THE PROTECTIVE EFFECT OF TETRAHYDROBIOPTERIN ON THE NITRIC OXIDE-MEDIATED INHIBITION OF PURIFIED NITRIC-OXIDE SYNTHASE

The nitric oxide synthases (NOS) are a class of enzymes responsible fo r the generation of NO via an oxygen and NADPH dependent oxidation of the amino acid arginine. These enzymes are ironheme proteins which con tain FAD and FMN and, enigmatically, require tetrahydrobiopterin (BH4) . NOS has recently been shown to be subject to inhibition by its produ ct, NO. Preliminary data by us indicate that a possible role for BH4 i s to prevent and/or reverse the NO-mediated inhibition of NOS. The obj ective of this study was to elucidate the mechanism by which BH4 prote cts NOS against NO inhibition. Protection of NOS from NO inhibition wa s observed by both BH4 and the BH4 regeneration system, dihydropteridi ne reductase (DHPR)/NADH. NO, rather than an oxidation product, appear s to be the inhibitory species. Protection by BH4 is not likely due to a simple chemical reaction between BH4 and NO or its oxidation produc t, NO2. The results are consistent with a protective mechanism by whic h BH4 may act as a nonstoichiometric reducing agent for a redox active enzyme component, such as the ironheme, to prevent NO ligation. (C) 1 995 Academic Press, Inc.