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Abstract
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 O2, nitrogenase activity and dissolved O2 concentration, in the absence of a gas phase. Leghaemoglobin allowed maximum rates of O2 uptake to continue to a much lower range of concentrations of free dissolved O2 than in the absence of the protein. This effect was diminished when the leghaemoglobin concentration was less than about 50 μ m. Nitrogenase activity at a given O2 concentration was not increased by raising the leghaemoglobin concentration above about 50 μ m. The fractional oxygenation of leghaemoglobin giving half the maximal O2 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 O2 from oxyleghaemoglobin was occurring at the maximum rate, than when similar maximum O2 uptake rates were being supported by higher concentrations of free dissolved O2. When myoglobin was used as the O2 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 O2 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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