Donkeys’ milk protein fraction investigated by electrophoretic methods and mass spectrometric analysis

Donkeys' milk has attracted a growing interest in human nutrition, particularly in the dietotherapy of patients affected by cows' milk protein allergy. In this paper, donkeys' milk protein fractions were analysed by isoelectric focusing (IEF) and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Four different IEF patterns were observed out of 51 individual milk samples. Thirty-six samples presented a common pattern, used as a reference, whereas 15 were characterized by the presence or absence of protein bands. A single sample, lacking two bands of the casein fraction, showed an apparent reduced amount of caseins, by SDS-PAGE as well as by chromatographic analysis. Mass spectrometry and high-performance liquid chromatography were used to characterize the IEF patterns. Compared with the reference, one pattern lacked two αs1-casein bands, another lacked two β-lactoglobulin II bands and one accounted for a different β-lactoglobulin II variant. No signals related to putative αs2- and κ-casein were observed.