Nomenclature of the Proteins of Cow's Milk: Fourth Revision

Abstract

This report reviews the nomenclature of the milk proteins of cow's milk in light of more recent advances in our knowledge. With the establishment of the primary structures of a number of these proteins, we now have a definite identification of alphas1-, kappa-, beta-, and the gamma-caseins as well as beta-lactoglobulin and alpha-lactalbumin. On the basis of new information on their primary structures and relationship to beta-casein polymorphs, changes in nomenclature have been recommended for proteins of the gamma-casein fraction. Although the primary structure serves as the unambiguous definition of proteins for which it is known, a more practical identification is necessary. We recommend that their behavior in gel electrophoresis under suitable conditions be employed for this purpose for all of the "major" milk proteins of raw skim milk except the immunoglobulins where, because of their heterogeneity and molecular genetics, physical parameters are less useful and their identification must be based upon antigenic determinants and their homology with their human counterparts. More work is needed and, with the accumulation of more information, additional changes in nomenclature can be expected for such proteins as the minor components of alphas- and kappa-caseins, alpha-lactalbumin, and the proteose-peptone fraction as well as further confirmation of the presence of immunoglobulins IgE and additional IgG subclasses. Additional components and genetic variants also can be expected.

Plasmin-mediated proteolysis of casein in bovine milk.

“…'y3-Casein A comigrates with yl-casein A2 in polyacrylamide gel electrophoresis (pH 9.6, 4 M urea) (21) and is not differentiated from y2-casein A2 in NaDodSO4 gel electrophoresis because they differ in size by only two amino acid residues (24). Polyaqrylamide gel electrophoresis (pH 3.0, 4.5 M urea) has been used to identify -y1-A2, y2-A2, and y3-A caseins as products of the degradation of /-casein A2 by bovine plasmin (10).…”

Lactose and major milk proteins are present in secretory vesicle-rich fractions from lactating mammary gland.

“…However, the polypeptide pattern of secretory vesicles were complex, as would be expected if both membrane and secretory proteins were present. In addition, the major caseins of bovine milk (as,, Mr 23,500, 45-55% of total milk protein; Total particulate 18 ± 4 Vesicle-depleted particulate (30) do not separate well from each other on sodium dodecyl sulfate/polyacrylamide gels. For this reason, a crude separation of caseins was performed and the putative casein-containing fraction was separated on urea/polyacrylamide gels (Fig.…”

“…Because the major proteins of bovine milk have been extensively characterized, vesicles from bovine mammary gland were used exclusively for identification of content proteins (30). When intact secretory vesicle-rich fractions were prepared directly and the proteins separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, bands comigrating with major milk caseins and whey proteins were evident ( Fig.…”

Responses of cows to abomasal infusion of lysine and methionine at two levels of dietary protein

“…Net uptake of individual AA by the gland was calculated as the AV concentration difference x mammary plasma flow. Amino acid balance across the mammary gland was calculated as uptake by the mammary gland divided by AA secreted in milk, corrected for milk protein AA derived from non-mammary synthesized protein (Whitney et al, 1976). Plasma flow and balance calculations were performed over the 8 h sampling period for the udder half in which the venous catheter was implanted.…”

“…Plasma flow was calculated according to the method described by Cant et al (1993) using the Fick principle and Phe and Tyr as internal markers. Whole milk Phe and Tyr concentrations, corrected for 4% of milk protein AA derived from non-mammary synthesized protein appearing in the milk (Whitney et al, 1976), were used in the model instead of casein-bound and milk-free Phe and Tyr. Net uptake of individual AA by the gland was calculated as the AV concentration difference x mammary plasma flow.…”