Title: Reactive nitrogen species
Key words: nitric oxide, nitrogen dioxide, free radicals, antioxidant, arginine, atherosclerosis, haemoglobin, nitrate, nitrite, peroxynitrite,
Date: May 2001
Category: Materia Medica
Author: Pam Brown
Reactive nitrogen species
Reactive nitrogen species
Nitric oxide (NO· ) and nitrogen dioxide (NO2.) are reactive nitrogen species which are radicals, ie they have an unpaired electron, usually associated with the nitrogen atom1. NO· is not very reactive with other non-radicals, but reacts rapidly with other radicals eg reaction with O2· - to form the non-radical peroxynitrite ONOO- (a reactive nitrogen or oxygen species)1. NO· is a colourless gas which diffuses easily within and between body cells, where it has important physiological actions1. NO· ’s ability to scavenge ONOO- and other damaging radicals may make it an important antioxidant in vivo2.
Nitrous oxide (HNO2), dinitrogen trioxide, dinitrogen tetroxide, nitronium ion, alkyl peroxynitrites, nitroxyl ion, nitrosyl cation and nitryl chloride are non-radical reactive nitrogen species1; these are not discussed further.
Nitric oxide (NO· )
In the body NO· is formed from L-arginine by oxidation, 5 electrons provided by NADPH, as shown below1. The reaction is catalysed by NO synthase enzymes, with FAD, FMN, haem and tetrahydrobiopterin cofactors, and requiring Ca2+ and calmodulin1.
L-arginine+2 NADPH+O2 ® L-citrulline+NO· +2 NADP+
Three different isoforms of NO synthase exist (nNOS/NOS1, iNOS/NOS2, eNOS/NOS3), producing NO· with different physiological functions1. iNOS is usually only induced in chronic inflammation, producing very high local concentrations of NO· , but may be found in normal lung endothelium1. Administration of oral L-arginine to postmenopausal women in the hopes of increasing endothelial NO production and decreasing atherosclerosis risk failed3.
NO· can also be formed by denitrifying bacteria in soil, which produce and release both NO· and nitrogen dioxide to the atmosphere1. NO· is also released from car exhausts. Cigarette smoke contains NO· , which may be responsible for transient bronchorelaxation in smokers4.
NO· reacts with oxygen in air to produce the reactive radical nitrogen dioxide. In aqueous solution NO· is oxidised mainly to nitrite ions NO2-. NO· reacts rapidly with other radicals2 eg tyrosyl which in turn may inhibit normal function of the ribonucleotide reductase enzyme which relies on this radical for catalytic action5. NO· also competitively inhibits binding of oxygen to cytochrome oxidase mitochondrial multienzyme complex6.
NO· readily binds to the Fe2+ haem group in guanylate cyclase and this is the basis for some of its physiological functions. Eg, some NO· formed in vascular endothelial cells inhibits aggregation of platelets, while some diffuses to vessel smooth muscle, combines with guanylate cyclase, increasing cGMP, lowering intracellular Ca2+, relaxing smooth muscle and dilating the vessel, lowering blood pressure4. Other physiological functions of NO· include roles in neurotransmission/neuromodulation, regulation cerebral blood flow and antioxidant activity in the nervous system7, killing of foreign organisms and non-specific immunity, penile erection, bladder control, lung vasodilation and normal peristaltic gastrointestinal function1. Reactive nitrogen species formed in the stomach may damage DNA and be carcinogenic8.
Much NO· formed in vivo eventually combines with the Fe2+ haem group of haemoblobin, forming nitrate and nitrite which is excreted in urine4. Some is lost as NO in lungs and nose, some forms stable compounds with proteins and some is used up reacting with free radicals4.
NO· reacts with O2· - faster than with haem, to form peroxynitrite1. This reaction is physiologically important for two reasons. Firstly, these two radicals can antagonise each other’s biological actions eg increased O2· - production near vascular endothelium can cause vasoconstriction and may be involved in the aetiology of hypertension9. Secondly peroxynitrate rapidly becomes protonated, resulting in depletion antioxidants, oxidation of LDL10, DNA strand breakage, DNA base damage and nitration of aromatic amino acids such as tyrosine11 in proteins, possibly resulting in enzyme inactivation and interference with signal transduction. Nitrated tyrosine is used as a fingerprint for formation of peroxynitrite in tissues4. The mechanisms by which peroxynitrite causes these reactions is complex11. The ratio of NO· to O2· - in tissues is important and the resulting effects are much more complex than summarised here1.