Nomenclature of the Proteins of Cows’ Milk—Sixth Revision

This report of the American Dairy Science Association Committee on the Nomenclature, Classification, and Methodology of Milk Proteins reviews changes in the nomenclature of milk proteins necessitated by recent advances of our knowledge of milk proteins. Identification of major caseins and whey proteins continues to be based upon their primary structures. Nomenclature of the immunoglobulins consistent with new international standards has been developed, and all bovine immunoglobulins have been characterized at the molecular level. Other significant findings related to nomenclature and protein methodology are elucidation of several new genetic variants of the major milk proteins, establishment by sequencing techniques and sequence alignment of the bovine caseins and whey proteins as the reference point for the nomenclature of all homologous milk proteins, completion of crystallographic studies for major whey proteins, and advances in the study of lactoferrin, allowing it to be added to the list of fully characterized milk proteins.

Review and update of casein chemistry

Of all food proteins, bovine milk proteins are probably the most well characterized chemically, physically, and genetically. The primary structures are known for most genetic variants of alpha s1-, alpha s2-, beta-, and kappa-caseins, beta-lactoglobulin, and alpha-lactalbumin. Secondary and tertiary structures of the whey proteins have been determined, and secondary structures of the caseins have been predicted from spectral studies. The caseins, although less ordered in structure and more flexible than the typical globular whey proteins, have significant amounts of secondary and, probably, tertiary structure. The amphipathic structure of the caseins is especially noteworthy; thus, these proteins most likely are divided into polar and hydrophobic domains. The presence of anionic phosphoseryl residue clusters in the calcium-sensitive casein polar domains is particularly significant because of their interaction with calcium ions, or calcium salts, or both, and the formation of micelles. Flexibility of casein structures is reflected by their susceptibilities to limited proteolysis, which dramatically changes functionality.

Purification and Characterization of a Substrate-Size-Recognizing Metalloendopeptidase from Streptococcus cremoris H61

During the ripening of Gouda-type cheese, two kinds of endopeptidases were found to participate in the degradation of αs1-CN(f1-23), a specific product from αs1-casein hydrolyzed by chymosin. One of the endopeptidases, lactic acid bacteria endopeptidase (LEP-II), which can recognize the size of its substrates, has already been purified and characterized (T. R. Yan, N. Azuma, S. Kaminogawa, and K. Yamauchi, Eur. J. Biochem. 163:259-265, 1987). The other endopeptidase, LEP-I, was purified to homogeneity by conventional chromatographic techniques from Streptococcus cremoris H61. The enzyme appeared to be monomeric, with an apparent molecular weight of 98,000, and its isoelectric point was 5.1. For the hydrolysis of αs1-CN(f1-23), the enzyme had an optimum pH and temperature of 7.0 to 7.5 and 40�C, respectively. Its activity was inhibited by such chelating agents as EDTA and 1,10-phenanthrolin, and it could be fully reactivated by Mn 2+ . Inhibitors specific for serine and thiol proteases had no effect on the protease activity. The enzyme showed a high affinity toward the Glu-Asn peptide bond of αs1-CN(f1-23) and αs1-CN(f91-100) but showed no hydrolysis activity toward αs1-CN(f1-52), αs1-CN(61-122), αs1-CN(136-196), αs1-casein, β-casein, κ-casein, α-lactalbumin, and β-lactoglobulin. The K m and V max of LEP-I for αs1-CN(f1-23) were 14.2 pM and 139 U, respectively.