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Amino Acid Composition of Six Distinct Types of ß-Casein

Eastern Regional Research Laboratory, Eastern Utilization Research and Development Division, ARS, USDA, Philadelphia, Pennsylvania

ABSTRACT

Aschaffenburg classified ß-caseins into three groups—A, B, and C—based on their mobilities at pH 7.15 in paper electrophoresis with a citrate-phosphate buffer-containing urea. A study of the high-voltage electrophoresis diagrams of tryptic hydrolysates of a large number of ß-caseins of Type A showed four distinct and different patterns. When representatives of these four types were purified by chromatography and analyzed for their amino acid composition, differences were found in histidine, proline, aspartic acid, glutamic acid, alanine, serine, valine, isoleucine, and leucine. Considering only the variation in charge-bearing groups, the four ß-caseins, on the basis of five glycines per molecule, might be abbreviated as: A¹(His₆Asp₉Glu₄₀), A^2-1 (His₅Asp₁₀Glu₃₉), A^2-2 (His₅Asp₁₀Glu₃₈), and A^2-3(His₅Asp₉Glu₃₈). When the ß-caseins were typed in 7% acrylamide gel at pH 9 by our usual procedure, no difference in mobility could be detected. However, if the acrylamide was increased to 10% in the gels, A^2-1(His₅Asp₁₀Glu₃₉) was faster and A^2-2(His₅Asp₁₀Glu₃₈) slower than the two other types. The majority of the ß-casein ¹ A samples typed agreed with A¹(His₆Asp₉Glu₄₀) and A^2-3(His₅Asp₉Glu₃₈) in mobility in the 10% gels. If the electrophoresis was in pH 3 acrylamide gels, the last two variants were easily distinguished, since the three A(His₅) variants ran with the same mobility, but the A(His₆) variant had a higher mobility. Thus, the four types can be distinguished by gel electrophoresis alone. An analysis of ß-caseins B and C is included for comparison. The ß-casein C was isolated by chromatography from an A/C ß-casein. The A and C analyses were identical, except for addition of one histidine and one lysine and subtraction of two prolines in the ß-casein C. ß-Caseins B are distinguished by the presence of five arginines; A and C have only four. (B and C have both 6-histidines.)