This proposal will focus on two major aspects of the particulate and soluble folate binding proteins of human milk to identify (1) the factors involved in determining their quantitative and qualitative content in milk, and (2) the functional role of these folate binding proteins in the uptake of folates in the intestine. Studies on the biosynthesis of particulate folate binding proteins with respect to processing of the polypeptide backbone, glycosylation and acylation that may have a direct bearing on the membrane insertion, folate binding capacity and possible conversion to the soluble folate binding protein will be investigated in cultured mammary cells. The effect of folate deficiency on the secretion of these folate binding proteins will be investigated in lactating rats. The influence of various components of human milk, saliva, gastric, pancreatic and intestinal secretions on the structural integrity and ligand binding capacity of these proteins and their role in the uptake of folates will be identified in rat intestinal cells and brush border vesicles. In addition, the possibility of spontaneous incorporation, lipid interaction, and binding to membrane components by these human milk folate binding proteins will be studied in vitro. Finally, the role of these proteins in absorption of folates in vivo will be tested in a rat model which will closely simulates food folate absorption. These studies may allow for a comprehensive understanding of the significance of folate binding proteins of human milk and their role in the absorption of folates.