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Properties of Caseins Modified by Treatment with Carboxypeptidase A

Eastern Regional Research Laboratory, Philadelphia, Pennsylvania¹

ABSTRACT

The two major components of cow's milk casein, _s1- and ß-casein, have hydrophobic amino acid sequences in the C-terminal portion of the polypeptide chain. _s1-Casein terminates in –Leu-Leu-Try (2), whereas ß-casein most probably terminates in –Ileu-Ileu-Val (3). This is the case with all of the genetic variants of these proteins so far examined. The major whey proteins also contain hydrophobic amino acids at the C-terminus; ß-lactoglobulin (1) ends in –Ileu, whereas -lactalbumin (7) terminates in –Leu. With the whey proteins, the C-terminal amino acid can be removed by the action of carboxypeptidase A, leaving a still crystallizable (1, 7) residual protein. The effect of removal of C-terminal amino acids from the caseins by the action of carboxypeptidase A has now been studied with _s1- and ß-casein to determine possible alterations of properties such as aggregation, stabilization by -casein, and electrophoretic mobility.

The genetic variants, _s1-A and ß-casein C, chosen for this study were prepared by the method of Thompson (5).

¹ Eastern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.