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Role of Phosphate Groups in the Calcium Sensitivity of _s2-Casein

Animal Products Processing Research Laboratory, Faculty of Agriculture, Kagoshima University, Kagoshima 890, Japan

ABSTRACT

In order to explain the higher calcium sensitivity of _s2-casein than other casein constituients, properties of _s2-casein and dephosphorylated _s2-casein were examined and compared with those of _s1-casein. _s2-Casein was more sensitive to calcium than _s1-casein. Aggregates of calcium _s1- and _s2-caseinates were both thoroughly solubilized by 4 M urea. Gel filtration of _s1-casein and of _s2-casein in 4 M urea showed the same elution pattern in the presence and absence of 10 mM calcium chloride. Dephosphorylated _s2-casein was insoluble at neutral pH although dephosphorylated _s1-casein was soluble. Dephosphorylated _s2-casein was solubilized by 4 M urea and alkalization to pH 11. The net proton charge, calculated from the primary structure, of dephosphorylated _s2-casein was very low at pH 7.0, although dephosphorylated _s1-casein has a considerable number of negative net charges. The calculated isoionic point of dephosphorylated _s2-casein, which contains one phosphate group, was 6.98. Ester phosphate groups of _s2-casein play an important role in its solubilization at neutral pH and neutralization of ionized phosphate groups by calcium ion increases hydrophobic interaction, which leads to precipitation.

¹ Technical Research Institute, the National Federation of Dairy Cooperative Association, Shinsayama, Sayama 350-13, Japan.

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