Isolation and Identification of β-Casein A1-4P and β-Casein A2-4P in Commercial Caseinates

Caseinate contained two modified β-casein (β-CN) fractions that together represented from 5 to 27% of the total β-CN depending on the type of caseinate analyzed (sodium, calcium, or potassium). Mass spectroscopy showed that the modified β-CN fractions had molecular weights of 23 940 ± 3 and 23 904 ± 2, ≈80 (or the mass of one phosphate group) less than that of the native β-CN fractions found in milk, β-CN A1-5P (24 028) and β-CN A2-5P (23 988). 31P 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 β-elimination. The two modified fractions identified as β-CN A1-4P and β-CN A2-4P are probably present in caseinate as a result of the dephosphorylation of the main β-CN gene products β-CN A1-5P and β-CN A2-5P, respectively. Keywords: Caseinate; β-casein; dephosphorylation; phosphatase