Methanosarcina barkeri is a strictly anaerobic, cytochrome-containing, methane-forming archaeon. We report here that the microorganism contains a catalase, which was purified and characterized. The enzyme with an apparent molecular mass of 190 kDa was shown to be composed of four identical subunits of apparent molecular mass of 54 kDa. The heme-containing enzyme did not exhibit peroxidase activity, which indicates that it is a monofunctional catalase. This is substantiated by the primary structure, which is related to that of other monofunctional catalases rather than to that of bifunctional catalase-peroxidases. The enzyme showed an [S]0.5V for H2O2 of 25 mM and an apparent Vmax of 200,000 U/mg; it was inhibited by azide ([I]0.5V = 1 microM) and cyanide ([I]0.5V = 5 microM) and inactivated by 1,2,4-aminotriazole. The activity was almost independent of the pH (between pH 4 and 10) and the temperature (between 15 degrees C and 55 degrees C). Comparison of the primary structure of monofunctional catalases revealed that the enzyme from M. barkeri is most closely related to the monofunctional catalase of Dictyostelium discoideum.