Milk amino acid composition

Gurbelash AT. 1969. Effect of hexachloroethane on the protein and amino acid composition of the blood in cattle milk. Veterinariia 51-52. 

Darragh, A. J., and Moughan, P. J. (1998). The amino acid composition of human milk corrected for amino acid digestibility. Br. J. Nutr. 80,25-34. 

TABLE 5.12 Amino acid composition (g/100 g milk dry matter basis) of morama bean milk compared with a commercial soybean milk Source Data adapted from Jackson et al. (2009) and Liu (1997)... 

Table 4.4 Amino acid composition of the major proteins occurring in the milk of western cattle (Swaisgood, 1982)... 

Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography purification up to 40 000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 kDa and an isoelectric point of 7.9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it in a number of characteristics. 

The pH optima of human milk lysozyme (HML), bovine milk lysozyme (BML) and egg-white lysozyme (EWL) are 7.9, 6.35 and 6.2, respectively. BML has a molecular weight of 18kDa compared with 15kDa for HML and EWL. The amino acid composition of BML is reported to be considerably different from that of HML or EWL. All lysozymes are relatively stable to heat at acid pH values (3-4) but are relatively labile at pH greater than 7. Low concentrations of reducing agents increase the activity of BML and HML by about 330%. 

Milk XO has a molecular weight of c. 300 kDa and consists of two subunits. The pH optimum is about 8.5 and the enzyme requires flavin adenine dinucleotide (FAD), Fe, Mo and an acid-labile compound as co-factors cows deficient in Mo have low XO activity. The amino acid composition of XO has been determined by a number of workers at least five genetic polymorphic forms have been reported. 

Gordon, W. G., Groves, M. L. and Basch, J. J. 1963. Bovine milk "red protein" Amino acid composition and comparison with blood transferrin. Biochemistry 2, 817-820. 

The amino acid composition of milk lipid globule membranes, as determined by several groups, has been summarized elsewhere (Patton and Keenan 1975). Some differences are evident in the data from differ-... 

Brunner, J. R., Duncan, C. W. and Trout, G. M. 1953. The fat-globule membrane of nonhomogenized and homogenized milk. I. The isolation and amino acid composition of the fat-membrane proteins. Food Res. 18, 454-462. 

The whey produced during cheese and casein manufacturing contains approximately 20% of all milk proteins. It represents a rich and varied mixture of secreted proteins with wide-ranging chemical, physical and functional properties (Smithers et al., 1996). Due to their beneficial functional properties, whey proteins are used as ingredients in many industrial food products (Cheftel and Lorient, 1982). According to Kinsella and Whitehead (1989), functional properties of foods can be explained by the relation of the intrinsic properties of the proteins (amino acid composition and disposition, flexibility, net charge, molecular size, conformation, hydrophobicity, etc.), and various extrinsic factors (method of preparation and storage, temperature, pH, modification process, etc.). 

Amino acid composition of cow and human milk proteins. Ibid., 10, 359 (1946). With D. Bolling. 

Dimick, P.S. 1976. Effect of fluorescent light on amino acid composition of serum proteins from homogenized milk. J. Dairy Sci. 59, 305-308. 

Proteins are macromolecules with different levels of structural organization. The primary structure of proteins relates to the peptide bonds between component amino acids and also to the amino acid sequence in the molecule. Researchers have elucidated the amino acid sequence in many proteins. For example, the amino acid composition and sequence for several milk proteins is now well established (Swaisgood 1982). 

In 1958 Yasunobu and Wilcox drew attention to certain similarities between a-lactalbumin and lysozyme (see Gordon, 1971). A few years later Brew and Campbell (1967) also drew attention to their marked similarity in molecular weights, amino acid composition, and the amino-and carboxy-terminal amino acid residues. They stated, To the extent that the properties mentioned reflect similar primary structures, the a-lactalbumins may have evolved by gradual modification from lysozyme, which is found in the milk of many species (p. 263). This proposal prompted Brew etal. (1967, 1970) to determine the amino acid sequence of bovine a-lactalbumin, which proved to have a high level of sequence identity with domestic hen egg-white lysozyme. Thus, these studies were in accordance with the proposal that the two proteins had diverged from a common ancestor (see also Hill etal., 1969, 1974). They stated that although lysozyme does not participate in lactose synthesis and a-lactalbu-... 

A rapid FTIR method for the direct determination of the casein/whey ratio in milk has also been developed [26]. This method is unique because it does not require any physical separation of the casein and whey fractions, but rather makes use of the information contained in the whole spectrum to differentiate between these proteins. Proteins exhibit three characteristic absorption bands in the mid-infrared spectrum, designated as the amide I (1695-1600 cm-i), amide II (1560-1520 cm-i) and amide III (1300-1230 cm >) bands, and the positions of these bands are sensitive to protein secondary structure. From a structural viewpoint, caseins and whey proteins differ substantially, as the whey proteins are globular proteins whereas the caseins have little secondary structure. These structural differences make it possible to differentiate these proteins by FTIR spectroscopy. In addition to their different conformations, other differences between caseins and whey proteins, such as their differences in amino acid compositions and the presence of phosphate ester linkages in caseins but not whey proteins, are also reflected in their FTIR spectra. These spectroscopic differences are illustrated in Figure 15, which shows the so-called fingerprint region in the FTIR spectra of sodium caseinate and whey protein concentrate. Thus, FTIR spectroscopy can provide a means for quantitative determination of casein and whey proteins in the presence of each other. 

Dietary protein sources differ widely in their proportions of the EAA. In general, complete proteins (those containing sufficient quantities of EAA) are of animal origin (e.g., meat, milk, and eggs). Plant proteins often lack one or more EAA. For example, gliadin (wheat protein) has insufficient amounts of lysine, and zein (com protein) is low in both lysine and tryptophan. Because plant proteins differ in their amino acid compositions, plant foods can provide a high-quality source of essential amino acids only if they are eaten in appropriate combinations. One such combination includes beans (low in methionine) and cereal grains (low in lysine). 

The amino acid composition of some )S-lg variants is shown in Table 4.4. It is rich in sulphur amino acids which give it a high biological value of 110. It contains 2 moles of cystine and 1 mole of cysteine per monomer of 18 kDa. The cysteine is especially important since it reacts, following heat denatura-tion, with the disulphide of K-casein and significantly affects rennet coagulation and the heat stability properties of milk it is also responsible for the cooked flavour of heated milk. Some /S-lgs, e.g. porcine, do not contain a free sulphydryl group. The isoionic point of bovine j3-lgs is c. pH 5.2. 

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