N-ε-fructosyllysine and N-ε-carboxymethyllysine, but not lysinoalanine, are available for absorption after simulated gastrointestinal digestion

Food processing leads to a variety of chemical modifications of amino acids in food proteins. Recent studies have shown that some modified amino acids resulting from glycation reactions can pass the intestinal barrier when they are bound in dipeptides. In this study, we investigated as to what extent modified amino acids are released from post-translationally modified casein during simulated gastrointestinal digestion. Casein was enriched with N-epsilon-fructoselysine, N-epsilon-carboxymethyllysine, and lysinoalanine, in different degrees of modification. The casein samples were subjected to a two-step proteolysis procedure, simulating gastrointestinal digestion. The digestibility of modified casein as measured by analytical size-exclusion chromatography (SEC) decreased with increasing degree of modification especially after enrichment of fructoselysine and lysinoalanine. Semi-preparative SEC of digested casein samples revealed that fructoselysine and carboxymethyllysine are released bound in peptides smaller than 1,000 Da, which is comparable to native amino acids. The glycation compounds should, therefore, be available for absorption. Lysinoalanine as a crosslinking amino acid, however, is mostly released into longer peptides of at least 30-40 amino acids which should strongly impair its absorption availability.