On the mechanism and biology of cytochrome oxidase inhibition by nitric oxide

doi:10.1073/pnas.0405368101

On the mechanism and biology of cytochrome oxidase inhibition by nitric oxide

^*Grupo de Bioquímica dos Oxidantes e Antioxidantes, Centro de Química e Bioquímica, da Faculdade de Ciências da Universidade de Lisboa, P-1749-016 Lisbon, Portugal; ^†Instituto de Investigação Científica Bento da Rocha Cabral, P-1250-047 Lisbon, Portugal; ^§Laboratory of Free Radical Biology, School of Pharmacy and Biochemistry, University of Buenos Aires, C1113AAD Buenos Aires, Argentina; and ^¶Department of Molecular Pharmacology and Toxicology, School of Pharmacy, University of Southern California, Los Angeles, CA 90033

Edited by Louis J. Ignarro, University of California School of Medicine, Los Angeles, CA, and approved September 28, 2004 (received for review July 24, 2004)

Abstract

The detailed molecular mechanism for the reversible inhibition of mitochondrial respiration by NO has puzzled investigators: The rate constants for the binding of NO and O₂ to the reduced binuclear center Cu_B/a₃ of cytochrome oxidase (COX) are similar, and NO is able to dissociate slowly from this center whereas O₂ is kinetically trapped, which altogether seems to favor the complex of COX with O₂ over the complex of COX with NO. Paradoxically, the inhibition of COX by NO is observed at high ratios of O₂ to NO (in the 40–500 range) and is very fast (seconds or faster). In this work, we used simple mathematical models to investigate this paradox and other important biological questions concerning the inhibition of COX by NO. The results showed that all known features of the inhibition of COX by NO can be accounted for by a direct competition between NO and O₂ for the reduced binuclear center Cu_B/a₃ of COX. Besides conciliating apparently contradictory data, this work provided an explanation for the so-called excess capacity of COX by showing that the COX activity found in tissues actually is optimized to avoid an excessive inhibition of mitochondrial respiration by NO, allowing a moderate, but not excessive, overlap between the roles of NO in COX inhibition and in cellular signaling. In pathological situations such as COX-deficiency diseases and chronic inflammation, an excessive inhibition of the mitochondrial respiration is predicted.

Footnotes

↵ ‡ To whom correspondence should be sent at the * address. E-mail: fantunes{at}fc.ul.pt.
Author contributions: F.A., A.B., and E.C. designed research, performed research, analyzed data, and wrote the paper.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviation: COX, cytochrome oxidase.