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Cardiovascular Research 1999 43(3):521-531; doi:10.1016/S0008-6363(99)00115-7
© 1999 by European Society of Cardiology
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Copyright © 1999, European Society of Cardiology

Enzymatic function of nitric oxide synthases

Penelope J Andrew and Bernd Mayer*

Institut für Pharmakologie und Toxikologie, Karl-Franzens-Universität Graz, Universitätsplatz 2, Graz A-8010, Austria

* Corresponding author. Tel.: +43-316-3805567; fax: +43-316-3809890 mayer{at}kfunigraz.ac.at

Nitric oxide (NO) is synthesised from L-arginine by the enzyme NO synthase (NOS). The complex reaction involves the transfer of electrons from NADPH, via the flavins FAD and FMN in the carboxy-terminal reductase domain, to the haem in the amino-terminal oxygenase domain, where the substrate L-arginine is oxidised to L-citrulline and NO. The haem is essential for dimerisation as well as NO production. The pteridine tetrahydrobiopterin (BH4) is a key feature of NOS, affecting dimerisation and electron transfer, although its full role in catalysis remains to be determined. NOS can also catalyse superoxide anion production, depending on substrate and cofactor availability. There are three main isoforms of the enzyme, named neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS), which differ in their dependence on Ca2+, as well as in their expression and activities. These unique features give rise to the distinct subcellular localisations and mechanistic features which are responsible for the physiological and pathophysiological roles of each isoform.

KEYWORDS Nitric oxide; Free radicals; Endothelial function; Endothelial factors; Vasoconstriction/dilation


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