Separation and Characterization of Mares' Milk _s1-, ß-, -Caseins, -Casein-Like, and Proteose Peptone Component 5-Like Peptides

¹ Laboratoire des BioSciences de l'Aliment, UC885 INRA, Faculté des Sciences et Techniques, Université Henri Poincaré-Nancy 1, B.P. 239, 54506 Vanduvre-lés-Nancy Cedex, France

The equine _s1- and ß-caseins (CN) were purified by chromatography on DEAE-cellulose and by reversed-phase HPLC. The _s1-, ß-, and -CN were characterized either by monodimensional urea-PAGE or sodium dodecylsulfate (SDS)-PAGE or by bidimensional electrophoresis. -Casein was characterized after electrophoresis by glycoprotein-specific staining. To identify _s1-CN without ambiguity, internal sequences were determined after trypsin or chymosin digestion of purified _s1-CN. These sequences, that could be estimated to correspond to 62% of the full protein, presented strong identities with regions of _s1-CN primary structures of other species. In particular, 51, 48, 43, and 40% identities were obtained with corresponding regions of sow, dromedary, cow, and human _s1-CN, respectively. On the other hand, trace amounts of equine -CN-like and proteose peptone component 5-like peptides were found in the whole CN. They were identified by microsequencing and corresponded to ß-CN peptides generated by plasmin action on the whole CN. The equine _s1, ß-, and -CN were separated by bidimensional electrophoresis in numerous isoelectric variants with apparent isoelectric point distributed between pH 4.4 to 6.3, 4.4 to 5.9, and 3.5 to 5.5, respectively. The ß- and -CN displayed a more acidic character in the mare than in the cow.

Key Words: bidimensional electrophoresis • equine casein • mare's milk • plasmin

Submitted on October 4, 2001
Accepted on November 15, 2001