Abstract
Characterization of the protein profile of the whey fraction from a milk sample taken from an individual donkey belonging to the ‘Ragusana’ species of the East of Sicily is reported. Direct RP‐HPLC/electrospray ionization (ESI)‐MS analysis of the whey fraction allowed the detection of some unknown components, together with the identification of already known whey proteins. Matrix‐assisted laser desorption/ionization (MALDI)‐TOF/MS and RP‐HPLC/ESI‐MS/MS analysis of the enzymatic digests of the unknown components resulted the identification and characterization of (1) two β‐casein fragments; (2) the sequence of donkey's serum albumin; and (3) the oxidized methionine forms of lysozyme B and α‐lactoalbumin. One of the two β‐casein fragments corresponds to the sequence Val^176^‐Arg^189^ of the horse's β‐casein. The second one corresponds the C‐terminal sequence Tyr^199^‐Val^226^ of the horse's β‐casein, with four amino acid substitutions (Q → R^203^, L/I → P^206^, F → L^210^ and P → A^219^). Both fragments, reasonably arising by endogenous proteases cleavage of the donkey's β‐casein, could be potential biologically active peptides. Direct mass spectrometric sequence characterization of the detected donkey's serum albumin reveals the presence of the amino acid substitution Val → Ile at position 497 with respect to the cDNA deduced sequence. The oxidized forms of lysozyme B and α‐lactoalbumin are selectively oxidized at methionine 79 and methionine 90, respectively. Copyright © 2007 John Wiley & Sons, Ltd.