SUMMARY: The effects of purified oxyleghaemoglobin added to suspensions of bacteroids prepared anaerobically from soybean root nodules, were studied in terms of uptake of dissolved O nitrogenase activity and dissolved O concentration, in the absence of a gas phase. Leghaemoglobin allowed maximum rates of O uptake to continue to a much lower range of concentrations of free dissolved O than in the absence of the protein. This effect was diminished when the leghaemoglobin concentration was less than about 50 μM. Nitrogenase activity at a given O concentration was not increased by raising the leghaemoglobin concentration above about 50 μM. The fractional oxygenation of leghaemoglobin giving half the maximal O consumption rate by bacteroids was usually about 0.2; with limiting leghaemoglobin concentrations it was higher. Rates of nitrogenase activity were invariably greater during periods when the discharge of O from oxyleghaemoglobin was occurring at the maximum rate, than when similar maximum O uptake rates were being supported by higher concentrations of free dissolved O When myoglobin was used as the O carrier protein in similar experiments, enhanced nitrogenase activity also accompanied the maximum rate of discharge from the carrier. This occurred at about four times the concentration of free dissolved O at which it occurred when leghaemoglobin was the carrier. These results are discussed in relation to current theories about the mechanism of leghaemoglobin action.


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