They looked at the amount of nitrogen fixed with different concentrations in the atmosphere, in Azotobacter vinelandii and chroococcum cells. They were looking for KN2: the concentration of nitrogen at which the amount of nitrogen fixed is half the maximum.
With inert gases (argon, helium) or with a partial vacuum, the concentration of nitrogen had to get down below 0.15 atm before the fixation decreased much. It gets to 50% around 0.01 atm. But when there was hydrogen, the rate decreased more quickly; the more, the faster. So hydrogen seems to inhibit the nitrogenase, but only at very high concentrations (over 20% of the atmosphere). This inhibition is competitive and reversible.
Reference:
With inert gases (argon, helium) or with a partial vacuum, the concentration of nitrogen had to get down below 0.15 atm before the fixation decreased much. It gets to 50% around 0.01 atm. But when there was hydrogen, the rate decreased more quickly; the more, the faster. So hydrogen seems to inhibit the nitrogenase, but only at very high concentrations (over 20% of the atmosphere). This inhibition is competitive and reversible.
Reference:
Wyss, O., Lind, C. J., Wilson, J. B. & Wilson, P. W. Mechanism of biological nitrogen fixation VII. Molecular H2 and the pN2 function of Azotobacter. Biochem J 35, 845–854 (1941).
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