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The Protective Effect of Tetrahydrobiopterin on the Nitric Oxide-Mediated Inhibition of Purified Nitric Oxide Synthase

https://doi.org/10.1006/bbrc.1995.1052Get rights and content

Abstract

The nitric oxide synthases (NOS) are a class of enzymes responsible for the generation of NO via an oxygen and NADPH dependent oxidation of the amino acid arginine. These enzymes are ironheme proteins which contain FAD and FMN and, enigmatically, require tetrahydrobiopterin (BH4). NOS has recently been shown to be subject to inhibition by its product, NO. Preliminary data by us indicate that a possible role for BH4 is to prevent and/or reverse the NO-mediated inhibition of NOS. The objective of this study was to elucidate the mechanism by which BH4 protects NOS against NO inhibition. Protection of NOS from NO inhibition was observed by both BH4 and the BH4 regeneration system, dihydropteridine reductase (DHPR)/NADH. NO, rather than an oxidation product, appears to be the inhibitory species. Protection by BH4 is not likely due to a simple chemical reaction between BH4 and NO or its oxidation product, NO2. The results are consistent with a protective mechanism by which BH4 may act as a nonstoichiometric reducing agent for a redox active enzyme component, such as the ironheme, to prevent NO ligation.

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    Tetrahydrobiopterin (BH4) is ubiquitously present in all tissues of higher organisms and is a cofactor required for the synthesis of several biomolecules [61]. In addition to its activity as a cofactor, BH4 serves as a reducing agent [62]. Schmidt et al. reported that treatment of porcine aortic endothelial cells with BH4 restores DEA/NO [NO donor]-stimulated cGMP production reduced in the presence of ODQ [63].

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