The redox siblings nitroxyl (HNO) and nitric oxide (NO) have often been assumed to undergo casual redox reactions in biological systems. However, several recent studies have demonstrated distinct pharmacological effects for donors of these two species. Here, infusion of the HNO donor Angeli's salt into normal dogs resulted in elevated plasma levels of calcitonin gene-related peptide, whereas neither the NO donor diethylamine͞NONOate nor the nitrovasodilator nitroglycerin had an appreciable effect on basal levels. Conversely, plasma cGMP was increased by infusion of diethylamine͞NONOate or nitroglycerin but was unaffected by Angeli's salt. These results suggest the existence of two mutually exclusive response pathways that involve stimulated release of discrete signaling agents from HNO and NO. In light of both the observed dichotomy of HNO and NO and the recent determination that, in contrast to the O2͞O 2 ؊ couple, HNO is a weak reductant, the relative reactivity of HNO with common biomolecules was determined. This analysis suggests that under biological conditions, the lifetime of HNO with respect to oxidation to NO, dimerization, or reaction with O2 is much longer than previously assumed. Rather, HNO is predicted to principally undergo addition reactions with thiols and ferric proteins. Calcitonin gene-related peptide release is suggested to occur via altered calcium channel function through binding of HNO to a ferric or thiol site. The orthogonality of HNO and NO may be due to differential reactivity toward metals and thiols and in the cardiovascular system, may ultimately be driven by respective alteration of cAMP and cGMP levels.Angeli's salt ͉ superoxide dismutase ͉ heme protein ͉ cGMP ͉ calcitonin gene-related peptide D uring the last two decades, discussion of the chemistry of nitric oxide (NO) in biological systems has primarily focused on the nitrosylation of heme proteins such as soluble guanylyl cyclase and the production of reactive nitrogen oxide species (RNOS) (1-3). The RNOS literature has largely been concerned with nitrogen dioxide (NO 2 ), dinitrogen trioxide (N 2 O 3 ), and peroxynitrite (ONOO Ϫ ), which are formed through reaction with molecular oxygen or superoxide (O 2 Ϫ ) (4-6). Recently, however, there has been increased interest in the one-electron reduction product of NO, nitroxyl (HNO͞NO Ϫ ; nitrosyl hydride͞nitroxyl anion). Of particular note are studies suggesting that oxidation of L-arginine by NO synthase (NOS) leads to production of nitroxyl rather than NO under certain conditions (7-10). In this light, elucidation of the chemical biology of nitroxyl has acquired new importance.Comparisons of the toxicological and pharmacological properties of nitrogen oxide donor compounds have revealed that NO and HNO in general elicit distinct responses under a variety of biological conditions. In vitro, HNO reacts with O 2 to generate potent oxidizing species capable of cleaving DNA, thereby augmenting oxidative damage (3, 11). The RNOS formed by NO autoxidation do not cause these cellular a...

A potential of about ؊0.8 (؎0.2) V (at 1 M versus normal hydrogen electrode) for the reduction of nitric oxide (NO) to its one-electron reduced species, nitroxyl anion ( 3 NO ؊ ) has been determined by a combination of quantum mechanical calculations, cyclic voltammetry measurements, and chemical reduction experiments. This value is in accord with some, but not the most commonly accepted, previous electrochemical measurements involving NO. Reduction of NO to 1 NO ؊ is highly unfavorable, with a predicted reduction potential of about ؊1.7 (؎0.2) V at 1 M versus normal hydrogen electrode. These results represent a substantial revision of the derived and widely cited values of ؉0.39 V and ؊0.35 V for the NO͞ 3 NO ؊ and NO͞ 1 NO ؊ couples, respectively, and provide support for previous measurements obtained by electrochemical and photoelectrochemical means. With such highly negative reduction potentials, NO is inert to reduction compared with physiological events that reduce molecular oxygen to superoxide. From these reduction potentials, the pKa of 3 NO ؊ has been reevaluated as 11.6 (؎3.4). Thus, nitroxyl exists almost exclusively in its protonated form, HNO, under physiological conditions. The singlet state of nitroxyl anion, 1 NO ؊ , is physiologically inaccessible. The significance of these potentials to physiological and pathophysiological processes involving NO and O 2 under reductive conditions is discussed. N itric oxide (NO) is an endogenously generated species with a diverse array of biological functions (1). NO is one of the primary regulators of vascular tone, is involved in signal transduction in both the peripheral and central nervous system, and is an integral part of the immune response system associated with macrophage and neutrophil activation. More recently, NO has been proposed to be involved in the regulation of mitochondrial function (2, 3). Problems in NO homeostasis have been implicated in the development of a variety of diseases and disorders such as hypertension and atherosclerosis (4), diabetes (5), and many neurodegenerative diseases (6). NO is also thought to be a cytoprotective agent, capable of inhibiting radical-induced damage and oxidative stress (7). To understand the actions of NO as a physiological messenger and a cytotoxic or cytoprotective effector molecule, it is essential to understand its basic chemical interactions with biological systems and its metabolic fate.NO and its reduced derivative NO Ϫ (and͞or its conjugate acid, HNO) have very different chemical properties and display distinct and often opposite effects in cells. For example, HNO͞ NO Ϫ has been found to be toxic under conditions where NO is cytoprotective (8). HNO͞NO Ϫ reacts with O 2 to generate potent oxidizing species, capable of damaging DNA and causing cellular thiol depletion, whereas NO does neither under similar conditions (9-11). HNO has been found to be a thiophilic electrophile (12), readily capable of modifying cellular thiol functions (13,14), whereas NO reacts only indirectly with thiols. HNO͞NO Ϫ h...