No abstract

Transglutaminasecatalyzes the formation of £-(y-glutamyl)lysyl cross-links within and between protein molecules. Therefore, transglutaminase polymerizes proteins through the formation of isopeptide bonds. Wehave reported that several high-concentration protein solutions formed firm gels when they were incubated with transglutaminase. This paper presents unambiguous evidence that asl-casein solution is gelled by the formation of £-(y-glutamyl)lysyl cross-links.Gelation of asl-casein was effected by the amount of transglutaminase and pHvalue, and asl-casein gel was not dissolved by various denaturants. Succinylated asl-casein, in which e-amino groups of lysine residues are blocked, was not gelled by transglutaminase. Therefore, it is confirmed that

The following properties of food proteins polymerized by guinea pig liver transglutaminase were investigated: (1) solubility, (2) emulsifying activity and emulsion stability, and (3) un frozen water content by pulsed NMR.Several food proteins (asl-and^-caseins, and soybean 7S and US globulins) were polymerized by this enzyme. Solubility and emulsifying activity of polymerized aslcasein were higher than those of the native protein in the range of pH 4~6. Un frozen water contents of polymerized soybean globulins were muchhigher than those of the native proteins. These results suggest that transglutaminase treatment maybe used for the production of newfood protein material with higher hydration ability. Transglutaminase(i^-glutaminyl-peptide :amine y-glutamyltransferase ; EC 2.3.2. 13) was found to catalyze the acyl transfer reaction between the y-carboxyl groups of glutaminyl residues in proteins and a variety of primary amines.1 j2) Whenprotein-bound lysyl residues act as acyl receptors, intra-and intermolecular £-(y-glutamyl)lysine isopeptide crosslinks are formed by the enzymereaction. Whitaker3) suggested that it would be useful to investigate the possibility of using transglutaminase for the formation of the crosslinks in protein leading to textured products. Recently During the purification, the enzyme activity of the fractionated and the purified samples was determined respectively by the colorimetric procedure, in which the detectable hydroxamate was formed, using TV-carbobenzoxy-L-glutaminylglycine.8) The method of Lowry et al.9) was used to determine the enzymeconcentration. The purified enzyme gave a single band on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE) and a single peak on high-performance liquid chromatography (TSK-GEL G3000SW column). Eight hundred grams of guinea pig livers yielded 1 18 mg of transglutaminase preparation with a specific activity of 12.7 unit/mg. The enzyme solution was stored at -30°C in lOmMTris-acetate buffer (pH 6) containing 1 mMEDTA and 0.16m KC1.Protein substrates. asl-and jc-caseins were prepared from fresh milk by the method of Zittle and Custer.10) Crude 1 IS and 7S globulins were prepared from defatted soybean flour by the method of Thanh et al.n) The obtained crude globulins were then chromatographed on a Sepharose 6B column (Pharmacia Fine Chemicals) at 5°C. These protein solutions were dialyzed overnight against distilled water, and then lyophilized.