35. NO and CO as signal molecules

Last updated on November 19, 2018 at 17:16

Summary

  • They can diffuse past biological membranes, so they have no need for membrane receptors
  • NO is synthesized from arginine by NO synthase
  • Three types of NO synthase exist, each in different tissues
  • NO is a vasodilator and a neurotransmitter
  • CO is synthesized from heme by heme oxygenase

NO synthase is a funny name

Nitric oxide, or NO, is a colourless gas that is toxic in high doses. It functions as a neurotransmitter and a vasodilator. It is synthesized by NO synthase, an enzyme there exist three forms of. Neuronal NO synthase is present in neural tissue, endothelial NO synthase is present in endothelium, while inducible NO synthase is present in macrophages and hepatocytes. The first produces NO to be used as a neurotransmitter, the second for it to be used as a vasodilator, while the third is stimulated by cytokines, like interferon-γ. iNOS produces NO as a defence as part of the oxidative burst of macrophages. iNOS and nNOS are soluble, which means they’re found in the cytosol, while eNOS is bound to the cell membrane.

NO in the cell activates soluble guanylyl cyclase, an enzyme that catalyses the conversion of GTP to cGMP. cGMP is a second messenger in many pathways. It activates Protein Kinase G, which is important in the vasodilatory response of NO.

CO – carbon monoxide

Carbon monoxide is synthesized by heme oxygenase, or HO. Two variations of HO exist, HO-1 and HO-2.

HO-2 is found in brain, endothelium and testis. It is always expressed, and regulates the amount of heme in the body.

HO-1 is found in many cells, but it’s synthesis is induced only by stimuli like stress, cytokines, hypoxia or ischemia.


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