Superoxide dismutase from the extremely halophilic archaebacterium Halobacterium cutirubrum

Halobacterium cutirubrum, a member of the archaebacteria, contains one superoxide dismutase (EC 1.15.1.1). This enzyme functions in the high-ionic-strength intracellular environment and protects the organism against the toxic effects of the superoxide anion. The enzyme has been purified to about 90% homogeneity by a four-step procedure which never removes it from conditions of high ionic strength. The subunits of the purified enzyme have a molecular weight of 25,000 and are possibly in tetrameric association. The enzyme shows anomalously high resistance to azide inhibition and sensitivity to inactivation by hydrogen peroxide. Metal analysis indicates 0.2 atom of Mn, less than 0.03 atom of Cu, and less than 0.001 atom of Fe per subunit. The low content of Mn may explain the low specific activity found for this enzyme compared with that of eubacterial enzymes. Optimum activity occurs in 2 M KCl; KCl gives about twice as much activity as NaCl over the range of 2 to 4 M. The enzyme appears to be related to those isolated from other archaebacteria but also exhibits several novel features.