Background and aims: Ulcerative colitis (UC) is characterised by refractory in flammatory ulceration and damage to the colon.The mechanisms underlying impaired healing have yet to be defined. As transglutaminase expression resulting in mat rix protein cross linking is associated with increased wound healing in a rat mo del of colitis, we hypothesised that different types of transglutaminase might a lso play a role in UC. Patients end methods: Endoscopic and histological indices were studied in 26 patients with UC (10 active and 16 inactive) and in 20 norma l controls undergoing colonoscopy. Transglutaminase activity was evaluated in pl asma (factor XIIIa) by a radioenzymatic metho- d. Factor XIIIa, tissue and keratinocyte transglutaminase protein content, and mRNA expression in the colon were evaluated by western blot analysis and semiqu antitative reverse transcription-polymerase chain reaction (RT-PCR),respectively. Colon ic location of transglutaminases and their reaction products, the -(γ-glutamy l)lysine bonds, was evaluated by immunohistochemistry using specific monoclonal antibodies.Results: Transglutaminase activity was significantly lower in the pla sma of patients with active UC (4.2 (2.4) mU/ml;p< 0.05 v controls) than in thos e with inactive UC and controls(10.6 (2.2) and 12.1 (1.7) mU/ml). As shown by we stern blot,protein levels of tissue transglutaminase and factor XIIIa were uncha nged in active UC compared with inactive disease and controls, while the keratin ocyte form was reduced in active UC. Tissue transglutaminase and factor XIIIa im munostaining was strongly present in damaged areas colocalising with isopeptide bonds. In contrast, the keratinocyte form was almost absent in active UC and loc alised in the upper part of the crypts in normal subjects. RT-PCR showed upregu lation of tissue transglutaminase mRNA in active UC (320%compared with controls ) while keratinocyte transglutaminase gene expression was downregulated in activ e UC. Conclusions: The results of the present study support the concept that, in the damaged colon,transglutaminases are needed in response to chronic injury an d underline the key role of these enzymes in mucosal healing. Background and aims: As characterized by refractory in flammatory ulceration and damage to the colon.The mechanisms underlying impaired healing have yet to be defined. As transglutaminase expression resulting in mat rix protein cross linking is associated with increased wound healing in a rat mo del of colitis, we hypothesised that different types of transglutaminase might a lso play a role in UC. Patients end methods: Endoscopic and histological indices were studied in 26 patients with UC (10 active and 16 inactive) and in 20 norma l controls undergoing colonoscopy. Transglutaminase activity was evaluated in pl asma (factor XIIIa) by a radioenzymatic metho- d. Factor XIIIa, tissue and keratinocyte transglutaminase protein content, and mRNA expression in the colon were evaluated by western blot analysis and semiquantitative reverse transcription- polymerase chain reaction (RT-PCR), respectively. Colon ic location of transglutaminases and their reaction products, the - γ-glutamy l lysine bonds, was evaluated by immunohistochemistry using specific monoclonal antibodies. Results: Transglutaminase activity was significantly lower in the pla sma of patients with active UC (4.2 (2.4) mU / ml; p <0.05 v controls ) than in thos e with inactive UC and controls (10.6 (2.2) and 12.1 (1.7) mU / ml). As shown by we stern blot, protein levels of tissue transglutaminase and factor XIIIa were uncha nged in active UC compared with inactive disease and controls, while the keratin ocyte form was reduced in active UC. Tissue transglutaminase and factor XIIIa im munostaining was strongly present in damaged areas colocalising with isopeptide bonds. In contrast, the keratinocyte form was almost absent in active UC and localised in the upper part of the crypts in normal subjects. RT-PCR showed upregu lation of tissue translutlutase mRNA in active UC (320% compared with controls) while keratinocyte transglutaminase gene expression was downregulated in activ e UC. Conclusio ns: The results of the present study that the that that that that in the damaged colon, transglutaminases are needed in response to chronic injury an d under the the key role of these enzymes in mucosal healing.