Studies of rates of consumption of dissolved O by suspensions of bacteroids (, strain CB1809) from soybean root nodules showed the presence of two different terminal oxidase systems. A high-affinity system, sensitive to inhibition by -phenylimidazole and by carbon monoxide, was most active when the dissolved O was between 0.01 and 0.1 μM. At 1 μM-0 or higher, this oxidase system had little activity and 0 was consumed largely by a low-affinity system insensitive to these inhibitors. At low concentrations of dissolved 0, bacteroid respiration rates appeared to be diffusion-limited. When purified oxyleghaemo-globin was added to such systems, this restriction was relieved and respiration was maintained to much lower concentrations of free dissolved 0 here nitrogenase activity was greatest.

Analysis of reactions which were terminated at various stages during the depletion of 0 from oxyleghaemoglobin showed that at low free 0 concentration, the high-affinity pathway produced up to five times greater bacteroid ATP concentrations than the low-affinity oxidase pathway operating about 1 μM free 0 in the absence of leghaemoglobin. At intermediate free 0 concentrations, ocfnring during the later stages of deoxygenation of oxymyoglobin, intermediate concentrations of ATP were found in the bacteroids.


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