CHEMICAL PHOSPHORYLATION OF BOVINE CASEIN - RELATIONSHIPS BETWEEN THEREACTING MIXTURE AND THE BINDING-SITES OF THE PHOSPHORYL MOIETY

The aim of this work was to determine the influence of pH on the chemi cal phosphorylation of bovine casein by POCl3. Experiments were perfor med at pH 5, pH 7 and pH 9. A pH-stat apparatus was used to maintain t he pH at a fixed value. The phosphorus content of modified casein was measured by the Fiske and Subbarow method. The amount of unreacted E-a mino groups of lysyl residues was determined using 2,4,6-trinitrobenze ne sulfonic acid. Polyacrylamide gel electrophoresis in a dissociating medium showed that many of the modified species were linked together by stable bonds. This observation is consistent with the very high vis cosity of the product in dissociating medium. An amino acid analysis w as performed to check the possible production of a lysinoalanine linka ge. The stability of the modified casein was investigated by partial h ydrolysis at pH 2, pH 5 and pH 10.5, followed by polyacrylamide gel el ectrophoresis. The results showed that the number and nature of phosph ate bonds depend on the pH at which the reaction was carried out. Copy right (C) 1996 Elsevier Science Ltd