Abstract
Four species of denitrifying bacteria were shown at pH 7.1 to catalyze nitrosation Reactions a and b, which are normally observed under acidic conditions,at rates comparable to those observed €or denitrification. The production in Reaction a of 14N'6N'80 but not 15N'4N'80 in 7 atom 5% HZ"O indicates that *'O exchange into nitrite occurred prior to nitrosyl transfer.The finding that theyield of 1sN14N'g0equaled the yield of 14N15N'60 plus 14N15N18i0n Reaction requires that 14*1SNzf0ormation proceeds through an effectively symmetrical intermediate. The appearance of '*O in the nitritepool and producNt zOwas followed for denitrification (yielding I6Nz0) and for Reaction a.With certain bacteria, I80 enrichments in N20 were much greater than those in nitrite. This indicates that nitrite, once having un- dergone '*Oexchange at an enzyme active site, is com- mitted to reactand seldom dissociates. In other cases, the l80enrichments of nitrite and N20 were similar throughout to suggest that dissociation can compete with reaction. In systems simultaneously catalyzing Reaction a and denitrification, the "0 enrichments of 14,16N20and I6N20 were similar, as required if both nitrosation and denitrification proceeded from a common enzyme-bound intermediate. The results are largely consistent with the involvement of a ferrous heme-nitrosyl compound, derived from nitriteand a heme of the dissimilatory nitrite reductase, as a common intermediate in denitrification, nitrosation, and"0exchange. Denitrification schemes are discussed and one involving mononitrogen intermediates through thenitroxyl (N'+) state is favored.