Release of pyrraline in absorbable peptides during simulated digestion of casein glycated by 3-deoxyglucosone

The glycation product 6-(2-formyl-5-hydroxymethyl-1-pyrrolyl)-l-norleucine (pyrraline), which is formed in the final stage of the Maillard reaction, is taken up in significant amounts with the daily diet. The high bioavailability of pyrraline known from human balance studies can be partly explained by the fact that pyrraline-containing dipeptides are substrates of the human peptide transporter 1 (PEPT1). In the present study we assessed whether pyrraline-containing peptides are formed during luminal digestion of glycated proteins. For this, casein and β-casein, respectively, were enriched with peptide-bound pyrraline by incubation with 3-deoxyglucosone (3-DG), and the modified casein samples were subjected to simulated gastrointestinal digestion. The digestibility of modified casein decreased with increasing pyrraline concentration as measured by analytical size-exclusion chromatography of the digested peptide mixtures. After digestion, 50–60 % of pyrraline was bound in peptides smaller than 1,000 Da isolated by ultrafiltration. Only <4 % of pyrraline was released as the free amino acid. High-pressure liquid chromatography in combination with mass spectrometry of small peptides from digestion mixtures, however, revealed the formation of a large number of different pyrraline dipeptides, which can theoretically be absorbed from the gut by the intestinal peptide transporter PEPT1. Moreover, the N-terminal arginine residue of β-casein modified with 3-DG to a pyrraline analogue at its amino group was also released. The high occurrence of potentially absorbable pyrraline peptides and the minor occurrence of non-absorbable free pyrraline after luminal digestion imply that the decrease in digestibility can yet enhance the bioavailability of pyrraline.