Role of NO and Nitrogen Intermediates in Regulation of Cell Functions

Prior to the 1980s, nitric oxide (NO) was best known as a toxic reactive free radical found in atmospheric pollutants and carcinogens [1], a by-product of microbial nitrogen metabolism [2,3] and a potent activator of the mammalian enzyme heme-containing soluble guanylate cyclase [4]. As early as 1916, evidence for a mammalian nitrogen metabolic pathway was reported with the finding that several types of animals, including humans, excreted more urinary nitrate than could be accounted for by dietary intake [5]. Further evidence was reported by Tannenbaum and co-workers [6,7] who showed that in vivo mammalian nitrate formation was substantially enhanced by administration of the immunostimulant lipopolysaccharide (LPS) [8]. Stuehr and Marletta [9] first demonstrated that murine macrophages stimulated in vitro with LPS expressed nitrogen oxide synthetic activity and produced nitrite (NO2) and nitrate (NO3 -). Simultaneously, other investigators were trying to elucidate the identity of a short-lived, diffusable endothelium-derived relaxing factor (EDRF) produced by acetylcholine-treated endothelial cells that was responsible for mediating smooth muscle cell relaxation [10]. Based on the similarities in the pharmacological properties of EDRF and NO generated from acidified nitrite, Furchgott suggested that EDRF may be NO in 1986 [11]. At the same time, Ignarro et al. also proposed that EDRF may be NO or a closely related species [12]. The following year, two independent groups of investigators [13,14] demonstrated EDRF was indeed nitric oxide (NO).