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Insulin-like growth factor-I (IGF-I) and IGF-II have been purified to homogeneity from chicken serum as a step towards the characterization of the roles for these peptides in the growth process. Chicken IGF-I had about half the efficacy of bovine/human IGF-I in a bioassay and in radioimmunoassays with bovine IGF-I as radioligand. Chicken IGF-II competed for the binding of bovine IGF-II to cell receptors while chicken IGF-I reacted minimally in this IGF-II radioreceptor assay. Further evidence of homology was obtained by N-terminal sequence analysis of the first 31 and 35 amino acids of chicken IGF-I and IGF-II respectively. Chicken IGF-I had the same N-terminal as human IGF-I, with the exception of the substitution of serine for asparagine at residue 26. Chicken IGF-II had a unique N-terminal tetrapeptide Tyr-Gly-Thr-Ala, but from residues 5–30 the sequence was identical to that reported for residues 6–31 of human IGF-II. Substitutions also occurred corresponding to residues 32, 33, 35 and 36 of human IGF-II. A variant form of chicken IGF-II that had the same N-terminal pentapeptide as human IGF-II was also detected.

J. Endocr. (1988) 117,173–181