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The free radical nitric oxide (NO) reacts with both anti- and prooxidants in plant cells. This article gives a brief overview of the critical functions of ascorbate and glutathione in limiting NO bioavailability, either alone or in concert with S-nitrosoglutathione reductase.
Other key areas include NO’s modulation of antioxidant enzymes and the interaction of NO with reactive oxygen species (ROS). Under stress, NO controls antioxidant enzyme and gene function, resulting in an increase or decrease in cellular redox state.
Chronic NO generation during salt stress, for example, activates the antioxidant system, enhancing salt tolerance in a variety of plants. Rapid NO buildup in response to intense stress stimuli, on the other hand, has been associated with the inhibition of antioxidant enzymes as well as a consequent increase in hydrogen peroxide levels.
Furthermore, in incompatible Arabidopsis thaliana-Pseudomonas syringae interactions, S-nitrosylation-triggered regulation of NADPH oxidases was demonstrated to stop ROS burst and cell death progression, revealing the diverse functions of NO during redox signalling.
Radicals derived from nitric oxide and oxygen
The importance of ONOO formation and the superoxide anion in biological systems was recognised in the early 1990s, and a function in the atherogenic cycle was quickly proposed. ONOO is now best thought of as a prototype reactive nitrogen species (RNS) having a specific role in the early development of an atherosclerotic lesion.
These hypotheses are based on studies that found 3-nitrotyrosine, a marker of RNS-mediated processes, in atherosclerotic lesions. ONOO is an oxidant that can convert low-density lipoprotein (LDL) into the pro-atherogenic state in addition to nitration processes.
For example, the activation or transcriptional regulation of enzymes that create this oxidant (e.gNAD(P)H oxidases) increases O2 generation. The pathways competent for consuming NO appear to be up-regulated in vascular disease. Increased ONOO-mediated oxidative damage is expected to develop and build in either circumstance.
However, it should be emphasised that new research suggests that a variety of mediators, including enzymes like myeloperoxidase, might mediate nitration and oxidation events in atherosclerotic lesions.
The reaction of NO with LOO is a termination reaction, similar to the reaction with O2. This response is seen as favourable in this situation since it leads to the halting and prevention of lipid peroxidation events.
This is similar to the traditional antioxidant effects of -tocopherol on LDL oxidation inhibition. However, NO is more effective on a molar basis.
NO-free radical termination reactions can also happen with radical species mediators in enzyme activities. In this scenario, NO can limit the production of pro-inflammatory chemicals by enzymes such as lipoxygenases and cyclooxygenases [32].
The following section discusses the reaction pathways between NO and peroxyl radicals, as well as the consequences they have on lipid peroxidation.
NO has been identified as a signal modulator involved in plant development and stress tolerance. Endogenous NO homeostasis in plants is altered by stress. 6.
Exogenous NO donor sodium nitroprusside (SNP) therapy can reduce Al toxicity in wheat root tips by successfully ameliorating Al-induced mitochondrial respiratory failure. 7.
Previous research by our lab demonstrated that NO inhibits PCD produced by Al in peanut root tips. NO may act as an antioxidant to delay PCD in barley aleurone layers9. The mechanism through which NO suppresses Al-induced PCD remains unknown.
Effects of Nitric Oxide on Antioxidant System
In vertebrates, nitric oxide (NO•) is a pervasive intracellular and intercellular messenger with a wide range of regulatory activities, activation of antioxidant enzymes in the nervous down in the central area, cardiovascular, and immunological systems (Moncada et al., 1991; Knowles and Moncada, 1994).
Due to the given physiological significance of a free radical NO•, various studies have been conducted to identify the enzyme accountable for its endogenous synthesis.
NOS (EC 1.14.13.39) catalyses the oxygen- and NADPH-dependent oxidation of L-arginine to NO• and citrulline in a complicated process requiring FAD, FMN, and tetrahydrobiopterin, as well as calcium and calmodulin in some circumstances (Knowles and Moncada, 1994).
NOSs have been identified as functional homodimers along with a subunit of 130 kDa that contains iron-protoporphyrin IX, flavin adenine dinucleotide (FAD), flavin adenine dinucleotide (FAD), flavin adenine dinucleotide (FAD), flavin adenine dinucleotide (FAD), flavin adenine dinucleotide (as prosthetic groups in mononucleotide (FMN) and tetrahydrobiopterin (BH4) (Knowles and Moncada, 1994; McMillan et al., 1996). In mammalian tissues, three different types of NOS isoforms get identified and understood: neuronal NOS (nNOS or Type I), inducible NOS (iNOS or Type II), and endothelial NOS (eNOS or type III).
Conclusion
While types I and II of Nitric Oxide have been present in the soluble fraction, type III has been proven to be membrane-associated, and none of the isoforms has a clear stem-cell expression profile (Sessa et al., 1992; McMillan et al., 1996). So far, NOS activity in animal organelles has only been discovered in rat liver mitochondria (Ghafourifar and Richter, 1997).
