A sensitive radioimmunoassay for somatomedin-C/insulin-like growth-factor I based on synthetic insulin-like growth factor 57-70.

We have developed a region-specific radioimmunoassay for somatomedin-C/insulin-like growth factor-I (IGF-I) directed against IGF-I 57-70. This sequence includes part of the A region and all of the carboxyterminal D region. In addition to being highly specific, this immunoassay is sensitive to as little as 1 ng/ml of IGF-I, and has half-maximum displacement by 10 ng/ml of IGF-I. Comparison to other existent immunoassays for IGF-I shows correlation coefficients of greater than 0.95. This immunoassay has the advantage of being directed against a known region of the IGF-I molecule which is on the exterior of the 3-dimensional structure, and can use a readily available synthetic peptide as both radioligand and standard.