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Alkylation of Reduced -casein

Eastern Regional Research Laboratory Eastern Utilization Research and Development Division Agricultural Research Service U. S. Department of Agriculture Philadelphia, Pennsylvania 19118

ABSTRACT

The study of polymorphism of -casein by the technique of gel electrophoresis requires the reduction of disulfide bonds. This is necessary because native -casein does not yield discrete bands after starch- or polyacrylamide-gel electrophoresis, whereas reduction of the protein yields material which produces discrete bands. In this way, differences in the electrophoretic distributions of components of reduced -casein prepared from milks of individual cows have been detected and ascribed to polymorphism (3, 6, 7).

Further investigation of reduced -casein requires the preparation of the proteins in a stable reduced form. Several alkylating agents can be utilized to stabilize the thiol groups after reduction of -casein. Implicit in these studies is the requirement that the reduction-alkylation procedure be complete for disulfide (cystine) or thiol (cysteine) groups, or both, and that the method does not lead to alkylation of other sites in the polypeptide chain. A method is presented for the complete reduction and alkylation of -casein utilizing iodoacetamide (IAA-NH₂) which, under the prescribed conditions, does not result in any detectable nonspecific alkylation.