A radioimmunoassay for insulin-like growth factor II specific for the C-peptide region.
Insulin-like growth factor II (IGF-II) is a human plasma peptide whose sequence is homologous to both insulin-like growth factor I/somatomedin C (IGF-I/SM-C) and human proinsulin in the A and B regions. However, there is no obvious homology in the C (connecting peptide) region. The synthetic 8-amino acid C-peptide segment of IGF-II (Ser-Arg-Val-Ser-Arg-Arg-Ser-Arg) was covalently linked to thyroglobulin to render it more antigenic. Antiserum against the IGF-II C-peptide was generated which had a titer of 1:2000 determined with [125I]IGF-II C-peptide. Half-maximum displacement was by 350 pg/ml IGF-II C-peptide or 80 ng/ml IGF-II. There was no displacement by IGF-I/SM-C, insulin, or a wide variety of peptides. There was also a high degree of species specificity of this antisera. Isoelectric focusing studies of immunoreactive IGF-II showed an apparent pI of 6-6.5. The mean (+/- SEM) level of IGF-II after acid chromatography of 28 normal adult males was 687.0 +/- 31.9 ng/ml. The mean of 8 acromegalics was 600.5 +/- 57.4 ng/ml, indistinguishable from normal. The IGF-II levels of 21 hypopituitary children were significantly lower (231.5 +/- 32.3 ng/ml). Thus, GH action appears to be necessary for normal levels of IGF-II, but excess GH does not cause an elevation above normal of IGF-II, unlike what is seen with IGF-I/SM-C. These structurally related IGF peptides have different control mechanisms and ultimately may play different functional roles. The availability of specific RIAs for the measurement of IGF-II will help to clarify its role in human physiology and disease states.
D000172:Acromegaly / Q000097:blood; D000328:Adult; D000918:Antibody Specificity; D000939:Epitopes / Q000276:immunology; D013006:Growth Hormone / Q000627:therapeutic use; D006801:Humans; D007018:Hypopituitarism / Q000097:blood / Q000188:drug therapy; D007328:Insulin / Q000097:blood* / Q000276:immunology; D008297:Male; D010446:Peptide Fragments / Q000276:immunology*; D010455:Peptides / Q000097:blood* / Q000276:immunology; D011863:Radioimmunoassay; D012016:Reference Values; D013002:Somatomedins / Q000097:blood* / Q000276:immunology; D013045:Species Specificity
D000939:Epitopes; D007328:Insulin; D010446:Peptide Fragments; D010455:Peptides; D013002:Somatomedins; D013006:Growth Hormone