[4] Purification of polypeptide growth factors from milk

There appear to be at least three growth factors for mouse BALB/c 3T3 cells in human milk. The purification of the predominant one is described in this chapter. Biochemical and immunological studies indicate that this growth factor is probably a form of human epidermal growth factor (EGF). Like EGF, the major human milk-derived growth factor has a molecular weight of about 6000, a pI of about 4.5, and is resistant to inactivation by dithiothreitol. (See this volume, Harper et al., for purification of human EGF.) In addition, Carpenter has shown that antibodies against human EGF will precipitate most of the growth factor activity for 3T3 cells found in human milk. The EGF-like species of growth factor cannot be detected in bovine milk. Instead, the major growth factor in bovine colostrum appears to be biochemically similar to platelet-derived growth factor (PDGF). Like PDGF, the bovine colostrum-derived growth factor has a molecular weight of about 30,000, a pI of about 10, is totally inactivated by dithiothreitol but is stable to treatments with guanidine-HCl, urea, and heat. Biochemical characterizations of milk-derived growth factors, EGF, and PDGF are summarized in Table III. At present, very little is known about the physiological role of these growth factors in milk. The availability of these growth factors in homogeneous form will facilitate the studies in understanding their possible involvement in the growth process.