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Identification and quantification of Epsilon-(Gamma-Glutamyl)lysine in digests of enzymatically cross-linked leguminous proteins by high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS)

: Schäfer, C.; Schott, M.; Brandl, F.; Neidhart, S.; Carle, R.


Journal of agricultural and food chemistry 53 (2005), No.8, pp.2830-2837
ISSN: 0021-8561
Journal Article
Fraunhofer IVV ()

A rapid and convenient method for the precise quantification of epsilon-(gamma-glutamyl)lysine isopeptide in lyophilized proteolytic digests of cross-linked plant protein samples was developed. The isopeptide was baseline-separated from three other isomers containing lysyl and glutamyl residues by reverse-phase high-performance liquid chromatography after exhaustive proteolytic digestion of the samples cross-linked by a microbial transglutaminase (MTG). Highly selective detection was performed by electrospray mass spectrometry in MS/MS mode. Demonstrating the applicability of the suggested analytical procedure, enzymatic cross-linking of protein isolates from soy [Glycine max (L.) Mort.], pea [Pisum sativum L], and the sweet lupin species Lupinus albus L. and Lupinus angustifolius L. was investigated after incubation with 0.01 g of MTG/100 g of protein for 0-240 min at 40 degrees C. The liquid chromatography-mass spectrometry (LC-MS) method was successfully applied to monitor the kinetics of epsilon-(gamma-glutamyl)lysine isopeptide formation. Since the calculated initial levels of c-(gamma-glutamyl)lysine in the genuine leguminous protein isolates were between 40 and 77 mu mol/100 g, an isopeptide detection limit of 0.5 mu g/mL, corresponding to approximately 50 mu mol/100 g of protein, was shown to suffice for quantifying the cross-linking rate enzymatically induced by MTG. Concentrations of epsilon-(gamma-glutamyl)lysine in the texturized proteins ranged from 100 to 500 mu mol/100 g of protein.