Regulation of the Expression of the Inducible Isoform of Nitric Oxide Synthase

All forms of NOS contain four prosthetic groups: flavin-adenine dinucleotide; flavin mononucleotide, tetrahydrobiopterin (BH₄), and a heme complex, iron protoporphyrin IX. All NOS isoforms are dependent on calmodutin; in the inducible isoform calmodulin is already present in a tightly bound form. The role of calmodulin is to control the electron transfer from NADPH to the flavins, possibly by causing a reorientation of the reductase and oxygenase domains of NOS, perhaps moving them into a favorable position for electron transfer among them. The function of BH₄ is unclear, but may substitute for peroxy-heme to carry out nucleophilic attack on hydroxyarginine or perhaps may act as a stabilizer of the active structure of NOS—that is, assembling dimeric forms of NOS in the case of bNOS and iNOS. The cellular and molecular mechanisms of iNOS induction, or its inhibition by pharmacological agents, are not fully understood. They clearly involve transcription of mRNA and novel protein biosynthesis and take at least several hours. The sequencing of the DNA regions upstream of the iNOS gene (that is, the promoter regions) revealed separate promoter regions for the induction of iNOS by endotoxin and interferon-γ (IFN-γ).