Elsevier

Nitric Oxide

Volume 14, Issue 2, March 2006, Pages 162-168
Nitric Oxide

Mitochondrial NO and reactive nitrogen species production: Does mtNOS exist?

https://doi.org/10.1016/j.niox.2005.05.011Get rights and content

Abstract

It is more than 10 years now that mitochondria are suspected to be sources of nitric oxide (NO). This hypothesis is intriguing since NO has multiple targets within the organelle and it is even suggested that mitochondria are the primary targets of NO in the cell. Most remarkably, nanomolar concentrations of NO can inhibit mitochondrial respiration, so even a small amount of NO in the mitochondrial matrix may regulate ATP synthesis. Therefore, the idea that mitochondria themselves are capable of NO production is an important concept in several physiological and pathological mechanisms. However, this field of research generates surprisingly few original papers and the published studies contain conflicting results. The reliability of the results is frequently questioned since they are seldom reproduced by independent investigators. Until 2003, all papers published in this field showed affirmative results but since then several studies directly challenged the existence of a mitochondrial nitric oxide synthase. The present review aims to summarize the most recent developments in mitochondrial NO production, highlights a few unsolved questions, and proposes new directions for future work in this research area.

Section snippets

The arginine-to-citrulline conversion pathway

Nitric oxide is mainly formed by the conversion of arginine to citrulline in the cell. This reaction is catalyzed by NOS enzymes, which need HEME, calcium–calmodulin, and flavins as obligatory cofactors. There are three known genes in the mammalian genome, which encode NOS enzymes. Endothelial NOS (eNOS) is mainly present in endothelial cells and it is responsible for the maintenance of vascular tone. The neuronal NOS (nNOS) is also a constitutive enzyme, its three main isoforms can be found in

What is the level of mitochondrial NO synthesis?

Cellular NO synthesis is a well-characterized mechanism, and several methodologies are available to investigate it. However, these methods such as the arginine-to-citrulline conversion assay are optimized for cytoplasmic extracts and not for the very different ionic concentrations, pH, and substrate and cofactor availability in the mitochondrial matrix. Furthermore, mitochondrial preparations always contain a small percentage (1–4%) of contamination, which may contain NOS from

Concluding remarks

About 10 years after the first observation of mitochondrial NO production, the existence of mtNOS is still questionable. This fact alone cautions us that if all these efforts were unable to convincingly prove an idea, and the later works do not show much progress compared to the earlier findings, mitochondrial NO synthesis is probably just a technical peculiarity without much physiological relevance.

However, during the search for mtNOS, new aspects emerged regarding the complex interaction of

Acknowledgments

This study was supported by grants from the American Heart Association (Mid-Atlantic Grant 99512724, Bugher Foundation Award 0270114N) and the NIH (HL30260, HL46558, HL50587, HL77731, and HD38964); the Hungarian OTKA (D-45933, T-049621, T-029169, T-037885, and T-037386), ETT (248/2003, 249/2003), TET (A4/2004) and GVOP (TST 0002/2004).

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