CASEINS (Latin. caseus cheese) — heterogeneous group of phosphoproteins of milk, one of the most valuable food proteins which part the full range of irreplaceable amino acids is; are also a source of food calcium and phosphorus. The drugs K received commercially. find broad application in biology and medicine. They are a part of artificial mixes for feeding of newborns, add them to composition of nutrient mediums for cultivation of microorganisms. To. and them hydrolyzates (see) apply to parenteral food of patients with various diseases which are followed by a hypoproteinemia including with disturbances of passability of intestines, gullet and stomach as a result of the stenosing processes of various nature. Parenteral food To. apply at an agastralny adynamy, coloenterites, a nutritional dystrophy, a cachexia, gastric, intestinal and pancreatic fistulas, burns, blood losses, purulent diseases and intestinal infections. Kazenna and their hydrolyzates are applied also to complex treatment acute and hron, pancreatitis, peritonitis and feverish states. For improvement of a ratio of amino acids hydrolyzates To. recommend to enter together with drugs of hydrolyzates of blood proteins. To. are applied in the same cases, as autohemotherapy (see). To. are a part of the ointments applied in dermatology and the glues used in surgery.
To. differ from other proteins of milk in property to be besieged (to pretsipitirovat) after acidulation of milk to pH 4,6 at 20 °.
In a stomach of mammals in the early post-natal period To. are exposed to curdling under the influence of proteolytic enzyme of rennin (chymosin; KF126.96.36.199). The formed product is called paracasein. Sometimes apply the name «caseinogen» to designation of complete protein, and call a product of its proteolysis casein.
On a share To. 78 — 85% of protein of cow's milk and apprx. 30% of protein of women's milk are necessary. To. are also main proteinaceous component of cheeses, cottage cheese and other dairy products.
The most studied To. — casein of cow's milk — are divided at a frontal electrophoresis into three fractions: alpha, beta and gamma caseins. The isolated fraction of alpha caseins can be divided into two components. One of them called αS-казеином is besieged in the presence of 0,4 M of CaCl 2 at pH 7,0 (i.e. it is sensitive to Ca). The second component called κ-casein, remains in the dissolved look even at high concentration of CaCl 2 in solution. αS-Казеин which makes 45 — 55% of proteins of milk it is divided by means of an electrophoresis in various gels on several components differing on mobility: αS1, αS2, αS3, αS4 and αS5. The main thing from them is αS1-αS5-казеин. Four genetic options αS1-казеина are found (And, In, With and D) also primary structure of all genetic as options - casein is defined. In all cases the molecule αS1 - casein consists of one polypeptide chain about a pier. it is powerful (weighing) from 22 000 to 24000 (depending on genetic option). The molecule αS1-казеина includes 8 phosphatic groups connected with the remains of serine orthomonoradio communication. kappa-Kazein meets in the form of two genetic options (And yes In). Molecules A - and V-options of the κ-casein making 8 — 15% of protein of milk also consist of one polypeptide chain. Pier. weight it is equal to 19 000. The molecule a kappa casein contains phosphoserine (1 rest) and, unlike others To., cysteine (2 rests). Each genetic option of κ-casein contains several components differing on the content of carbohydrates. Carbohydrates of κ-casein are presented by trisaccharide alfa-N-atsetilneyraminil->(2-6) - beta-galaktozil-(>1 — 3 or 6) - N-atsetilgalaktozaminom. The main component of κ-casein is deprived of carbohydrates.
Beta Casein, to-rogo falls to the share 25 — 35% of protein of milk, meets in the following genetic options: A1, A2, A3, B, Bz, C, D and E. For the most part of these options primary structure is defined. It is established that beta casein is constructed of one polypeptide chain about a pier. weight apprx. 24 000. 4 — 5 phosphatic remains are a part of a molecule of beta casein (depending on option).
The gamma Caseins consisting of three components (gamma1-, gamma2-and gamma3-caseins), a pier. which weight (weight) are respectively equal 20 500, 11 800 and 11 500, make 3 — 7% of protein of milk. Comparison of primary structure of gamma casein with structure of beta casein demonstrates that gamma caseins are C-trailer fragments of beta caseins. Gamma1-Kazein contains the amino-acid remains occupying 29 — 209 situation in a molecule of beta casein; gamma2-casein contains respectively 106 — 209 amino-acid remains, and gamma3-casein contains 108 — 209 remains. The number of genetic options at gamma caseins are less, than at beta caseins. It is connected with the fact that in some genetic options of beta caseins amino-acid replacements are localized in N-trailer area, edges is not a part of the C-trailer fragment which is gamma casein.
Molecules K. in milk are present at the basic in the form of the mixed micelles having the sizes 40 — 300 nanometers and a pier. weight 10 6 — 10 9 . Micelles To. it is possible to receive also in vitro, mixing alfas-and κ-caseins in the presence of Ca salts. Loss of alfas-caseins in a deposit at the same time does not happen because of stabilizing action of κ-casein. Apparently, the same stabilizing role is played by κ-casein and in natural micelles To. This role of κ-casein was established during the studying of curdling of milk when it was shown that subject to the attack by rennin in micelles To. κ-casein is. Enzyme hydrolyzes in a molecule of κ-casein one peptide bond (between the rest of phenylalanine in the 105th situation and methionine in the 106th situation), chipping off C-trailer 64-chlenny macropeptide. After hydrolysis of κ-casein of a micelle To. lose stability and there occurs coagulation of milk. Other proteolytic enzymes cause curdling of milk, also affecting first of all κ-casein. It is supposed that the mechanism of coagulation is connected with reduction of a negative charge of micelles after hydrolysis of κ-casein and eliminating of macropeptide.
At decrease in ion concentration of Ca 2+ in milk proteolysis K. rennin does not lead to their curdling. It is caused, obviously, by increase in a negative charge of micelles To., not connected with Ca 2+ , and strengthening of their mutual pushing away. Curdling To. does not occur and if proteolysis is conducted by rennin at low temperatures (+4 °). In this case lack of coagulation is connected with the increase in a charge of micelles caused by an exit of beta caseins from structure of micelles. At temperature increase beta casein is returned to the micelles hydrolyzed by rennin, their charge decreases and they coagulate.
Process of curdling of milk has big fiziol, value since it promotes detention To. in a stomach and to their best digestion by proteolytic enzymes. Proteolysis K. it is facilitated in connection with features of their structure. Judging by data physical. - chemical researches, a molecule K. contain few structured sites and have generally disorder conformation, edges at the majority of other proteins it is observed only after their denaturation. Disorder of conformation does molecules K. especially subject to proteolysis.
See also Milk, dairy products .
Bibliography: Vasilyev P. S., etc. Preparation, structure and properties of a hydrolyzate of casein as means of parenteral proteinaceous food, in book: Aktualn, vopr, parenteral food, under the editorship of V. A. Korzan, page 151, Riga, 1972; Chernikov M. P. and A. Ya. O Camp to structure of caseins of cow's milk, Prikl. biochemical, and mikrobiol., t. And, century 2, page 241, 1975; Milk proteins, ed. by H. A. Me Kenzie, v. i \22222222, N. Y., 1970 — 1971; Whitney R. M. a. o. Nomenclature of the proteins of cow’s milk, fourth revision, J. Dairy Sci., v. 59, p. 795, 1976.
I. M. Karmansky.