Amino Acid Composition of Six Distinct Types of β-Casein

Open Access
Publisher Website
Google Scholar
Abstract
The [beta]-caseins were classified into 3 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 hydroly-sates of a large number of [beta]-caseins of type-A showed 4 distinct and different patterns. Differences were found in histidine, proline, aspartic acid, glutamic acid, alanine, serine, valine, isoleucine and leucine when representatives of these 4 types were purified by chromatography and analyzed for their amino-acid composition. Considering only the variation in charge-bearing groups, the 4 [beta]-caseins, on the basis of 5 glycines/molecule, might be abbreviated as: A1 (His6Asp9Glu40), A2-1 (His Asp10Glu39), A2-2 (His Asp10Glu38) and A2-3 (His5Asp9Ghigr,). When the [beta]-caseins were typed in 7% acrylamide gel at pn 9, no difference in mobility could be detected. However, if the acrylamide was increased to 10% in the gels, A2-1 was faster and A2-2 was slower than the other 2 types. The majority of the [beta]-casein2 A samples typed agreed with A1 and A2-3 in mobility in the 10% gels. If the electrophoresis was in pH 3 acrylamide gels, the last 2 variants were easily distinguished since the 3 A(His5) variants had the same mobility but the A(His6i) variant had a higher mobility. An analysis of [beta]-caseins B and C is included for comparison. The A and C analyses were identical except for the addition of 1 histidine and 1 lysine and the subtraction of 2 prolines in the [beta]-casein C. [beta]-Caseins B are distinguished by the presence of 5 arginines; A and C have only 4; B and C have 6-histidines.