CRYSTALLIZATION OF SUPEROXIDE DISMUTASE FROM A HALOPHILIC ORGANISM. Terence P. Lo, Ilona L. Canestrelli, John A. Tainer and Elizabeth D. Getzoff. Department of Molecular Biology, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California, 92037, USA

Superoxide dismutase from the phototrophic purple sulfur bacterium Ectothiorhodospira halophila has been isolated, purified and crystallized. The importance of this enzyme lies in its ability to scavenge toxic superoxide free radicals. Although these free radicals are a normal byproduct of metabolism, they are responsible for the breakdown of biological macromolecules and thus contribute to such conditions as aging and cancer. In humans, Cu,Zn superoxide dismutase has been implicated in the neurodegenerative disease Familial Amyotrophic Lateral Sclerosis, and so has been the subject of intense scrutiny.

Ectothiorhodospira halophila is a halophilic bacterium found in sunny, hypersaline lakes, and is able to survive in solutions having a salt concentration approaching 4 M. Ectothiorhodospira halophila is also slightly thermophilic, having optimum growth at 42 degrees Celsius. The enzyme was purified directly from the bacterium by assaying for superoxide dismutase activity. While superoxide dismutases from a variety of sources have been found to be highly thermostable, the enzyme from Ectothiorhodospira halophila is extremely thermostable, retaining activity even after boiling. In addition, this superoxide dismutase is highly resistant to cleavage by proteases.

Superoxide dismutase from Ectothiorhodospira halophila has been crystallized in the orthorhombic space group P2₁2₁2₁ with unit cell dimensions a=58, b=74, c=105. The search for a molecular replacement solution is underway.