Hi friends, im here with the last blog of this month "Nitric Oxide and its Physiological Role". Nitric Oxide (NO) is an important signaling molecule involved in many physiological and pathological processes. The discussion in my blog is mainly focused on the generation of NO in the physiological setting, its metabolism, signaling mechanisms and some of its important functions in our body such as its role in cardiovascular system, nervous systems and in host defence mechanisms. Biosynthesis:- The synthesis of nitric oxide inside the body is from the amino acid L-Arginine. The reaction requires the presence of O2 and NADPH, and is catalyzed by the enzyme "Nitric Oxide Synthase" (NOS). This enzymatic reaction also involves co-factors such as heme, tetrahydrobiopterin and flavin adenine dinucleotide (FAD). There are three isoforms of this enzyme, viz. * Inducible Form (iNOS or NOS-II) - This enzyme is mainly expressed under pathological conditions in macrophages and Kupffer cells, neutrophils, fibroblasts, vascular smooth muscle and endothelial cells. * Endothelial (eNOS or NOS-III) - Constitutive form, mainly expressed in endothelial cells under physiological conditions and also present in cardiac myocytes, renal mesangial cells, osteoblasts and osteoclasts, airway epithelium. * Neuronal (nNOS or NOS-I) - Constitutive form, expressed in nerve terminals. The intracellular calcium-calmodulin complex controls the activity of eNOS and nNOS and thereby the nitric oxide synthesis. Substances that trigger the increase in intracellular calcium, ultimately leads to the increase in calcium-calmodulin which activates the eNOS or nNOS and Phosphorylation of specific residues on eNOS makes it more or less active and this can alter NO synthesis. In blood vessels the synthesis of nitric oxide is mainly due to the shear stress that is detected by the endothelial mechanoreceptors and signaled via a serine-threonine protein kinase called Akt or protein kinase
B. Agents that increase eNOS activity are: beta agonists via protein kinase-A mediated phosphorylation, and insulin via tyrosine kinase activation. iNOS activity is independent of intracellular calcium. Bacterial lipopolysaccharides or cytokines (interferon-g) released in response to pyrogens induce iNOS. TNF-a and IL-1 has a synergistic effect with interferon-g. Inactivation of NO:- NO is very labile and this is due to its rapid reaction with metals and reactive oxygen species (ROS). NO undergoes Oxidation to form nitrate when it reacts with heme and hemeproteins, oxyhemoglobin. Often reactions of NO with hemoglobin may result in the formation of partial S-nitrosylated products of hemoglobin which are distributed all through vasculature. It is also inactivated by Superoxide radical and enzymes such as Superoxide Dismutase (SOD) protect the NO from inactivation. Signaling Mechanisms:- Nitric Oxide exerts its effects by covalent modification of proteins. The major effector targets are: 1. Metalloproteins- "Guanylyl cyclase" (GC) the enzyme that synthesizes cyclic GMP from guanosine triphosphate (GTP) is the primary target of NO. The enzyme is activated when NO is bound to the heme present in GC, and results in increased intracellular cGMP levels. cGMP activates Protein kinase G which phosphorylates specific proteins this result in reduced intracellular calcium levels. This cause smooth muscle relaxation and platelet aggregation inhibition. NO also has a cytotoxic effect when present in large quantities in cells. It inhibits Metalloproteins involved in cellular respiration. Inhibition of heme containing cytochrome P450 enzyme by NO is a major process underlying in Inflammatory Liver Disease. 2. Thiols- On reaction with thiol groups present in certain proteins, Nitric oxide form certain adducts known as "Nitrosothiols". These adducts may have an activating/inhibiting effect on the activity of these proteins. These changes are termed as S-nitrosylation or S-nitrosation and require metals or oxygen. Some of the important targets of this reaction are * H-Ras- Regulator of cell proliferation (Activated by S-nitrosylation) * Glyceraldehyde-3-phosphate dehydrogenase- Metabolic enzyme (Inhibited) * Glutathione- Forms S-nitrosoglutathione (Carrier of NO) 3. Tyrosine nitration- NO interacts with superoxide radical effectively to form Peroxynitrite (ONOO-). Many of the inflammatory and degenerative diseases are associated with increased levels of Peroxynitrite levels due to this reason only. This has harmful consequences like DNA damage, nitration of tyrosine, and oxidation of cysteine. When proteins undergo tyrosine nitration with the formed Peroxynitrite it leads to the activation/inactivation of that protein function which results in tissue damage. Hence protein tyrosine nitration is used as a marker for detection of Oxidative and Nitrosative stress. Physiological Role of NO:- 1. Cardiovascular System- Synthesized in Vascular smooth muscle and endothelium where it is involved in control of blood pressure and regional blood flow. Increased levels of NO leads to hypotension and decreased levels leads to Atherogenesis, thrombosis. 2. Platelets- Involved in inhibition of platelet aggregation. 3. Immune System- The iNOS expressed in Macrophages, Neutrophils, and Leucocytes synthesizes NO in these cells where it provides defence against various viruses, bacteria, fungi, protozoa and other parasites. Mechanisms involved include nitrosylation of nucleic acids, combination with heme containing enzymes involved in cellular respiration. 4. Nervous System- As a neurotransmitter in CNS it is primarily implicated in long-term Potentiation, plasticity (which is concerned with memory, appetite, nociception). Increased levels of NO leads to Excitotoxicity and cell death. PNS it plays role in gastric emptying and penile erection. Insufficient levels of NO in this leads to erectile dysfunction. "This Blog is free from plagiarized material" References:- 1. Rang and Dale PHARMACOLOGY, 6th Edition, Elsevier Publications, Pg: 265-273. 2. Basic and Clinical Pharmacology, 11th Edition, Bertram G. Katzung, Tata McGraw Hill Education Pvt Limited, Pg: 331-337.