Abstract
An almost stoichiometric conversion of nitrite to nitrous oxide was observed during the nitrite reduction by Paracoccus denitrificans cells in a medium of pH 6.4. The N 2O accumulated in the reaction medium and was decomposed only after nitrite had been consumed; when the pH of the medium was higher than 7.3–7.4, nitrous oxide did not accumulate. The activity of N 2O reductase was, in the whole range of pH 6.4–9.2, higher than the activity of NO − 2 reductase, both activities showing the maximum at the pH higher than 8.0. Using an artificial donor, TMPD plus ascorbate, the maximum activity of NO − 2 reductase, but not N 2O reductase was shifted by about two pH units to acidic region. The activity of nitrite reductase declined in the presence of N 2O only at higher pH values. Cytochrome c, as a common electron donor for both N 2O and NO − 2 reductase, was more oxidized at pH < 7.3 in the presence of NO − 2 than in the presence of N 2O, the opposite being true at pH > 7.3. The increased flux of electrons to cytochrome c has for a constant pH value (6.4) no effect on their distribution over NO − 2 and N 2O. The results indicate that the distribution of electrons in the terminal part is determined by the different pH optima for NO − 2 reductase and N 2O reductase, and by a mutual dependence of activities of the two reductases due to the competition for redox equivalents from a substrate.
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