N_2O Reduction by Azotobacter vinelandii with Emphasis on Kinetic itrogen Isotope Effects

概要

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A nitrogen-fixing bacterium Azotobacter vinelandii was successfully grown in a specially designed system with constant partial pressures of N_2O (0.2 atm) and O_2 (0.2 atm) in a nitrogen-free liquid medium. Reduction of N_2O proceeded with the evolution of N_2 in the gas phage. Large nitrogen isotope fractionation was found for both processes, reduction of N_2O to N_2 and N_2O-fixation. The kinetic isotope fractionation factors of these reactions were at most 1.039 and 1.034, respectively. Furthermore, an unexpected inverse isotope effect (organic-N, the end-product, is more enriched in ^<15>N than N_2, the intermediate) strongly suggested that N_2O was directly assimilated within the bacterial cells. Simultaneous assimilation of N_2O and N_2 was also confirmed by using a ^<15>N tracer technique. Three independent pathways were demonstrated for the nitrogen fixing system investigated in this study: (1) a direct reduction of N_2O to ammonium (apparently 8-electron reduction), (2) reduction of N_2 to ammonium (6-electron reduction) and (3) N_2O reduction to N_2 (2-electron reduction).
日本植物生理学会の論文