DAIRY CHEMISTRY AND BIOCHEMISTRY L-chonn FOb l2 CHO sulfide bond IgD Figure 4.29 Models of IgG, IgA, IgD, IgE and IgM. (a)Structural model of IgG, before and after fragmentation by pepsin and papain and reduction with a sulphydryl reagent. Solid bl chain portion= variable regions; light chain portion constant regions. Small black lines represent disulphide and e(--SH) groups. Small black dots in Fc regions represen arious parts of the model are labelled. (b) The structure of four classes of immunol e shown with monomeric IgA, dimeric IgA and secretory IgA. Location of the J-cl etory component (SC) and carbohydrate is approximate From Larson, 1992)
198 DAIRY CHEMISTRY AND BIOCHEMISTRY , H-chain Figure 4.29 Models of IgG, IgA, IgD, IgE and IgM. (a) Structural model of IgG, before and after fragmentation by pepsin and papain and reduction with a sulphydryl reagent. Solid black chain portion = variable regions; light chain portion = constant regions. Small black lines represent disulphide and half-cystine (-SH) groups. Small black dots in Fc regions represent attached carbohydrate groups. The various parts of the model are labelled. (b) The structure of four classes of immunoglobulins are shown with monomeric IgA, dimeric IgA and secretory IgA. Location of the J-chain, secretory component (SC) and carbohydrate is approximate. (From Larson, 1992.)
MILK PROTEINS eternal IgG selectively seali IgG selectively trons ferred rans ferred ROUP工RP立 A COLOSTRAL lgs Ig A, I gM, IoG IgAlgG, IgM gG, IgA, IgM IgG1,1gM,IgA Absorption b gu Probably Moderate, selective Extensive, selective one 19 days in rats, mice 1248h h figure 4-30 Transfer of maternal immunoglobulins to the foetus and of representative mammalian species. Group I species transfer Ig in utero before birth II species tr both in utero before birth and via colostrum after birth. Group II via colostrum after birth. The size of the immunoglobulin notation (IgA, IgM, Igg dicates the relative percentage composition of the im group II may have Igg as the predominant Ig in colostrum. Significant IgG, also may be present in the colostrum of some Group Ill species. The relative absorption of immuno- globulins in the gut of the neonate is also shown.(From Larson, 1992) 4.11 Minor milk proteins Milk contains numerous minor proteins, including perhaps 60 indigenous enzymes, some of which, e.g. lipase, proteinase, phosphatases and lac- toperoxidase, are technologically important( Chapter 8). Most of the minor proteins have biological functions and probably play very significant roles (section 4.16) 4. 12 Non-protein nitrogen Nitrogen soluble in 12% TCa is referred to as non- protein nitrogen (NPN), of which milk contains 250-300 mg1 -, i.e. 5-6% of total milk nitrogen The NPN is a very heterogeneous fraction(table 4.7)
MILK PROTEINS 199 selective, 12-48h Probably Moderate, selective Extensive, selective Extensive, nonnone 19 days in rots, mice 12- 48h Figure 4.30 Transfer of maternal immunoglobulins to the foetus and neonate of representative mammalian species. Group I species transfer Ig in utero before birth. Group I1 species transfer Ig both in utero before birth and via colostrum after birth. Group 111 species transfer Ig only via colostrum after birth. The size of the immunoglobulin notation (IgA, IgM, IgG, IgG,) indicates the relative percentage composition of the immunoglobulins in colostrum. Species in group I1 may have IgG as the predominant Ig in colostrum. Significant IgG, also may be present in the colostrum of some Group I11 species. The relative absorption of immunoglobulins in the gut of the neonate is also shown. (From Larson, 1992.) 4.11 Minor milk proteins Milk contains numerous minor proteins, including perhaps 60 indigenous enzymes, some of which, e.g. lipase, proteinase, phosphatases and lactoperoxidase, are technologically important (Chapter 8). Most of the minor proteins have biological functions and probably play very significant roles (section 4.16). 4.12 Non-protein nitrogen Nitrogen soluble in 12% TCA is referred to as non-protein nitrogen (NPN), of which milk contains 250-300mgl-', i.e. 5-6% of total milk nitrogen. The NPN is a very heterogeneous fraction (Table 4.7)
DAIRY CHEMISTRY AND BIOCHEMISTRY Table 4.7 Non-protein nitrogen of cow's milk N(mg 1") Creatinine mino nitrogen Unaccounted 88.1 he"unaccounted'N includes some phospholipids, amino sugars, nucleo- tides, hippuric acid and orotic acid. The a-amino n includes free amino acids and small peptides; almost a complete range of amino acids, including ornithine, has been identified in milk, but glutamic acid predominates All the components of NPn are present in blood, from which they are probably transferred into milk. The technological and nutritional signifi cance of NPN is not known but the amino acids are likely to be important for the nutrition of starter micro-organisms, especially of weakly proteoly strains. Urea, which is the principal component of the NPn (6 mmoll-),is strongly correlated with the heat stability of milk; the urea content of milk from cows on pasture is twice as high as that from cows on dry feed and hence the heat stability of the former is considerably higher. The level of NPn in freshly drawn milk is fairly constant but it does increase on ageing, especially if significant growth of psychrophilic bacteria, which may be strongly proteolytic, occurs 4. 13 Comparison of human and bovine milks As mentioned in section 4.1, milk is species-specific, designed to meet the nutritional and physiological requirements of the young of that species. There are about 4300 species of mammal but the milks of only about 170 have been analysed, and data for only about 40 of these are considered eliable. Not surprisingly, human and bovine milks have been studied most intensely. In many respects, the milks of these two species are at the opposite nds of a spectrum. It will be apparent from the foregoing discussion that the proteins in human and bovine milks differ markedly, both qualitatively and quantitatively. Some of the more important differences are summarized in Table 4.8. At least some of these differences are probably nutritionally and physiologically important. It is perhaps ironic that human babies are the least likely of all species to receive the milk intended for them
200 DAIRY CHEMISTRY AND BIOCHEMISTRY Table 4.7 Non-protein nitrogen of cow's milk Component N (mg I-') Ammonia 6.7 Urea 83.8 Creatinine 4.9 Creatine 39.3 Uric acid 22.8 a-Amino nitrogen 37.4 Unaccounted 88.1 The 'unaccounted' N includes some phospholipids, amino sugars, nucleotides, hippuric acid and orotic acid. The x-amino N includes free amino acids and small peptides; almost a complete range of amino acids, including ornithine, has been identified in milk, but glutamic acid predominates. All the components of NPN are present in blood, from which they are probably transferred into milk. The technological and nutritional significance of NPN is not known but the amino acids are likely to be important for the nutrition of starter micro-organisms, especially of weakly proteolytic strains. Urea, which is the principal component of the NPN (6mmoll-'), is strongly correlated with the heat stability of milk; the urea content of milk from cows on pasture is twice as high as that from cows on dry feed and hence the heat stability of the former is considerably higher. The level of NPN in freshly drawn milk is fairly constant but it does increase on ageing, especially if significant growth of psychrophilic bacteria, which may be strongly proteolytic, occurs. 4.13 Comparison of human and bovine milks As mentioned in section 4.1, milk is species-specific, designed to meet the nutritional and physiological requirements of the young of that species. There are about 4300 species of mammal but the milks of only about 170 have been analysed, and data for only about 40 of these are considered reliable. Not surprisingly, human and bovine milks have been studied most intensely. In many respects, the milks of these two species are at the opposite ends of a spectrum. It will be apparent from the foregoing discussion that the proteins in human and bovine milks differ markedly, both qualitatively and quantitatively. Some of the more important differences are summarized in Table 4.8. At least some of these differences are probably nutritionally and physiologically important. It is perhaps ironic that human babies are the least likely of all species to receive the milk intended for them
MILK PROTEINS 201 Table 4. 8 Some important differences between bovine and human milk proteins Constituent Bovine Human Casein: NCN 50%of NCN 20% of total n Very high (6% TN: 3000× bovine) Trace NPn (as % TN) Taurine High ow Ig type IgG1>IgG,>IgA Iga>IgG, >IgG2 NCN, Non-casein nitrogen: NPN, non-protein nitrogen; TN, total nitrogen 'A low level of x. - casein has recently been demonstrated in human milk(Martin et al, 1996). 4.14 Synthesis and secretion of milk proteins The synthesis and secretion of milk proteins have been studied in consider able detail; reviews include mercier and Gaye(1983), Mepham(1987)and Mepham et al. (1992) 4. 14.1 Sources of amino ae Arteriovenous(Av) difference studies and mammary blood flow measure ents( Chapter 1)have shown that in both ruminants and non-ruminants, mino acids for milk protein synthesis are obtained ultimately from blood plasma but that some interconversions occur. The amino acids can be divided into two major groups 1. those for which uptake from blood is adequate to supply the require ments for milk protein synthesis and which correspond roughly to the essential amino acids(EAA); and 2. those for which uptake is inadequate, i.e. the non-essential amino acids (NEAA) Studies involving AV difference measurements, isotopes and perfused gland preparations indicate that the eaa may be subdivided into those for which uptake from blood and output in milk proteins are almost exactly balanced Group I)and those for which uptake significantly exceeds output(Group I I). Group II amino acids are metabolized in the mammary gland and provide amino groups, via transamination, for the biosynthesis of those
MILK PROTEINS 20 1 Table 4.8 Some important differences between bovine and human milk proteins Constituent Bovine Human Protein concentration (YO) Casein : NCN Casein types /?-Lactoglobulin Lactotransferrin Lysozyme GI ycopeptides NPN (as YO TN) Taurine Lactoperoxidase Immunoglobulins (Ig) Ig type (colostrum) 3.5 80: 20 50% of NCN Trace Trace Trace 3 Trace High Very high IgG, > IgG, > IgA ZSl = p > a,, = K 1 40: 60 None 20% of total N Very high (6% TN; High 20 High Low Lower IgA > IgG, > IgG, p > K > CiSly 3000 x bovine) NCN, Non-casein nitrogen; NPN, non-protein nitrogen; TN, total nitrogen. "A low level of ?,,-casein has recently been demonstrated in human milk (Martin et al., 1996). 4.14 Synthesis and secretion of milk proteins The synthesis and secretion of milk proteins have been studied in considerable detail; reviews include Mercier and Gaye (1983), Mepham (1987) and Mepham et al. (1992). 4.14.1 Sources of amino acids Arteriovenous (AV) difference studies and mammary blood flow measurements (Chapter 1) have shown that in both ruminants and non-ruminants, amino acids for milk protein synthesis are obtained ultimately from blood plasma but that some interconversions occur. The amino acids can be divided into two major groups: 1. those for which uptake from blood is adequate to supply the requirements for milk protein synthesis and which correspond roughly to the essential amino acids (EAA); and 2. those for which uptake is inadequate, i.e. the non-essential amino acids (NEAA). Studies involving AV difference measurements, isotopes and perfused gland preparations indicate that the EAA may be subdivided into those for which uptake from blood and output in milk proteins are almost exactly balanced (Group I) and those for which uptake significantly exceeds output (Group 11). Group I1 amino acids are metabolized in the mammary gland and provide amino groups, via transamination, for the biosynthesis of those
02 DAIRY CHEMISTRY AND BIOCHEMISTRY aSmeGLUCOSE ACETATE ARG GLY ALA CYS Ribosomes Milk proteins UREA ORN ARG ell 卡 POLYAMINES 6 AMINO GLUTAMATE Y SEMI ALDEHYDE RANSAMINATION A PYRROLINE 5-CARBOXYLATE Figure 4.31 Summary diagrams of amino acid metabolism in mammary tissue. (a) amino acid )amino acid nitrogen interrelationships(from Mepham, Gaye and
202 DAIRY CHEMISTRY AND BIOCHEMISTRY PI M H n r ci VAL ILE GLU LEU $? 1 CYS + GLU + GLY [Plrsmsl UREA POLYAMINES GLUTAMATE y SEMI ALDEHYDE Figure 4.31 Summary diagrams of amino acid metabolism in mammary tissue. (a) Amino acid carbon interrelationships, (b) amino acid nitrogen interrelationships (from Mepham, Gaye and Mercier, 1982)