Selective inhibition of nitrogen fixation in Clostridium pasteurianum.

Hydrogen is a selective inhibitor for nitrogen fixation, the effect in aerobes being one of competition with nitrogen (Wilson, 1951; Rosenblum and Wilson, 1950; Pratt and Frenkel, 1959). Also, carbon monoxide and nitrous oxide are selective fixation inhibitors (Hino, 1955) in Clostridium pasteurianum. The present work was undertaken to provide additional examples of selective inhibitors for nitrogen fixation with the purpose of using them in research to identify substances involved in the fixation mechanism. Mediators and intermediates of nitrogen fixation should be recognizable through their ability to counteract effects of competitive inhibitors. The approach consisted of comparing effects of organic compounds in nitrogen-fixing and ammonia-assimilating cultures of C. pasteurianum strain W-5, followed by tests with supplements to offset inhibitory effects specific to the nitrogenfixing cultures alone. Out of these experiments have come several inhibitors counteracted by biotin and one counteracted by sodium molybdate. None has so far been found requiring iron as counteractant. The goal of finding inhibitors requiring an unknown factor from natural extracts as the counteractant has yet to be achieved.