Protein and Nonprotein Nitrogen Components in Human Milk, Bovine Milk, and Infant Formula: Quantitative and Qualitative Aspects in Infant Nutrition

During the last decade, the discussion regarding the quantity of protein in feeding term and preterm infants had led to numerous controversial publications. The protein quality of bovine milk has been extensively studied as a dairy product, but the processing of infant formula has revealed new questions about the type of heat treatment to be used to produce microbiologically safe and highly nutritious formulae and to the impact of protein hydrolysis on nutrient bioavailability, e.g., during the production of hypoantigenic formula. Feeding preterm infants with donor milk also requires processing of human milk, which affects the functional properties of certain milk components. From the qualitative point of view, some major milk proteins seem to be well characterized. Functional aspects of side chains like carbohydrates in lactoferrin, secretory IgA (sIgA), casein, and others, however, need further investigation. In addition, during the gastrointestinal transit of proteins in milk or infant formula, a variety of smaller peptides and amino acids are formed, which may exert local or systemic functions, e.g., as bioactive or immunomodulatory components. Here, we will focus on proteins in human and bovine milk; the protein quantity necessary for appropriate growth; the content and utilization of nonprotein nitrogen; the nutritional bioavailability of milk proteins from human milk; and the effect of processing of infant formula, including hypoantigenic products, on the formation of bioactive peptides and on possible functions of the glycan part of some proteins. COMPARISON BETWEEN HUMAN MILK, BOVINE MILK, AND INFANT FORMULA The total milk protein consists of two major groups of constituents, caseins and whey proteins; some minor components such as enzymes or growth factors; and proteins within the milk fat globule membrane (1-4). In mature human milk, the ratio of whey protein to casein is about 60:40 to 50:50, whereas in bovine milk, casein constitutes ≈80% of the total protein (1,5,6). The difference in the gross protein composition of human and bovine milk after SDS-gel electrophoresis is shown in Fig. 1. Whereas lactoferrin, sIgA, serum albumin, and α-lactalbumin are the main components in human whey, β-lactoglobulin is the dominant soluble protein in bovine milk (compare lanes 2 and 3 with lane 6). The latter has been detected in human milk as well, but only in minute quantities (7,8), possibly originating from the cow's milk protein intake of the mother. The caseins in human milk are β- and κ-casein; α-casein, which is regularly found in bovine milk, seems to be absent in human milk (compare lanes 4 and 5 with lane 7). Table 1 summarizes the composition, the concentration, and the degree of glycosylation of the major proteins in human and bovine milk. An increasing number of publications demonstrate that from a biochemical and physiological point of view, the glycosylation of proteins and lipids plays an important role in the function of these components. It is currently discussed that glycoproteins and glycolipids, for example, exert antiinfective properties to prevent the attachment of pathogenic microorganisms to epithelial cells, that oligosaccharides and other carbohydrates affect the interaction of cell adhesion molecules with endothelial cells, and that negatively charged components like acidic oligosaccharides and glycoconjugates enhance mineral and trace element absorption (9-11). The latter effect would be comparable to the possible impact of phosphopeptides from human or bovine milk on the absorption of, e.g., calcium or zinc. Further functional aspects of the carbohydrate moiety of glycoconjugates are discussed in the final section on current topics in human milk protein research. Immunoglobulins, lactoferrin, and casein are glycosylated to a different degree in both human and bovine milk. For example, in κ-casein, all sugar chains are of the O-glycan type (12); no carbohydrate parts linked to asparagine have been detected so far. A characteristic of human κ-casein compared with the bovine subunit is not only the higher amount of carbohydrates (40-60% vs. 10%), and therefore the higher variety of possible sugar linkages, but the presence of ≤10 prosthetic sugar groups in human casein (only one in bovine casein) (12-14). There are pronounced differences in the casein subunit patterns for premature milk, colostrum, and mature milk. In general, κ-casein subunits in human milk are not detectable until day 3-4 postpartum (5). In early premature milk the glycosylated components (κ-casein) are present at very low concentrations or are missing (Fig. 2)(15,16). The glycosylated subunits are also lower in colostrum (>day 3 postpartum) compared with mature milk of mothers with term infants, although a subunit pattern more similar to that of mature milk is established. β-casein seems to exist with a similar pattern in premature milk, but the concentrations of most subunits are higher in colostrum and milk of mothers with term infants (Fig. 2). The marked increase in casein concentration and the concomitant decrease in whey proteins during the first days of lactation leads to a changing proportion of casein as a percentage of total protein. The ratio of whey proteins to casein is high (≈90:10) at the onset of lactation and changes rapidly within days to a ratio of 60:40 or even 50:50 in mature milk (5,16). These observations suggest a different hormonal regulation of β-casein and κ-casein synthesis. Whether both-protein synthesis and side chain modifications (glycosylation and phosphorylation)-of the subunits are controlled the is other milk all casein subunits are a of amino and for the casein as be to enhance calcium absorption and with In addition, like other glycosylated casein subunits may the growth of or with or to prevent adhesion to epithelial proteins are to be in the in synthesis and in of the glycosylated part of and on the will be discussed There are other proteins and in huma