Positional isotopic equivalence of nitrogen in N2O produced by the denitrifying bacterium Pseudomonas stutzeri. Indirect evidence for a nitroxyl pathway.

During the concomitant reduction of [15N]nitrite and 14NO, the denitrifying bacterium Pseudomonas stutzeri produced considerable amounts of isotopically mixed N2O (14,15N2O) but did not isotopically intermix the nitrite and NO pools (Garber, E. A. E., and Hollocher, T. C. (1981) J. Biol. Chem. 256, 5459-5465). It was determined that the mass spectrometric abundance of 14N15NO was about equal to that of 15N14NO in 14,15N2O by examination of the abundances of 14NO+, 15NO+, and 15N2+ that arose from the fragmentation of N2O+ species in the mass spectrometer. This positional isotopic equivalence requires that N2O arose from [15N]nitrite and 14NO by a process in which loss of oxygen occurred with equal probability from 15N and 14N precursors and suggests that at some point the precursors were identical if monomeric or effectively symmetrical if dimeric. One pathway, which is consistent with the available data and for which there is chemical precedence, is the reduction of nitrite and NO to nitroxyl (NOH or HNO), dimerization of free nitroxyl to a dinitrogen intermediate of short half-life, and dehydration of this intermediate to form N2O.