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.

Animals↗

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Superoxide, hydrogen peroxide, and oxygen tolerance of oxygen-sensitive mutants of Escherichia coli.

Oxygen-intolerant mutants of Escherichia coli K12 were selected by a replica plating technique after treatment with the mutagen, N-methyl-N'-nitro-N-nitrosoguanidine, to a lethality of 99.5%. One group of mutants had lost the ability to induce both peroxidase and catalase when exposed to oxygen but retained the ability to induce the manganese-superoxide dismutase. The second group of mutants had lost the ability to induce the activity of all these enzymes. Failure to induce peroxidase and catalase was associated with enhanced susceptibility of the bacteria to the lethal effect of oxygen. When a member of the first group of mutants was prevented from producing the manganese-superoxide dismutase by the presence of puromycin, its susceptibility to the lethal effects of oxygen was greatly increased. Two types of revertants were seen. In one group the ability to induce enzyme activity was recovered and was accompanied by the return of oxygen tolerance. Members of the other group lost the ability to respire and, therefore, no longer produced O2- AND H2O2. These results indicated that enzymic scavenging of both H2O2 and O2- provides an important defense against oxygen toxicity. The parallel loss of peroxidase and catalase, which was seen in all mutants, suggests that these enzymes constitute a precursor-product pair in E. coli. The parallel loss in two of these mutants of peroxidase, catalase, and the manganese-superoxide dismutase suggests a control linkage for these enzymes, the basis of which remains to be explored.

Escherichia coli↗

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