Endothelial NOS

Three isoforms of NO synthesizing enzymes ( nitric oxide synthase (NOS)) were isolated, purified, and cloned neuronal NO synthase ( neuronal nitric oxide synthase (nNOS) or isoform (I), immunological or inducible NOS ( inducible (immunological) nitric oxide synthase (iNOS) or isoform (II), and endothelial NOS ( endothelial nitric oxide synthase (eNOS) or isoform... 

The release of NO from the endothelium is induced by various chemical substances, including acetylcholine polypeptides such as substance P, bradykinin, and arginine vasopressin histamine ATP/ADP a2-adrenoceptor agonists thrombin and Ca2+ iono-phores. NO formed in response to mechanical stimuli like shear stress or transmural pressure plays an important role in maintaining basal blood flow. Endothelial NO causes vasodilatation, decreased... 

Endothelial NO Synthase (eNOS) expression is relatively specific for endothelial cells. However, the isozyme has also been detected in certain neurons of the brain, in syncytiotrophoblasts of human placenta and in LLC-PKi kidney tubular epithelial cells. 

Forstermann U (2006) Janus-faced role of endothelial NO synthase in vascular disease - Uncoupling of NADPH oxidation from NO synthesis and its pharmacological reversal. Biol Chem 387 1521-1533... 

Forstermann U, Miinzel T (2006) Endothelial NO synthase in vascular disease — from marvel to menace. Circulation 113 1708-1714... 

The inactivation of enzymes containing the zinc-thiolate moieties by peroxynitrite may initiate an important pathophysiological process. In 1995, Crow et al. [129] showed that peroxynitrite disrupts the zinc-thiolate center of yeast alcohol dehydrogenase with the rate constant of 3.9 + 1.3 x 1051 mol-1 s-1, yielding the zinc release and enzyme inactivation. Later on, it has been shown [130] that only one zinc atom from the two present in the alcohol dehydrogenase monomer is released in the reaction with peroxynitrite. Recently, Zou et al. [131] reported the same reaction of peroxynitrite with endothelial NO synthase, which is accompanied by the zinc release from the zinc-thiolate cluster and probably the formation of disulfide bonds between enzyme monomers. The destruction of zinc-thiolate cluster resulted in a decrease in NO synthesis and an increase in superoxide production. It has been proposed that such a process might be the mechanism of vascular disease development, which is enhanced by diabetes mellitus. 

NADH-ubiquinone reductase) and the second one was complex II (succinate-ubiquinone reductase). Chiesi and Schwaller [101] found that quercetin and tannin inhibited neuronal constitutive endothelial NO synthase. 

Calcium antagonists are able to affect nitric oxide production and suppress the peroxyni-trite-induced damage. Thus, nifedipine enhanced the bioavailability of endothelial NO in porcine endothelial cell cultures supposedly through an antioxidative mechanism [288], Pretreatment with nisoldipine, a vascular-selective calcium blocker of dihydropyridine-type, of confluent bovine aortic endothelial cells suppressed the peroxynitrite-induced GSH loss and increased cell survival [289]. 

The interaction of estrogen receptors with signaling systems of the cell membrane that respond to growth factors and mediate nongenomic, fast actions of estrogens will be reviewed as well. These mechanisms have a growing importance in the comprehension of phenomena like the induction of endothelial NOS (nitric oxide synthase) by estrogens (Rubanyi et al. 2002). 

Experiments with isolated vessels have confirmed the enhancing effect of raloxifene on endothelial NOS (Rahimian et al. 2002) with a similar behavior for tamoxifen (Hutchison et al. 2001). 

Fig. 12.2 Immunogold localization of endothelial NO synthase (NOS3) in skeletal muscles, a Immunogold labeling of NOS3 in subsarcolemmal mitochondria, b Inside of the sarcoplasm NOS3 is localized along the contractile fibers, in the sarcoplasmic reticulum and in mitochondria. EPON embedding, etching with sodium ethoxide, antigen retrieval in citrate buffer, pH 6.0. 12 nm immunogold.

Endothelial NOS was found to be associated with cell membranes and this property was associated with the finding that the cDNA contained the structure for binding myristic acid55. Busconi and Michel56 used bovine aortic endothelial cells to demonstrate that the myristoylated protein was found. When the second amino acid, glycine, was converted to alanine, the addition of myristic acid was prevented and the enzyme remained soluble. Sessa and coworkers51 carried out the same experiments simultaneously and found the same results. However, in both cases the enzyme was not purified, although the protein... 

The term NOS is used to denote a family of three related but distinct isoenzymes neuronal NOS (nNOS) endothelial NOS (eNOS, endothelium and platelets) and inducible NOS (iNOS, endothelium, vascular smooth muscle and macrophage). In addition to reduced nicotinamide adenine dinucleotide phosphate (NADPH) shown in Figure 5.5, NOS enzymes also require flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and tetrahydrobiopterin (BH4) as coenzymes. 

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