Insulin‐like growth factor I (IGF‐I), a protein of 70 amino acid residues and 3 cystine bridges, has been synthesized by two solid phase Boc methods. The first method used N‐methylpyrrolidinone as the solvent with single coupling cycles while the second synthesis used dimethylformamide and dichloromethane as the solvents with a double‐coupling protocol. In both cases, trifluoroacetic acid/trifluoromethanesulphonic acid cleavage of the peptide from the resin was employed. Purification of the cleavage products followed by removal of the S‐acetamidomethyl protecting groups gave reduced peptides which were then oxidized under conditions favouring the formation of the correct disulphide bonds. The purified synthetic IGF‐I peptides were full agonists of natural IGF‐I in a radioimmunoassay, in an IGF‐I radioreceptor assay, in a bioassay which measures the stimulation of protein synthesis in rat L6 myoblasts and in an IGF‐binding protein competitive binding assay. Moreover, in each of these assays, the synthetic IGF peptides were found to be at least 70% as potent as natural IGF‐I
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