Isolation and Identification of beta-Casein A(1)-4P and beta-Casein A(2)-4P in Commercial Caseinates

Caseinate contained two modified beta-casein (beta-CN) fractions that together represented from 5 to 27% of the total beta-CN depending on the type of caseinate analyzed (sodium, calcium, or potassium). Mass spectroscopy showed that the modified beta-CN fractions had molecular weights of 23 940 +/- 3 and 23 904 +/- 2, approximately 80 (or the mass of one phosphate group) less than that of the native beta-CN fractions found in milk, beta-CN A(1)-5P (24 028) and beta-CN A(2)-5P (23 988). (31)P NMR verified mass spectroscopy results showing that the modified fractions contained four instead of five phosphorylated serine residues. Molecular weight differences between the modified and unmodified fractions also indicated that the dephosphorylation was a result of enzyme, acid, or alkali hydrolysis and not alkali hydrolysis that proceeds through beta-elimination. The two modified fractions identified as beta-CN A(1)-4P and beta-CN A(2)-4P are probably present in caseinate as a result of the dephosphorylation of the main beta-CN gene products beta-CN A(1)-5P and beta-CN A(2)-5P, respectively.