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Abstract
The somatomedins are unique among the polypeptide hormones in that they circulate in the plasma as large complexes, bound specifically to one or more carrier proteins. These studies were aimed at understanding the role of carrier proteins in the regulation of the biological activity of the somatomedins themselves. Multiplication-stimulating Activity (MSA) and its specific carrier protein, MCP, have been used in this investigation. MSA is synthesized and secreted in association with its carrier protein by a cloned Buffalo rat liver cell line (BRL-3A) in serum-free culture and both polypeptides are readily purified from cell conditioned medium.
The MSA carrier protein was characterized by SDS-polyacrylamide gel electrophoresis. MCP migrates as a single polypeptide with a molecular weight of 31,500 under non-reducing conditions and 34,000 when fully reduced. Immunoprecipitation studies using polyclonal anti-MCP antibodies have shown that large quantities of MCP are secreted by BRL-3A cells and a subclone, BRL-3A2. MSA polypeptides are also secreted by both cell lines.
High molecular weight serum complexes have been demonstrated by specific binding of radioiodinated MSA to normal rat serum. A small complex similar in size to one of the complexes seen in serum (40,000-60,000 daltons) was reconstructed from purified MCP and MSA. Chemical crosslinking of these purified polypeptides allowed the demonstration of a 42,000 dalton complex following SDS-polyacrylamide gel electrophoresis.
Normal adult and fetal rat serum exhibited significant differences upon immunoautoradiographic analyses. Immunoreactive material could be detected in fetal rat serum only. These studies suggest that MSA and MCP are fetal forms of a somatomedin and its carrier protein and may therefore play an important role in fetal development.
