Inter-species comparison of somatomedin structure using immunological probes

Abstract

To explore the structural differences in various animal somatomedins, we examined the sera of 20 animal species using two region-specific radioimmunoassays, a radioimmunoassay specific for somatomedin-C and a placental membrane radioreceptor assay. Using small peptide portions of the insulin-like growth factor-I (IGF-I) C chain and D chain made by solid-state methods, we generated antisera in rabbits and developed radioimmunoassays to these regions. For a radioimmunoassay directed against intact somatomedin-C, we used the antibody distributed by the NIH (NIH-radioimmunoassay). For the detection of somatomedin peptide content, we used a placental membrane radioreceptor assay. 125I-Labelled IGF-I was used as the radioligand in all assays and an insulin-free, partially purified somatomedin preparation was used for the standard curves. All samples were chromatographed in 0·25 m-formic acid to remove any somatomedin-binding protein before assay. The correlation between the radioreceptor assay and the NIH-radioimmunoassay was good (r2 = 0·84) but the slope was significantly (P<0·001) different from a value of 1 which would be expected if the two assays were detecting equal amounts of somatomedin activity. In all the species, the somatomedin level measured by radioreceptor assay was greater than that measured by the NIH-radioimmunoassay, suggesting the presence of a somatomedin-like protein which has retained its ability to bind to the somatomedin receptor but which is sufficiently different to give it reduced affinity in the NIH-radioimmunoassay. The somatomedins of rat, hamster and mouse (all rodents of the superfamily Myomorpha) were not detectable using the radioimmunoassays for the C and D chains of IGF-I, although they were high in the radioreceptor assay. This suggests that these regions have a different structure from the analogous regions in the guinea-pig (a rodent of the superfamily Caviomorpha) and implies a significant structural change in a somatomedin-like molecule during the evolution of the order Rodentia. This is analogous to the marked differences in the known sequence of the pro-insulins in these two superfamilies.