Regulation of transglutaminases by nitric oxide.

Nitric oxide (NO) is an inorganic diffusible molecular messenger that plays several central roles in pathophysiology. NO can affect the biological activity of proteins through the direct or indirect (via intermediate S-nitrosothiols) S-nitrosylation of protein thiol groups. Transglutaminases (TGases), Ca(++)-dependent enzymes that modify proteins by cross-linking reactions, require a cysteine residue in the active site as a prerequisite for their catalytic activity. Therefore, NO may regulate enzymatic activity of TGases and their biological effects, via S-nitrosylation of their crucial thiol groups. We here review the effects of NO on coagulation factor XIII (fXIII, or plasma TGase) and TGase 2 (or tissue transglutaminase). NO has an inhibitory effect on fXIII, thus suppressing the gamma-chain cross-linking in fibrin gels, and subsequent clot formation. Tissue transglutaminase, an apoptotic effector molecule, also represents a molecular target for NO. Accordingly, the inhibition of tissue transglutaminase enzymatic activity by NO is able to prevent the induction of apoptosis.