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. a-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 hens egg white. The most abundant component of the casein fraction of human milk is 3-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 3-casein bears a strong structural homology to that of bovine milk 3-casein. Human c-casein is also in the form of genetically-determined polymorphs, however, it has not been purified and characterized. Human K- 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 K-casein in human milk. The nature of the glycomacropeptide varies depending on whether purified K-casein or whole casein fractions are subjected to the action of rennin. The amino acid sequence of a glycomacropeptide produced from K-casein-enriched fraction of human milk casein was structurally homologous to the glycomacropeptides from other species. Biological Properties: Secretory IgA is derived from human colostrum and is predominately in dimeric form, the mononers being linked by immunoglobulin J chain. Applications: Suitable for use in ELISA. Other applications have not been tested. Storage and Stability: May be stored at 4C for short-term only. Aliquot to avoid repeated freezing and thawing. Store at -20C. Aliquots are stable for 6 months after receipt at -20C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer.
