Milk is composed of two main protein fractions, whey and casein. If milk is allowed to curdle, the liquid that remains is the whey protein fraction and the solid curds are the casein protein fraction. Casein is a collective name for a group of phosphoproteins, namely αS1-, αS2-, β- and κ-casein, that make up around 80 % of the total milk proteins. Whey protein represents around 20% of the total protein content of milk, and also contains vitamins and minerals, carbohydrate as lactose, and fat. Over 85 % of whey protein is made up of just 5 major proteins, with the remaining 15 % containing hundreds of low abundance proteins (figure 1). Whey protein from cow’s milk also contains a number of growth factors. These include transforming growth factor β (TGF-β), insulin like growth factor (IGF) I and II, platelet derived growth factor (PDGF) and fibroblast growth factor I (FGF-1).
Figure 1. The main proteins present in milk1.
The major proteins in whey include α-lactalbumin, β-lactoglobulin, glycomacropeptide, proteose peptone 3, immunoglobulins and serum albumin. Of these proteins, all are synthesised in the epithelial cells of the mammary glands with the exception of the immunoglobulins and the serum albumin. The proteins in whey serve a number of different physiological roles, although these roles are not fully understood. Whey α-lactalbumin has been shown to stimulate the immune system in animal studies and can stimulate human lymphocytes in vitro. The high tryptophan content of α-lactalbumin has also been shown to be able to increase plasma tryptophan levels and morning alertness in humans, consistent with a role in serotonin synthesis. The β-lactoglobulin protein is the major protein of whey and is the cause of cow’s milk allergies. However, less in know about its physiological role, although it may enhance pregastric lipase activity by binding inhibitory fatty acids.
Glycomacropeptide in whey is produced through the action of the enzyme chymosin on casein during cheese making, and is therefore only present during this process. Glycomacropeptide contains high levels of branched chain amino acids, but does not contain the aromatic amino acids phenylalanine, tryptophan or tyrosine. This makes it an important protein source for individuals with phenylketoneuria, as it is one of the only sources of phenylalanine free protein. Proteose peptone 3 is only found in whey protein, but is absent from human milk. In cell culture models, proteose peptone 3 shows immune stimulatory and anti-microbial properties. The immunoglobulins in whey give the neonate passive immunity, and are therefore of vital importance during early development. Bovine serum albumin is a rich source of essential amino acids, and shows anti-cancer and blood pressure lowering activity. Bovine serum albumin may also play a role in motility in the gut.
The low abundance proteins in whey include lactoferrin and lactoperoxidase, which make up 1% and 0.5% of the total protein, respectively. Lactoferrin is an iron binding glycoprotein that demonstrates a remarkable number of clinically relevant benefits. Lactoferrin may promoted intestinal iron uptake, regulate bone cell activity, protect from microbial infection, regulate myelopoiesis (blood cell production), as well as modulate inflammation and the systemic immune response. The antimicrobial effects of lactoferrin are interesting, as it is secreted by neutrophils and is found in high concentrations at locations of bacterial infections. Most secretion exposed to high bacterial concentrations contain high lactoferrin concentrations such as milk, tears, nasal secretions, colostrum, saliva, bile and pancreatic juice. Lactoperoxidase also displays a protective effect against microbes, although much of the work is based on cell culture studies. Other low abundance proteins may possess beneficial effects, but these have not yet been identified.
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