Pathways of NOS Activation - Research Paper Example

They are associated with various cofactors, including tetrahydrobiopterin, FAD, FMN and iron protoporphyrin IX. Although there has been some debate over whether the production of NO by NOS is actually significant, however, for the time being, it is assumed to be so. Nitric oxide plays a wide range of physiological roles in various tissues. It acts as a neurotransmitter in the nervous system, regulates blood vessel tone in the vascular system, and assists bacterial killing in the immune system.

It is also, however, a free radical, and has cytotoxicity when produced in large amounts during pathological processes ( (Aktan). The regulation of NOS that allows specific amounts of NO to be produced at particular times, is a complex phenomenon. Three isoforms of NOS are recognized, which have 50-60% sequence homology but are encoded by different genes and have different localization, regulation and catalytic properties. Although other splice variants have also been identified, their physiological role is not yet clear.

The three main isoforms are: nNOS (NOS-I), iNOS ane eNOS. Their functional roles and regulation are described in detail below. nNOS, also known as NOS-I, is NOS found in neuronal tissue and was the first isoform to be identified. nNOS has a particular propensity for the chemical reaction where uncoupled oxidation of NADPH is done, to yield superoxide. Although all three isoforms can produce this reaction, it is more rapidly catalysed by nNOS. iNOS is inducible NOS, also known as NOS-II. iNOS can be induced in a wide variety of cells all over the body, including hepatocytes, endothelium, monocytes, mast cells, macrophages and smooth muscle cells.

However, it is known to be localized predominantly at the perinuclear region of the CD45-positive neutrophils in humans (Wheeler). Common stimuli for iNOS production include inflammatory cytokines including IFN-gamma (which induces iNOS through the JAK-STAT pathway), bacterial cell products, and other molecules released in response to infection, such as Toll-like receptors and CD14. Transcription factors such as NF-kB, that are upregulated in inflammatory states by cytokines such as IFN-g and TNF-a, induce expression of iNOS.

In particular, it is induced within macrophages and neutrophils during bacterial infection, producing large quantities of nitric oxide that has effects on the immune, inflammatory and cardiovascular systems, including bacterial killing. Thus, iNOS is part of the human host cell’s defense against bacterial and viral pathogens. NO has a negative feedback effect on iNOS production, it inhibits NF-kB to decrease iNOS expression (Aktan). eNOS is also known as NOS-III, it is the isoform that was first found to be expressed by vascular endothelial cells.

It localizes primarily in the perinuclear Golgi region of endothelial cells. Fluid shear stress appears to be a major stimulating factor for eNOS expression by vascular endothelial cells, the resultant NO dilates the blood vessel and thus helps reduce the shear stress. Mechanisms by which fluid shear stress affects eNOS production are further explained below. A number of regulatory and activation pathways for NOS have been discovered. For example, calmodulin is an intracellular protein essential to the activation of all three isoforms of NOS.

It forms a complex with calcium, which binds to NOS and increases the rate of the chemical reaction by facilitating electron transfer from NADPH