Amine-binding capacities of food proteins in transglutaminase reaction and digestibility of wheat gliadin with ϵ-attached lysine

Several food proteins were examined for their capacity as substrate in transglutaminase reaction by using a fluorescent amine, monodansyl cadaverine, as donor substrate and following the increase in fluorescence of the reaction mixture. Wheat gluten, or one of its component proteins, gliadin, was the best acceptor among the proteins tested. Compared with intact protein, a preparation of wheat gliadin, modified by transglutaminase-mediated incorporation of ¹⁴C]lysine, showed a marked decrease in in-vitro digestibility. In rat feeding tests, however, the luminal leavings and excreta collected 24 h after administration of gliadin with ?-attached ¹⁴C]lysine contained less than one-tenth of the radioactivity originally administered. The results presented support the previous notion that occurrence of the ?-γ isopeptide linkage in protein does not significantly impair the in-vivo digestibility of the protein and nutritional availability of isopeptide bound lysine moieties.