Tetrahydrobiopterin nitric-oxide synthase activity

P. D., Loftus, M., Stuehr, D. J., Expression of human inducible nitric oxide synthase in a tetrahydrobiopterin (H4B)-deficient cell line H4B promotes assembly of enzyme subunits into an active dimmer, Proc. Natl. Acad. Sci. USA 92 (1995), p. 11771-11775... 

Bec, N., Gorren, A. C., Voelker, C., Mayer, B., Lange, R., Reaction of neuronal nitric-oxide synthase with oxygen at low temperature. Evidence for reductive activation of the oxy-ferrous complex by tetrahydrobiopterin, J. Biol. Chem. 273 (1998), p. 13502-13508... 

Hevel, J. M., and Marietta, M. A. (1992). Macrophage nitric oxide synthase Relationship between enzyme-bound tetrahydrobiopterin and synthase activity. Biochemistry 31, 7160-7165. 

BH4 = Tetrahydrobiopterin CAM = Cytotoxic activated macrophage cNOS = Constitutive nitric oxide synthase CPR = Cytochrome P450 reductase EDRF = Endothelial-derived relaxation factor EPR = Electron paramagnetic resonance spectroscopy IL-1 = Interleukin-1 iNOS = Inducible nitric oxide synthase EPS = Lipopolysaccharide, or endotoxin NMMA = ISp-monomethyl-L-arginine NOS = Nitric oxide synthase ROS = Reactive oxygen species SOD = Superoxide dismutase TNF = Tumor necrosis factor. 

Gross, S. S., Jaffe, E. A., Levi, R., and Kilbourn, R. G. (1991). Cytokine-activated endothelial cells express an isotype of nitric oxide synthase which is tetrahydrobiopterin-dependent, calmodulin-independent and inhibited by arginine analogs with a rank-order of potency characteristic of activated macrophages. Biochem. Biophys. Res. Commun. 187,823-829. Gryglewski, R. J., Bolting, R., and Vane, J. R. Mediators producted by tbe endothelial cell. Hypertension 12, 530-548. 

O2. Endothelial nitric oxide synthase (eNOS) also produces O2 but production of O2 vs. NO is determined by the amount of fully reduced tetrahydrobiopterin (BH4) available, with BH4 suppressing 02 in favor of NO. On a cautionary note, 02 production by many systems may represent a non-physiological activity expressed only when native activity is compromised in some way. 

Nitrosothiols are difficult to control as donors, since the release of NO is catalyzed by traces of free metal ions, in particular copper(I) Cu" ". The endogenous synthesis of NO may be enhanced by activating existing nitric oxide synthases with supplementation of cofactors like tetrahydrobiopterin, with folic acid or vitamin C, or by phosphorylation of certain serine groups of the enzyme. Alternatively, the expression of NOS enzyme in tissues or cell cultures may be artificially enhanced by gene therapy, i.e., genetic transfection with viral vectors. Finally, under favorable conditions NO levels may be increased by blocking the... 

Figure 3.2 Beneficial effects of folic acid on vascular wall. Folic acid circulates in human body as 5-methyltetrahydrofolate (5-MTHF). 5-MTHF lowers circulating homocysteine (Hey) levels, thus reducing systemic oxidative stress and Hcy-induced activation of prothrombotic mechanisms. In addition, vascular 5-MTHF has a favourable effect on intracellular Hey metabolism, attenuating Hcy-induced activation of NADPH oxidase isoforms (NOXs) in the vascular wall. Furthermore vascular 5-MTHF scavenges per se peroxynitrite (ONOO ) radicals in the vascular wall preventing the oxidation of vascular tetrahydrobiopterin (BH4) associated with endothelial nitric oxide synthase (eNOS) uncoupling and diminished vascular nitric oxide (NO) bioavailability. In total through these effects 5-MTHF lowers vascular oxidative and nitrosative stress. Thus by modulating vascular redox, 5-MTHF inhibits activation of proinffammatory pathways which orchestrate vascular wall inflammation and perpetuate endothelial dysfunction and atherogenesis development (unpublished).

Frohlich LG, Kotsonis P, Traub H, Taghavi-Moghadam S, Al-Masoudi N, Elofmann H, Strobel H, Matter El, Pfleiderer W, Schmid HW (1999) Inhibition of neuronal nitric oxide synthase by 4-amino pteridine derivatives structure-activity relationship of antagonists of 6(R) 5,6,7,8-tetrahydrobiopterin cofactor. J Med Chem 42(20) 4108 121. doi 10.1021/... 

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