Nitric oxide (NO) has earned the reputation of being a signaling mediator with many diverse and often opposing biological activities. The diversity in response to this simple diatomic molecule comes from the enormous variety of chemical reactions and biological properties associated with it. In the last few years, the importance of steady state NO concentrations have emerged as a key determinant of its biological function. Precise cellular responses are differentially regulated by specific NO concentration. We propose 5 basic distinct concentration levels of NO activity; cGMP mediated processes ([NO] <1-30 nM; Akt phosphorylation ([NO] = 30-100 nM); stabilization of HIF-1α ([NO] = 100-300 nM); phosphorylation of p53 ([NO] > 400 nM) and nitrosative stress (1 µM). In general, lower NO concentrations promote cell survival and proliferation, while higher levels favor cell cycle arrest, apoptosis, and senescence. Free radical interactions will also influence NO signaling. One of the consequences of reactive oxygen species (ROS) generation is to reduce NO concentrations. This antagonizes the signaling of nitric oxide and in some cases results in converting a cell cycle arrest profile to a cell survival one. The resulting reactive nitrogen species (RNS) that are generated from these reactions can also have biological effects and increase oxidative and nitrosative stress responses. A number of factors determine the formation of NO and its concentration, such as diffusion, consumption, and substrate availability which are referred to as Kinetic Determinants for Molecular Target Interactions. These are the chemical and biochemical parameters that shape cellular responses to NO. Herein we discuss signal transduction and the chemical biology of NO in terms of the direct and indirect reactions.

The role of redox molecules, such as NO and ROS, as key mediators of immunity has recently garnered renewed interest and appreciation. To regulate immune responses, these species trigger the eradication of pathogens on the one hand and modulate immunosuppression during tissue-restoration and wound-healing processes on the other. In the acidic environment of the phagosome, a variety of RNS and ROS is produced, thereby providing a cauldron of redox chemistry, which is the first line in fighting infection. Interestingly, fluctuations in the levels of these same reactive intermediates orchestrate other phases of the immune response. NO activates specific signal transduction pathways in tumor cells, endothelial cells, and monocytes in a concentration-dependent manner. As ROS can react directly with NO-forming RNS, NO bioavailability and therefore, NO response(s) are changed. The NO/ROS balance is also important during Th1 to Th2 transition. In this review, we discuss the chemistry of NO and ROS in the context of antipathogen activity and immune regulation and also discuss similarities and differences between murine and human production of these intermediates.

Inducible nitric oxide synthase (NOS2) is involved in wound healing

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...