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Enzymic modification of αs1-casein with peptidylarginine deiminase: preparation of less acid-coagulable and less calcium-sensitive casein

Summary Enzymic modification with peptidylarginine deiminase (EC 3.5.3.15) enabled five out of six arginyl residues in αs1-casein to be convcrted to citrullyl residues, only the N-terminal arginyl residue remaining unaffected. An increase in the net negative charge was confirmed by PAGE. The isoelectric point was decreased from 4·46 for the intact αsl-casein to 4·30 for the deiminated type, while simultaneously lowering the acid-precipitation starting point from pH 5·17 to pH 4·62. The deiminated αs1-casein self-associated less in the absence of Ca and was less Ca-sensitive than the native type, although its Ca-binding ability was slightly enhanced. In the presence of 25 mM-CaC12 and K-casein, Ca-induced precipitation of αs1-casein did not occur, the solution of the mixture remaining transparent. Deimination of αs1-casein resulted in altering its characteristics, possibly by interfering with interactions through hydrophobicity and/or hydrogen bonding. The positive charge of the arginyl residues might play an important role in casein micelle formation.