Nitric oxide and the respiratory enzyme.

Available information on the molecular mechanisms by which nitric oxide (NO) controls the activity of the respiratory enzyme (cytochrome-coxidase) is reviewed. We report that, depending on absolute electron flux, NO at physiological concentrations reversibly inhibits cytochrome-coxidase by two alternative reaction pathways, yielding either a nitrosyl- or a nitrite-heme a3 derivative. We address a number of hypotheses, envisaging physiological and/or pathological effects of the reactions between NO and cytochrome-c-oxidase.