A novel calcium-independent enzyme capable of incorporating putrescine into proteins

A Ca++-independent enzyme capable of incorporating [3H]-putrescine into proteins was detected in the rat intestine mucosa. The Ca++-independent incorporation of [3H]-putrescine into proteins was temperature-, pH-, time-, and dose-dependent. However, this enzyme was absent in the gastric mucosa. Similar to testicular Ca++-dependent transglutaminase, the optimal pH of intestinal Ca++-independent enzyme was 9.0. At 10-5 M or less putrescine concentrations, the Ca++-independent enzyme in an intestinal cytosol preparation showed a greater activity than did the Ca++-dependent transglutaminase. However, at higher putrescine concentrations, the latter showed a greater activity than did the former. Both the intestinal Ca++-dependent and independent enzymes were inhibited by cystamine, thermal labile at 50°C and precipitated by 30 to 50% saturation of ammonium sulfate. The fact that these two enzymes shared many similar characteristics, with the exceptions of Ca++-requirement, suggests that they may have similar active site and intrinsic molecular function(s).