Lactalbumin
One of the major differences of human milk over milk from other mammals is the presence of alpha-lactalbumin as the major whey protein. Alpha-lactalbumin is a major whey protein found in cow's milk. The term "whey protein" includes a group of milk proteins that remain soluble in "milk serum" or whey after the precipitation of casein, another milk protein, at pH 4.6 and 20.degree. C. .alpha.-Lactalbumin has these characteristics. Whey proteins are generally known to be used as a substitute for breast milk and as a protein source in nutritional mixtures both for human and animals since whey protein has a high nutritional value and is also highly efficient in protein utilization, when compared to casein or soya protein. The proteins in whey are divided into three principal groups: lactalbumins, globulins and peptones. The lactalbumins include beta-lactoglobulin, alpha-lactalbumin and serum-albumin, and represent the major fraction of the proteins of whey. The beta-lactoglobulin is a protein characteristic of ruminants. Alpha-lactalbumin is found in the milk of all mammals and represents a major protein in human milk. It is largely used in the preparation of humanized milk, and for non-allergenic milk preparations containing casein and alpha-lactalbumin for feeding infants allergic to the beta-lactoglobulin of cow's milk. Monomeric alpha-lactalbumin is the most abundant protein in human milk whey. This 14 KDa protein has been extensively characterised and the crystal structure has been resolved. The native monomer is a component of the lactose synthase complex, and alters the substrate specificity of the galactosyl transferase from N-acetylglucosamine to glucose, with subsequent synthesis of lactose. Alpha-lactalbumin has been used as an index of mammary gland function in response to hormonal regulation in bovine explant culture and as an index of udder development. Alpha-lactalbumin interacts with galactosyl transferase and therefore plays an essential role in the biosynthesis of milk sugar lactose. Lactose is an important component in milk, and contributes to milk osmolality. It is the most constant constituent in cow's milk. Alpha-lactalbumin is useful as an index of lactogenesis in cultured mammary tissue. It is therefore believed that alpha-lactalbumin is an important protein in controlling milk yield and can be used as an indicator of mammary function. Beta-lactoglobulin is absent from human milk. The protein content of bovine whey contains about 50% to 55% of beta-lactoglobulin and about 18% of alpha-lactalbumin. Beta-lactoglobulin is particularly rich in the essential amino acids valine and threonine. Alpha-Lactalbumin is particularly rich in the essential amino acids tryptophan, lysine and cystine compared to other bovine milk proteins. As a consequence of the differing amounts of the specific whey proteins in bovine milk and human milk and the amino acid compositions of these proteins, bovine milk and human milk differ substantially in their amino acid profiles. This difference remains in infant formulas made with bovine whey in which the composition of the specific whey protein is not modified.
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