Gah (transglutaminase type II; tissue transglutami-nase) is a bifunctional enzyme with transglutaminase (TGase) and guanosine triphosphatase (GTPase) ac-tivities. The GTPase function of Gah is involved in hormonal signaling and cell growth while the TGase function plays an important role in apoptosis and in cross-linking extracellular and intracellular proteins. To analyze the regulation of these dual enzymatic ac-tivities we examined their calcium-dependence and thermal stability in enzymes from several cardiac sources (mouse heart, and normal, ischemic and di-lated cardiomyopathic human hearts). The GTP bind-ing activity of Gah was markedly inhibited by Ca2+ whereas the TGase activity was strongly stimulated, suggesting that Ca2+ acts as a regulator, switching Gah from a GTPase to a TGase. The TGase function of Gah of both mouse and human hearts was more thermo-stable in the presence of Ca2+.

Keywords: a1-Adrenoreceptor, Ca2+, G-Protein, Heart, Transgluta-minase II.