Human Milk and Colostrum Proteins: A Review

Human milk contains a number of factors that are protein in nature, which apparently enhance the resistance of the breast-fed infant against disease. Among such factors are lactoferrin, immunoglobulin A, the Lactobacillus bifidus growth-promoting glycoproteins, and lysozyme. Lactoferrin is believed to exert its bacteriostatic action mainly in concert with specific antibacterial immunoglobulins. The immunoglobulins may, in turn, act in concert with the large numbers of white blood cells in human milk and colostrum. α-Lactalbumin, a major component of human milk, is important both from a nutritional point of view, and as a component of the enzyme system biosynthesizing lactose. It is structurally homologous to lysozyme of both human milk and hen's egg white. The most abundant component of the casein fraction of human milk is β-casein, which exists in the form of genetically-determined polymorphs. The polymorphs apparently differ in their phosphate content only. The amino acid sequence of the N-terminal region of human milk β-casein bears a strong structural homology to that of bovine milk β-casein. Human α-casein is also in the form of genetically-determined polymorphs; however, it has not been purified and characterized. Human κ-casein is a glycoprotein which releases a glycomacropeptide when acted upon by rennin. Though it has not been characterized to any great extent, there is evidence that there may be several forms of κ-casein in human milk. Also the nature of the glycomacropeptide varies depending on whether purified κ-casein or whole casein fractions are subjected to the action of rennin. The amino acid sequence of a glycomacropeptide produced from κ-casein-enriched fraction of human milk casein was structurally homologous to the glycomacropeptides from other species.