Preparation and Properties of Trifluoroacetylated κ-Casein

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Abstract
Blocking the free amino groups of ~-ca- sein with trifluoroacetyl-groups resulted in markedly altered properties of the protein. Trifluoroacetylated K-casein was not able to stabilize a,~-casein in the presence of calci- um ions, but this ability was fully restored after removal of the blocking groups. Tri- fluoroacetylated-K-casein was susceptible to rennin action, but the liberated para- s-casein was completely soluble and did not yield the typical para-K-casein clot. This loss of characteristic properties is attributed to structural alterations pro- duced by the change in ionic character or by steric effects of the blocking trifluoro- acetyl-groups. The stability of the casein micellar system can be attributed directly to the unique prop- erties of ~-casein. Through complex formation with calcium-sensitive caseins, K-casein imparts physical stability to casein micelles in the pres- ence of calcium ions. Although much informa- tion regarding the chemistry of the caseins has accumulated, the physicochemical forces responsible for the interactions between ~- casein and the calcium-sensitive caseins have not yet been elucidated. Various functional groups of K-casein have been studied, to determine their participation in complex formations. K-Casein modified by renmval of phosphate groups (5) or sialic acid moieties (6) substantially retained the ability to form complexes with a.~-casein. Similarly, studies with reduced and alkylated K-caseins (3, 8) have indicated that neither free sulfhy- dry) groups nor intact disulfide bonds are essential for complex formation. In the present investigation, we report the blocking of the free amino groups of K-casein using S-ethyl-trifluorothioacetate, and its effect on the stabilizing properties, rennin sensitiv- ity, and sedimentation behavior.