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).