Comparison of Milk Proteins Using Preparative Isoelectric Focusing Followed by Polyacrylamide Gel Electrophoresis

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Abstract
The major proteins in milks from bovine, caprine, porcine, and murine animals and from humans were compared using a two-dimensional analysis method. In the first dimension, proteins were separated by their isoelectric points using preparative isoelectric focusing in pH gradient of 3 to 10. Twenty fractions from each sample were then analyzed by urea-PAGE and SDS-PAGE. Two-dimensional gels showed characteristic patterns for each milk. Major bovine milk proteins were identified and used as reference for proteins of other mammals. Additionally, some peptides resulting from plasmin hydrolysis were characterized. Caprine milk proteins showed a pattern similar to that of bovine milk except for the absence of alpha s1-caseins. alpha-Lactalbumin of bovine and caprine milks resolved as two bands in an immunoblot using bovine alpha-lactalbumin antibody. Each band corresponded to normal and glycosylated alpha-lactalbumin. Human, porcine, and murine milk proteins were totally different from those of ruminant milks on the two-dimensional gels. Two-dimensional analysis using preparative isoelectric focusing, followed by PAGE, was a useful method to compare major milk proteins in several mammals because of the rapid simultaneous separation into 20 fractions. This fractionation allows additional analytical procedures for more efficient comparison of chemical and physical properties of the proteins.