. These data suggest that serum binding protein-bound peptide is cleared from the serum more slowly than free peptide. The tissue distributions of [125I]IGF-I and [125I]B-chain mutant are similar 10 min after dosing, with more than 80% of the tissue-sequestered intact radioactive peptides in the kidney. Despite decreased serum half-lives, the B-chain mutant and [Gln3,Ala4,Tyr15,Leu16]IGF-I are, respectively, 4 times and twice as active as IGF-I in stimulating the incorporation of [14C]glucose into glycogen in rat diaphragm in vivo. This effect of IGF- I is thought to be mediated by the type 1 IGF receptor in muscle, since the same doses of peptide that stimulated glycogen synthesis more than 30-fold did not stimulate the incorporation of [14C]glucose into total lipid in adipose tissue, an effect known to be mediated by the insulin receptor. These data support the hypothesis that serum- or tissue- derived binding proteins impair the ability of IGF-I to exert its effects through the type 1 IGF receptor in vivo.


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