SUPEROXIDE DISMUTASES: STRUCTURE, FUNCTION, EVOLUTION

Superoxide dismutases are enzymes which catalyze the decomposition of superoxide, a cytotoxic free radical that is produced in normal oxidative metabolism and also implicated in the etiology of certain inflammatory diseases. Three different metalloenzyme forms are known, dismutases containing manganase, or iron, or both copper and zinc as metal cofactors. As a superoxide scavenger, Cu-Zn dismutases are used to treat veterinary inflammations and show promise in clinical trials in humans for managing rheumatoid arthritis, osteoarthritis, and the side effects of radiation therapy. Our long-term objectives are to identify the chemical basis of dismutase structure and catalysis, and to characterize the phylogenetic distribution and evolutionary relationships among the three forms. These data are prerequisities for any therapeutic uses of dismutases to be successful. Our objectives will be pursued on both the protein level, through determining the amino acid sequences of dismutases representing the three metalloenzyme forms, and on the DNA level, through isolating and sequencing the gene for a bacteriocuprein form of dismutase, and using it to study the distribution of bacteriocuprein in other bacteria and in primitive eukaryotes. Bacteriocupreins are Cu-Zn dismutases found in bacteria; all other Cu-Zn dismutases are found in eukaryotes. Bacteriocupreins have been found in only two species, Photobacterium leiognathi in 1974 (A.M. Michelson, Paris), and Caulobacter crescentus in 1981 (in my laboratory). Our specific protein structure aims are: (a) to complete the ongoing amino acid sequence analyses of the Mn dismutase of chicken liver mitochondria and of the Photobacterium bacteriocuprein, (b) to perform partial sequence analyses on the Mn dismutase from chimpanzee liver, the Fe dismutase from E. coli, and the Cu-Zn dismutase of the fish, Leiognathus nuchalis, in which the P. leiognathi lives as a symbiont. Our specific aims in characterizing dismutase genes are: (a) to isolate the gene for bacteriocuprein from C. crescentus and to determine its sequence, and (b) to use it as a hybridization probe to examine bacteria which lack expressed bacteriocuprein activity: certain strains of P. leiognathi, enteric bacteria related to it, bacteria related to Caulobacter, and Paracoccus denitrificans, for which protein studies have given equivocal results about the presence of a bacteriocuprein.