| Popis: |
As shown previously, higly purified hydroxylamine oxidoreductase (hydroxylamine:oxygen oxidoreductase, EC 1.7.3.4) from Nitrosomonas catalyzes the aerobic oxidation of NH2OH to a mixture of NO2− and NO3− in the presence of phenazine methosulfate. The present work shows that N2O and NO are also products of the oxidation of NH2OH. A nitrite reductase, present in less purified samples of the hydroxylamine oxidoreductase is shown to catalyze the reduction of NO2− to a mixture of NO and N2O with leucopyocyanine as electron donor. The possible reduction, by purified hydroxylamine oxidoreductase, of NO2− or NO3− to form NO (with NH2OH-reduced enzyme or phenazine methosulfate as potential electron donors) was eliminated; 15NO was shown not to be produced in the presence of 15NO2− and 15NO3−. NO thus appears to be a product of NH2OH oxidation by purified hydroxylamine oxidoreductase and is thus a possible intermediate in the production of nitrite. Mn2+ in the concentration range 10−6–10−5 M progessively and completely inhibits the formation of nitrite and nitrate by purified hydroxylamine oxidoreductase while concomitantly stimulating the rate of dehydrogenation of hydroxylamine three-fold. The presence of Mn(II) results in decreased formation of NO and increased formation of N2O. The present results are consistent with a mechanism of NH2OH oxidation in which the oxidation of an intermediate compound of the oxidation state of (HNO) occurs by dehydrogenation rather than direct addition of O. The dehydrogenation step is inhibited in the presence of Mn(II). |
