A Mass Spectrometry Study of the Peptides Produced During the [i]in vivo[/i] digestion of α-lactalbumin
Résumé
An α-lactalbumin beverage was consumed by ten healthy subject and samples removed from their stomach via a naso-gastric tube at three minute intervals. The stomach pH initially increased, but after 3 minutes it began to decrease dropping to ~pH 2.0. Gastric emptying occurred after approximately 30 minutes. The digestion of
α-lactalbumin was followed gel permeation chromatography. The peak for intact α-lactalbumin was predominant during the first 6 minutes of digestion and ~90% of the protein present is greater than 10 kDa. After 9 minutes, corresponding with a significant decrease in pH, only ~5% of the protein was greater than 10 kDa and nointact α-lactalbumin remained. During an STSM stay in INRA, Rennes some of the digests were analysed by mass
spectrometry, 113 different peptides were identified. The greatest diversity of peptides is present after 12 minutes of digestion, however the portion of the protein sequence covered by the peptides identified remains constant, even though the number of peptides identified in the digests decreases over time. The peptides
identified are not distributed evenly through the α-lactalbumin sequence. The terminals give rise to a greater number of peptides than the centre of the protein. The intensity of the signal obtained by the mass spectrometer for each peptide was measured. The majority peptides coming from the c-terminal portion of the protein (residues 81-123) were their maximum concentrations between 0 and 9 minutes of digestion. By this time the protein has just unfolded and the pH of the stomach is not low enough for optimum pepsin activity, this indicates that (1-40) do not reach their maximum concentration until later in the digestion when α-lactalbumin has unfolded and pH has dropped to 2.0.