Mechanism of Biological Nitrogen Fixation

Recently Phelps and Wilson (1) reported the occurrence of hydrogenase, the enzyme which activates molecular hydrogen, in nitrogen-fixing bacteria. Because HZ specifically inhibits nitrogen fixation by red clover plants inoculated with Rhizobium trijolii as well as by Azotobacter (2,3), possession of hydrogenase by these organisms may be more than merely fortuitous. The possibility must be considered that hydrogenase is associated with nitrogenase, the enzyme concerned in the first step of the fixation reaction. Evidence which supports the view of a relationship was their observation that cultures of the root nodule bacteria taken directly from nodules of the pea plant fixing nitrogen contained hydrogenase but not those grown on laboratory media (on which they are unable to fix Nz). This significant finding, however, could not always be confirmed with cultures from the nodules, apparently because the factors which affect hydrogenase activity in nitrogen-fixing bacteria were not known. In the preliminary investigations we followed the technique described by Stephenson and her associates (4) for studies of hydrogenase in Escherichia coli and related species, but this may not be satisfactory, since the metabolism of the nitrogenfixing bacteria is quite different from that of the colon group. We have therefore determined the chief properties of hydrogenase as it occurs in Axotobacter in order to develop a method which will insure consistent results in the further study of its possible relationship to nitrogen fixation.