calcium-activated potassium channels. The fall in concentration of Ca2+ ensures that the myosin light-chain kinase (MLCK) can no longer phosphorylate the myosin molecule, thereby stopping the crossbridge cycle and leading to relaxation of the smooth muscle cell. NO is also generated by phagocytes (monocytes, macrophages, and neutrophils) as part of the human immune response. Phagocytes are armed with inducible nitric oxide synthase (iNOS), which is activated by interferon-gamma (IFN-γ) as a single signal or by tumor necrosis factor (TNF) along with a second signal. On the other hand, transforming growth factor-beta (TGF-β) provides a strong inhibitory signal to iNOS, whereas interleukin-4 (IL-4) and IL-10 provide weak inhibitory signals. In this way, the immune system may regulate the resources of phagocytes that play a role in inflammation and immune responses. NO is secreted as free radicals in an immune response and is toxic to bacteria and intracellular parasites, including Leishmania and malaria; the mechanism for this includes DNA damage and degradation of iron sulfur centers into iron ions and iron-nitrosyl compounds. Two important biological reaction mechanisms of NO are S-nitrosation of thiols, and nitrosylation of transition metal ions. S-nitrosation involves the (reversible) conversion of thiol groups, including cysteine residues in proteins, to form S-nitrosothiols (RSNOs). S-Nitrosation is a mechanism for dynamic, post-translational regulation of most or all major classes of protein. The second mechanism, nitrosylation, involves the binding of NO to a transition metal ion like iron to modulate the normal enzymatic activity of an enzyme such as cytochrome P450. Nitrosylated ferrous iron is particularly stable, as the binding of the nitrosyl ligand to ferrous iron (Fe(II)) is very strong. Hemoglobin is a prominent example of a heme protein that may be modified by NO by multiple pathways. There are several mechanisms by which NO has been demonstrated to affect the biology of living cells. These include oxidation of iron-containing proteins such as ribonucleotide reductase and aconitase, activation of the soluble guanylate cyclase, ADP ribosylation of proteins, protein sulfhydryl group nitrosylation, and iron regulatory factor activation. NO has been demonstrated to activate NF-κB in peripheral blood mononuclear cells, an important transcription factor in iNOS gene expression in response to inflammation. NO can be problematic under certain circumstances if it reacts with superoxide to produce the damaging oxidant peroxynitrite. Pharmaceutical initiatives include: Nitroglycerin and amyl nitrite serve as vasodilators because they are converted to nitric oxide in the body. The vasodilating antihypertensive