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