Determination of the dipeptide epsilon-(gamma-glutamyl)lysine in transglutaminase crosslinked proteins by amino acid analysis.

After enzymatic hydrolysis, dipeptide epsilon-(gamma-glutamyl)lysine could be detected via amino acid analysis in food samples crosslinked by transglu taminase. The chromatogram showed the dipeptide to be located between methi onine and isoleucine. In the range of 1-20 nmol/40 mu l, a linear correlati on between peak area and epsilon-(gamma-glutamyl)lysine content could be pr oved. Comparing the lysine contents of caseinate and of crosslinked caseina te, both enzymatically degraded, the proportion of the lysine incorporated in isopeptide bonds could be determined at 1.6 %. The tendency of the epsil on-(gamma-glutamyl)lysine contents measured in milk and quarg samples confi rmed the findings previously stated by using SDS-PAGE and gel chromatograph y. The epsilon-(gamma-glutamyl)lysine contents of yoghurt samples incubated with different transglutaminase activities did not correspond to the enzym e-substrate ratio.