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Acylated ß-Caseins: Electrostatic Interactions and Aggregation

Eastern Utilization Research and Development Division, USDA, Philadelphia, Pennsylvania

ABSTRACT

ß-Casein was both acetylated and succinylated, to investigate alterations of selected properties brought about by removal of positive charges and, in the case of succinylated ß-casein, addition of negative charges. The results were used to evaluate the role of electrostatic interactions in the aggregation of ß-casein. Mobility during alkaline polyacrylamide gel electrophoresis of both derivatives was greater than ß-casein, succinylated ß-casein having the greatest mobility. Succinylated ß-casein and acetylated ß-casein, in that order, required a higher concentration of NaCl than did ß-casein for elution from DEAE-cellulose at pH 7.0. The calcium ion senstivity of acetylated ß-casein was decreased in comparison to ß-casein. Succinylated ß-casein was insensitive to calcium ions at pH 7 in 0.1 M CaCl₂ at a 0.3% concentration. Sedimentation patterns at pH 6.86, 20 C, revealed that succinylated ß-casein did not form a fast-sedimenting peak, usually associated with aggregation. The fast-sedimenting peak of acetylated ß-casein had a lower sedimentation coefficient than did ß-casein. These alterations of the above properties of ß-casein are interpreted to result from an increase in the net negative electrostatic charge of the modified protein in neutral solution. For succinylated ß-casein, the net negative electrostatic charge is sufficient to prevent precipitation by calcium ions and to prevent aggregation under conditions that favor aggregation of ß-casein.