Characterization of mare caseins. Identification of alpha(S1)- and alpha(S2)-caseins

Whole mare (Mongolian and French breeds) casein was obtained from skim milk by isoelectric precipitation (pH 4.6) at 22 degrees C. In another series o f experiments, equine caseins were fractionated after isoelectric precipita tion at 4 degrees C, according to their sensitivity to temperature, on one hand, the pH of the resulting pellet was adjusted to 6.5 before centrifugat ion (45 000 g for 30 min) at 4 degrees C; on the resulting casein fraction which precipitated was named the cold precipitated (CP) fraction. On the ot her hand, the supernatant obtained from the centrifugation of skim milk at pH 4.6 and 4 degrees C was warmed to 30 degrees C before being centrifuged at 30 degrees C, 45 000 g, for 30 min. The resulting casein fraction which precipitated was named the thermally precipitated (TP) fraction. Equine cas eins were then purified by high resolution gel chromatography on an anion-e xchange column followed by reversed phase-high performance liquid chromatog raphy. The casein fractions were analyzed by urea- and SDS-polyacrylamide g el electrophoresis, their amino acid compositions were determined and their first 15 N-terminal amino acids were sequenced. This analysis showed the p resence of alpha(s)-like caseins isolated from the CP fraction, alpha(s1)-L ike-casein showed 4 major double bands by electrofocusing with a pI range o f 4.3-4.8. In the same conditions, alpha(s2)-like casein showed 2 major ban ds with a pi range of 4.3-5.1. The TP Fraction revealed the existence in eq uine milk of 6 subfractions of beta-like caseins, occurring in the followin g ratios: 1:5:18:20:15:10, differing at least in their degree of phosphoryl ation (as shown also by the action of acid phosphatase). Since no evidence of the presence of kappa-casein was found in equine milk, it is proposed th at part of its functions in the milk of the Equidae could be assumed by the population of less phosphorylated beta-caseins.