Superoxide dismutases: defence against endogenous superoxide radical

Attempts to measure the rate of O2- production, in whole cells or in intact subcellular organelles, are frustrated by the endogenous superoxide dismutase (SOD). Streptococcus faecalis contains a single manganese-SOD which was isolated and used as an antigen in the rabbit. A precipitating and inhibiting antibody was obtained and used to suppress the SOD in crude lysates of S. faecalis. It allowed the demonstration that 17% of the total oxygen uptake by such lysates, in the presence of NADH, was associated with O2- production. O2- attacks unsaturated lipids and breaches the integrity of membranes. When the membranes are free of lipid hydroperoxides, then both O2- and H2O2 are required and singlet oxygen appears to be the proximal attacking species. When the membrane contains some lipid hydroperoxide, then O2- is itself sufficient and seems to generate an alkoxy radical, by reacting with the lipid hydroperoxide. It appears likely that attack on membranes is one of the reasons for the cytotoxicity of O2-. In Escherichia coli the manganese-SOD is derepressed by O2-. This enzyme is not made in the absence of oxygen and in aerobic conditions any change which results in enhanced production of O2- calls forth an increased synthesis of this enzyme. Increased levels of SOD, however achieved, correlate with greater resistance towards oxygen toxicity. It is generally true that respiring cells contain more SOD than non-respiring cells. Among obligate anaerobes there is a correlation between SOD-content and tolerance towards oxygen. It is not known whether the SOD in obligate anaerobes is a retained primitive characteristic or one recently acquired by plasmid transfer. There is an exception to the rule that copper-zinc-SOD is found in eukaryotes but not in prokaryotes, and that is the symbiotic bacterium Photobacterium leiognathi. This symbiont may have obtained the Cu-ZnSOD gene from the host fish.