Nitrite reductase (cd(1)NIR) from Pseudomonas aeruginosa, which catalyses the reduction of nitrite to nitric oxide (NO), contains a c-heme as the electron acceptor and a d(1)-heme where catalysis occurs. Reduction involves binding of nitrite to the reduced d(1)-heme, followed by dehydration to yield NO; release of NO and re-reduction of the enzyme close the cycle. Since NO is a powerful inhibitor of ferrous hemeproteins, enzymatic turnover demands the release of NO. We recently discovered that NO dissociation from the ferrous d(1)-heme is fast, showing that cd(1)NIR behaves differently from other hemeproteins. Here we demonstrate for the first time that the physiological substrate nitrite displaces NO from the ferrous enzyme, which enters a new catalytic cycle; this reaction depends on the conserved His369 whose role in substrate stabilization is crucial for catalysis. Thus we suggest that also in vivo the activity of cd1NIR is controlled by nitrite. (C) 2007 Elsevier Inc. All rights reserved.