ISOLATION AND CHARACTERIZATION OF MILK GROWTH-FACTOR, A TRANSFORMING-GROWTH-FACTOR-BETA-2-RELATED POLYPEPTIDE, FROM BOVINE-MILK

A strategy was developed for the purification of a biologically active polypeptide growth and migration factor from skimmed bovine milk. This 25-kDa dimeric molecule, termed milk growth factor (MGF), was isolated by a method consisting of a combination of strong cation-exchange chromatography, low-pressure hydrophobic-interaction chromatography, hydrophobic-interaction HPLC and size-exclusion HPLC steps, which separated the protein according to its properties of charge, hydrophobicity and size, respectively. On average, a total purification of 10(6) - 10(7) -fold and a yield of approximately 115 +/- 78 ng/MGF/milk was obtained using the method described. All purification steps were performed with novel combinations of ethanol and volatile acidic salt (ammonium acetate) solutions in order to retain biological activity of the protein. These conditions, together with the easy removal of salt by lyophilisation, facilitated the detection of biological activity in fractions collected at each step of the purification by means of a sensitive in vitro fibroblast-migration assay in which the half-maximal activity was obtained at a concentration of approximately 17 +/- 4 pg/ml (i.e. approximately 1 pM) of the pure protein. Biological activity of the dimeric protein was unaffected by heat treatment or exposure to acid (pH 2.0), but was lost upon reduction to its monomeric form. Amino acid composition and sequence analyses demonstrate that MGF is related to transforming growth factor type beta-2.