Characterization of mare caseins. Identification of $\alpha_{{\bf S1}}$- and $\alpha_{{\bf S2}}$- 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 α slike caseins isolated from the CP fraction. α s1 -Like-casein showed 4 major double bands by electrofocusing with a pI range of 4.3-4.8. In the same conditions, α s2 -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.

Comparison of the digestion of caseins and whey proteins in equine, bovine, caprine and human milks by human gastrointestinal enzymes

“…Another factor may be that equine milk contains β-caseins with variable degree of phosphorylation. The less phosphorylated forms of β-caseins can act on the surface of the casein micelles and thereby play the role of the κ-casein [25]. Even though a certain amount of the human casein was degraded in the gastric juice, the degradation pattern was very different from the profile observed for the equine caseins.…”

“…Human milk contains 1.0 g·L −1 κ-casein [24]. It has been suggested that the high content of β-caseins and the variable degree of phosphorylation of equine milk compensate for the low content of κ-casein in the micelle formation [25].…”

A Proteomic Study on Donkey Milk

“…In conclusion, the heterogeneity shown in the whole casein analysis by 2-DE may be due to a variable degree of phosphorylation and to genetic variants of αs1-and β-caseins [32][33][34].…”

Separation and characterization of major milk proteins from Algerian<br>Dromedary (Camelus dromedarius)

“…Separation of the protein fraction into caseins and whey proteins was conducted according to a modified method from Ochirkhuyag et al (2000). Specifically, whole casein was obtained from skimmed milk by isoelectric precipitation (pH 4.3) at 22°C using 1N HCl.…”