| Abstract | The role of uptake hydrogenase was studied in Rhizobium leguminosarum bacteroids from the nodules of Pisum sativum L. cv. Homesteader. Uptake hydrogenase activity, measured by the ³H₂ uptake method, was dependent on O-consumption and was similar to H₂ uptake measured by gas chromatography. Kₘ for O₂ of 0.0007 atm (0.0709 kPa) and a Kₘ for H₂ of 0.0074 atm (0.7498, kPa) were determined. H₂ increased the rate of endogenous respiration by isolates with uptake hydrogenase (Hup⁺) but had no effect on an isolate lacking uptake hydrogenase (Hup⁻). A survey of 14 Hup⁺ isolates indicated a wide range of H₂ uptake activities. Four of the isolates tested had activities similar to or higher than those found in two Hup⁺ Rhizobium japonicum strains. H₂ uptake was strongly coupled to ATP formation in only 5 of the 14 isolates. H₂ increased the optimal O₂ level of C₂H₂ reduction by 0.01 atm and permitted enhanced C₂H₂ reduction at O₂ levels above the optimum in both a coupled and an uncoupled isolate. At suboptimal O₂ concentrations a small enhancement of C₂H₂ reduction by H₂ was seen in two out of three isolates in which H₂ oxidation was coupled to ATP formation. Thus, the main function of uptake hydrogenase in R. leguminosarum appears to be in the protection of nitrogenase from O₂ damage. |
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