Nitric Oxide in Asthma: Pathogenic, Therapeutic, or Diagnostic?

Nitrogen monoxide, more commonly called nitric oxide or simply NO, is a fascinating free radical molecule, which participates in a broad range of important physiologic processes, including vasodilation, neurotransmission, and host defense. NO is generated from L -arginine by the enzyme nitric oxide synthase (NOS), of which there are three known isoforms. Types I and III NOS, found predominantly in neurons and endothelium, respectively, are constitutively expressed and are dependent upon calcium for activity. The third isoform, NOS II, can be expressed by a wide range of cells primarily after it has been induced by certain cytokines, microbes, or microbial products (1, 2). All three isoforms are dimeric, with each subunit containing iron protoporpyrin IX, tetrahydrobiopterin, flavin adenine dinucleotide, flavin mononucleotide, and binding sites for