The cream fraction of human milk is composed of HMFGs that consist of a neutral fat droplet surrounded by a membrane of phospholipids, cholesterol, and glycoproteins. Two of these glycoproteins, lactadherin and mucin (MUC-1) are involved in preventing infection, while butryophilin appears to be a structural component. We have now shown that significant amounts of these HMFG glycoproteins are also present in skim milk (>50%), being associated with large molecular weight material, possibly phospholipid (PL) micelles (skim milk contains 20-50% of milk PL). Dissociation of lactadherin and butyrophilin from the complex using gel exclusion chromatography requires detergent, heat and reducing conditions. Both mucin and butyrophilin have transmembrane domains and lactadherin is lipophilic, presumably covalently linked to phospholipids or fatty acids. All three glycoproteins survive in the stomach of human milk-fed preterm infants, mucin and lactadherin surviving longer (at least 4 hours) than butyrophilin. The mucin and lactadherin concentrations are strongly correlated with phospholipid content of milk (P ≤ 0.05, P ≤ 0.02) but not with neutral lipid content. In contrast, butyrophilin is strongly correlated with neutral lipid content of milk (P ≤ 0.001), suggesting some structural or functional role of butyrophilin with lipids of the milk fat globule core. The resistance of mucin and lactadherin to degradation in the stomach is important considering their role in preventing infection.

Supported in part by grant Nos. HD30444, CA61258, and CA39932.