Characterization of an Insulin-Like Growth Factor-I/ Somatomedin-C Radioimmunoassay Specific for the C Peptide Region*

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Abstract
Insulin-like growth factor-I (IGF-I) and somatomedin- C (SM-C) have been shown to be functionally identical by a number of criteria. We have synthesized the 12 amino acid C-peptide region of IGF-I (Gly-Tyr-Gly-Ser-Ser-Ser-Arg-Arg- Ala-Pro-Glu-Thr) and developed a RIA based on antibodies against this synthetic peptide. IGF-I and SM-C were indistinguishable in this RIA. No other peptides competed for this antiserum. The SM-C/IGF-I values of acid-chromatographed serum were strongly age dependent. The mean of children 1–5 yr old was 0.67 ± 0.033 U/ml (mean ±SD; n =23), whereas the mean of children 12–17 yr old was 2.01 ±0.66 U/ml (n =39) and the mean of 38 adults 26–85 yr old was 1.05 ±0.34. The SM-C/ IGF-I values measured by this RIA were also growth hormone dependent. Thus, this region-specific RIA provides a clinically useful assessment of serum SM-C/IGF-I levels. (J Clin Endocrinol Metab55: 927, 1982)