Nitric oxide (NO) is synthesized in animals and simpler life forms by sequential oxidation of a terminal guanidino nitrogen of L-arginine (L-Arg) (KERWIN et al. 1995; GRIFFITH and STUEHR 1995) (Fig. 1).This pathway was first described in mouse macrophages and bovine endothelium in 1987, although enzymatic hydroxylation at other atoms in L-Arg had been previously demonstrated. The reaction is catalyzed by the NO synthases (NOSs), which all utilize reduced nicotinamide adenine dinucleotide phosphate (NADPH) and O2 as cosubstrates. Three NOS isoforms have evolved to function in animals, and each gene is located on a different chromosome (NATHAN and XIE 1994). Although alternatively spliced NOSs are sometimes expressed, they are often missing structural elements critical for full function (BRENMAN et al. 1996; EISSA et al. 1998). Two of three NOSs are constitutively expressed in cells, and they synthesize NO in response to increased Ca2+ or, in some cases, in response to Ca2+-independent stimuli such as shear stress (FLEMING et al. 1998). These NOSs function in signal transduction cascades by linking temporal changes in Ca2+ level to NO production, which serves as an activator of soluble guanylate cyclase (IGNARRO and MURAD 1995). The constitutive enzymes are designated nNOS and eNOS (or NOS I and III, respectively), after the cell types in which they were originally discovered (rat neurons and bovine endothelial cells). An inducible NOS (iNOS or NOS II) is constitutively expressed only in select tissues, such as lung epithelium (DWEIK et al. 1998), and is more typically synthesized in response to inflammatory or proinflammatory mediators (HIERHOLZER et al. 1998; KRONCKE et al. 1995). Expression of iNOS may be beneficial in host defense or in modulating the immune response, but its expression is also linked to a number of inflammatory diseases (reviewed in MACMICKING et al. 1997; KRONCKE et al. 1995; NATHAN 1997).
