Characterization of mare caseins. Identification of αs1- and αs2-caseins

Abstract

Whole mare (Mongolian and French breeds) casein was obtained from skim milk by isoelectric precipitation (pH 4.6) at 22 °C. In another series of experiments, equine caseins were fractionated after isoelectric precipitation at 4 °C, according to their sensitivity to temperature, on one hand, the pH of the resulting pellet was adjusted to 6.5 before centrifugation (45 000 g for 30 min) at 4 °C; on the resulting casein fraction which precipitated was named the cold precipitated (CP) fraction. On the other hand, the supernatant obtained from the centrifugation of skim milk at pH 4.6 and 4 °C was warmed to 30 °C before being centrifuged at 30 °C, 45 000 g, for 30 min. The resulting casein fraction which precipitated was named the thermally precipitated (TP) fraction. Equine caseins were then purified by high resolution gel chromatography on an anion-exchange column followed by reversed phase-high performance liquid chromatography. The casein fractions were analyzed by urea- and SDS-polyacrylamide gel electrophoresis, their amino acid compositions were determined and their first 15 N-terminal amino acids were sequenced. This analysis showed the presence of αs-like caseins isolated from the CP fraction. αs1-Like-casein showed 4 major double bands by electrofocusing with a pI range of 4.3-4.8. In the same conditions, αs2-like casein showed 2 major bands with a pI range of 4.3-5.1. The TP fraction revealed the existence in equine milk of 6 subfractions of β-like caseins, occurring in the following ratios: 1:5:18:20:15:10, differing at least in their degree of phosphorylation (as shown also by the action of acid phosphatase). Since no evidence of the presence of κ-casein was found in equine milk, it is proposed that part of its functions in the milk of the Equidae could be assumed by the population of less phosphorylated β-caseins.