Purification and primary structure of a polypeptide with multiplication-stimulating activity from rat liver cell cultures. Homology with human insulin-like growth factor II

A low molecular weight polypeptide with multiplication-stimulating activity (MSA) was isolated from serum-free media conditioned by a clone of normal Buffalo rat liver cells. The purification of MSA was achieved by gel permeation chromatography of the acid-soluble growth-promoting activity on Bio-Gel P-10 in 1 M acetic acid and followed by reversed phase high pressure liquid chromatography on muBondapak C18 support using a linear gradient of acetonitrile in 0.05% trifluoroacetic acid. The primary structure of MSA has been determined. MSA is a single chain polypeptide of 67 residues, with a calculated molecular weight of 7,484, and displays 93% homology with the functionally related human insulin-like growth factor II (IGF-II). A comparison between the sequences of rat MSA and human IGF-II reveals only five amino acid substitutions. Based on the extensive amino acid sequence homology, we propose the term rat IGF-II for this newly isolated polypeptide.