Size heterogeneity of human serum somatomedin

Somatomedin (SM) in unextracted human serum was studied by Sephadex chromatography, starch gel electrophoresis, and in vitro SM bioassays. Repeated rechromatography of the various serum fractions revealed SM activity in a variety of indicated molecular size (IMS) areas; greater than 90,000, (very large); 90,000-20,000, (large); 20,000-9000, (intermediate); 9000-2000, (small); and less than 2000, (very small). Chromatography of unextracted serum, at pH 7.4, revealed 65% of SM as very large SM. Most of this very large SM remained stable at neutral pH; less than 25% appeared in smaller molecular sizes after acid eluent rechromatography. The latter significantly reassociated with other serum proteins in alkaline conditions. The large IMS SM was relatively stable on rehcromatography, whereas most of the intermediate IMS SM was converted to acidic small IMS SM proteins. The small IMS SM was stable, as a basic protein, after repeated acidic rechromatography. Most of the very small IMS SM occurred in a molecular size of less than 500. These results suggest the presence of multiple molecular size forms of SM in unextracted serum. Some of the larger SM represents an SM-serum protein aggregation. The acidic SM, with a molecular size near insulin, may be derived from a larger SM. The very small SM may be a nonpolypeptide substance(s).