NO mediates its effects by covalent modification of proteins. There are three major targets of NO (Figure 19–1): 1. Metalloproteins—NO interacts with metals, especially iron in heme, a prosthetic group in certain proteins. The major heme-containing target of NO is soluble guanylyl cyclase (sGC), an enzyme that generates cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). When NO binds the heme in sGC, the enzyme is activated, resulting in an elevation in intracellular cGMP levels. cGMP activates protein kinase G (PKG), which phosphorylates specific proteins. In blood vessels, NO is released from endothelial cells to increase cGMP and PKG activity in vascular smooth muscle cells. The elevated PKG activity results in the phosphorylation of proteins that lead to reduced cytosolic calcium levels. Since calcium triggers contraction of smooth muscle, the NO-mediated reduction in cytosolic calcium leads to vasorelaxation.
NO also has cytotoxic effects, especially when it is produced at high levels, eg, in activated macrophages. Excessive NO production results in NO complexing with metals in metalloproteins involved in cellular respiration, including the citric acid cycle enzyme aconitase and the electron transport chain protein cytochrome oxidase. Additionally, inhibition of heme-containing cytochrome P450 enzymes by NO is a major pathogenic mechanism in inflammatory liver disease.
2. Thiols—Although the primary target of NO, especially in the context of vasorelaxation, is sGC, some of the effects of
NO+NADPH, O2 Superoxide Activation of guanylyl cyclase Heme iron in Guanylyl cyclase NOSL-NMMA Arginine Citrulline O2, metals +H3N NH2 NH O +H3N NH2 NH +H3N +H3N COO– COO– COO– COO– OH S N O
FIGURE 19–1 Synthesis and reactions of nitric oxide (NO). L-NMMA (see Table 19–3) inhibits nitric oxide synthase (NOS). NO binds to the iron in hemoproteins (eg, guanylyl cyclase), resulting in the activation of cyclic guanosine monophosphate (cGMP) synthesis and cGMP target proteins such as protein kinase G. Under conditions of oxidative stress, NO can react with superoxide to nitrate tyrosine. GTP, guanosine triphosphate.