The activity of microbial transglutaminase (MTG) and the corresponding secondary structure, measured by circular dichroism (CID), was analyzed before and after treatment at different temperatures (40 and 80 degrees C) and pressures (0.1, 200, 400, 600 MPa). Irreversible enzyme inactivation was achieved after 2 min at 80 degrees C and 0.1 MPa. Enzyme inactivation at 0.1, 200, 400, and 600 MPa and 40 degrees C followed first-order kinetics. The enzyme showed residual activity of 50% after 12 min at 600 MPa and 40 degrees C. Mobility of aromatic side chains of the enzyme molecule was observed in all temperature- and/or pressure-treated samples; however, high-pressure treatment at 600 MPa induced a loss of tertiary structure and a significant decrease in the alpha-helix content. The relative content of beta-strand substructures was significantly increased after 30 min at 600 MPa and 40 degrees C or 2 min at 0.1 MPa and 80 degrees C. We conclude that the active center of MTG, which is located in an expanded 8-strand domain, is resistant to high hydrostatic pressure and pressure-induced inactivation is caused by destruction of cc-helix elements with a corresponding influence on the enzyme stability in solution.