Superoxide dismutase analyses of Azotobacter vinelandii and other aerobic, free-living nitrogen-fixing bacteria
The extraordinarily high respiratory activity of Azotobacter Vinelandii presents this organism with the potential for accumulating toxic intermediates of oxygen-reduction. Additionally, the nitrogenase complex in these aerobic, free- living, molecular nitrogen-fixers may be susceptible to specific intermediates of oxygen-reduction, such as the superoxide free radical. Superoxide dismutase (SOD) has been proposed to be a defense against the accumulation of toxic intermediates of oxygen-reduction. SOD activity and metallotypes were analyzed in cell-free extracts of A. Vinelandii strain O, grown under different physiological conditions. Additionally, selected members of the family of free-living, aerobic, nitrogen-fixing bacteria Azotobacteraceae were examined for SOD. Late-log phase cultures of A. Vinelandii grown under nitrogen-fixing conditions with either glucose or acetate as sole energy source, possessed very high SOD specific activities with a characteristic, single, iron-containing SOD (Fe-SOD) activity band detected by polyacrylamide gel electrophoresis and activity staining. A possible, second, mangan-ese-containing SOD (Mn-SOD) activity band could sporadically be detected, but only in trace amounts. The Mn-SOD activity was more prominent during the stationary phase of growth of A. Vinelandii strain O and when A. Vinelandii strain O was cultured under non-nitrogen-fixing conditions. SOD specific activities were markedly reduced in A. Vinelandii strain 0 grown under non-nitrogen-fixing conditions. An A. Vinelandii mutant, strain 3, which is not capable of fixing molecular nitrogen, possessed SOD specific activities comparable to A. Vinelandii strain 0 grown under non-nitrogen-fixing conditions. Also, the Azotobacter mutant contained the same electrophoretic SOD activity bands seen in A. Vinelandii strain 0 grown under non-nitrogen-fixing conditions. A survey of Azotobacteraceae for SOD showed generally high specfiic activities for the organisms examined, although there was a broad range of SOD activity noted. There was also variability within the family regarding the types of SODs present and the relative mobilities of the SOD activity bands on polyacrylamide gels. The differences in SOD observed for the family Azotobacteraceae correlate well with recent genetic studies of members of this family.