15N tracer studies on the role of NO in denitrification.

P. stutzeri was anomalous.While it conformed to Scheme 1 in that isotope was not exchanged between nitrite and NO, nor transferred from one to the other unidirectionally, it nevertheless exhibited extensive scrambling of 14N and 15N in product N20.It is possible in P. stutzeri that the reductive pathways of nitrogen from nitrite and NO share in common a free mononitrogen intermediate which is not NO.These studies rule out the involvement of free NO as an obligatory intermediate in the denitrification of nitrite among the five bacteria examined to date.The results can be rationalized in part if enzyme-bound NO were to arise from nitrite and partition variously between release as free NO and further reduction to NzO.Acid-and NO2-catalyzed chemical exchange reactions between nitrite and NO were used to measure the rate of equilibration of NO between gas and aqueous phases and to assure that phase equilibration was rapid relative to denitrification.The mechanism of the NOzcatalyzed exchange reaction probably involves nucleophilic displacement of nitrite on N203 which is formed from NO and NO2.An 13N experiment carried out with