Insulin-like growth factors 1 and 2 in bovine colostrum. Sequences and biological activities compared with those of a potent truncated form

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Abstract
1. Insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) together with a truncated form of IGF-1 were purified to homogeneity from bovine colostrum. 2. Two forms of IGF-1 were totally resolved from IGF-2 in the purification by h.p.l.c. involving cation-exchange and reverse-phase columns. 3. The complete amino acid sequences for all three forms of IGF were determined. The sequence of bovine IGF-1 was found to be identical with that of humans IGF-1, and that of the variant lacked the N-terminal tripeptide Gly-Pro-Glu (-3N:IGF-1). Bovine IGF-2 was found to differ in three residues of the C-domain compared with human IGF-2, with serine, isoleucine and asparagine substituted for alanine, valine and serine respectively at positions 32, 35 and 36. 4. Protein synthesis in L6 rat myoblasts was stimulated and protein degradation inhibited in a co-ordinate response with all three IGFs. The relative potency in both processes was -3N:IGF-1 > IGF-1 > IGF-2. A similar order of potency was obtained for the stimulation of DNA synthesis by -3N:IGF-I and IGF-1. The approximately 10-fold effect on biological activity of removing the N-terminal tripeptide is unexpected in view of current information on IGF-1 structure and function.