Bos taurus b-casein: protein structure, gene polymorphism, effect on the human gastrointestinal tract (review)

High-quality food products play an important role in a healthy lifestyle in the modern world. Cow’s milk and milk products contain all the essential nutrients. The main components of milk are water, fat, protein, lactose, minerals, vitamins, etc. (P.C. Wynn et al., 2013). With the advent of A2 milk on the market, which reduces the symptoms of lactose intolerance and has good digestibility, the population’s demand for this product began to grow. A2 milk is obtained from cows of the A2 genotype for β-casein. Cow's milk is 3.5 % protein (P. Feng et al., 2020). Casein is the most important protein component of milk and makes up about 80 % of the total protein composition of cow’s milk. Casein consists of four fractions, the αs1, αs2, β and κ. β-Casein is a protein that is one of the main ones in cow’s milk and makes up most of all casein. β-Casein consists of 209 amino acid residues, of which 16.7 % is proline, evenly distributed over the polypeptide, which limits the formation of the a-helix (S. Pattanayak, 2013). More than 95 % of casein in milk is in micellar form. The β-casein gene has 13 allelic variants, among which types A1 and A2 are the most studied, differing in the sequence of amino acids in the primary structure. Since the primary structure of the protein is different, A1 and A2 β-caseins broken down in the human gastrointestinal tract form different bioactive peptides. For A1 allele, β-casomorphin-7, a peptide consisting of seven amino acid residues is formed. The level of this peptide is 4 times higher in A1 milk than in A2 milk. When using milk containing β-casein type A1, 12 h after consumption, people may experience bloating, abdominal pain, flatulence, heaviness in the stomach, changes in the frequency and consistency of stools, in some cases, symptoms of celiac disease. Consumption of milk containing type A1 β-casein leads to a significantly longer transit time through the gastrointestinal tract (6.3 hours longer), inflammation of the small intestine and inflammation of the gastric mucosa compared to drinking milk containing A1 type of β-casein. People with lactose intolerance have adverse gastrointestinal symptoms after drinking milk, which may be associated with the presence of β-casein A1 in milk, and not with lactose itself (H. Brüssow, 2013; D. Hu et al., 2014). Unlike variant A1, β-casein A2 increases 2-gold the natural production of glutathione, one of the most important antioxidants of the human body.