Spectrophotometric assay using o-phtaldialdehyde for the determination of transglutaminase activity on casein

In this work, the possibility of using a simple and quick method was tested for determining transglutaminase activity on casein using a spectrophotometric assay. The enzyme activity was estimated on the basis of the decrease of o-phthaldialdehyde reactive ε-amino groups of lysine following the formation of isopeptide bonds. The lysine residues involved in the formation of isopeptide bonds when the reaction reaches its plateau are equal to 0.126 μmol per mg of casein. This value results as equal to 0.205 μmol per mg of casein when N-carbobenzoxy-glutaminyl-glycine is added to the reaction medium as a small size acyl group donor. The electrophoretic analysis of the reaction products emphasised a different kinetic formation of casein polymers with the two substrate solutions used. This proposed method has resulted as accurate, with a mean coefficient of variation of 4.6%.