Transferrin is a major mouse milk protein and is synthesized by mammary epithelial cells.

We have identified a major mouse milk protein as transferrin (Tf) using immunoprecipitation, 2-dimensional electrophoresis, Ouchterlony diffusion and V-8 protease digests. We show that Tf is synthesized by mammary epithelial cells themselves and that its synthesis and secretion is regulated distinctly from that of other milk proteins. In culture, the kinetics of Tf synthesis and secretion are distinct from that of beta-casein; furthermore, Tf is relatively insensitive to lactogenic hormones whereas beta-casein is hormone-dependent. In vivo, however, Tf is regulated by pregnancy. While the virgin gland produces small amounts of Tf, its production is greatly increased during pregnancy and lactation. Thus, Tf synthesis in the mammary gland is modulated by as yet unknown factors in vivo. These observations are discussed in terms of Tf's possible role in mammary gland growth, differentiation and function.