| Abstrakt: |
For Azospirillum brasilense Sp7, the energy transformation efficiencies were measured in anaerobic respirations with either nitrate, nitrite or nitrous oxide as respiratory electron acceptors by determining the maximal molar growth yields and the H-translocations using the oxidant pulse method. In continuous cultures grown with malate limiting, the maximal molar growth yields (Y-values) were essentially the same with O or NO but were 1/3 and 2/3 lower with NOor NO, respectively, as respiratory electron acceptors. Both the maximal molar growth yields and the maintenance energy coefficients were surprisingly high when Azospirillum was grown with nitrite as the sole electron acceptor and source for N-assimilation. Growth under N-fixing conditions drastically reduced the Y-values in the NO and O-respiring cells. In the H-translocation measurements, the $$\vec H^ + $$ /oxidant ratios were 5.6 for O→HO, 2.5-2.8 for NO→NO, 2.2 for NO→NO and 3.1 for NO→N respirations when the cells were preincubated with valinomycin and K. All the values were enhanced when the experiments were performed with valinomycin plus methyltriphenylphosphonium (=TPMP) cation. The uncoupler carbonyl cyanide-m-chlorophenyl-hydrazone diminished the H-excretion indicating that this translocation was due to vectorial flow across the membrane. In the absence of any ionophore, nitrate and nitrite respirations were accompanied by a H-uptake $$(NO_3^ - \to N_2 = - 2.9 \vec H^ + /NO_3^ - and NO_2^ - \to N_2 = - 2.5 \vec H^ + /NO_2^ - )$$ . Any significant H-translocation could not be detected in NO- and O-respirations under these conditions. It is concluded that nitrate reduction proceeds inside the cytoplasmic membrane, whereas nitrite is reduced extramembraneously. The data are not conclusive for the location of nitrous oxide reductase. The maximal molar growth yield determinations and the absence of any H-uptake in untreated cells indicate a cytoplasmic orientation of the enzyme similar to the terminal cytochrome oxidase of respiration. The low H-extrusion values for NO-respiration compared to O-respiration in cells treated with valinomycin plus TPMP are, however, not in accord with such an interpretation. [ABSTRACT FROM AUTHOR] |
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