35. NO and CO as signal molecules

Learning objectives

• Why do gaseous hormones not require a cell surface receptor?
• Describe the half-life of NO
• Which enzyme synthesises NO?
• Describe the functions and differences of the various types of NOS
• Describe the pathway of NO as a vasodilator
• Describe the breakdown of cGMP
• Which enzyme synthesises CO?
• Describe the mechanism of toxicity of CO

Nitric oxide

Nitric oxide, or NO, is a colourless gas that is toxic in high doses. In physiological concentrations functions as a neurotransmitter and a vasodilator.

Because NO is a gas it freely diffuses across cell membranes and therefore doesn’t require a cell surface receptor to mediate its actions.

NO rapidly reacts with the free radical superoxide to form peroxynitrite (ONOO-), causing it to have a half-life of only a few seconds.

Synthesis

NO is synthesized by NO synthase (NOS), an enzyme which converts arginine to citrulline + NO. There are exist three forms of NO synthase, the neuronal type, the endothelial type, and the inducible type.

Neuronal NO synthase (nNOS) is present in neural tissue. It produces NO to be used as a neurotransmitter.

Endothelial NO synthase (eNOS) is present in endothelium. It produces NO to be used as a vasodilator, which is explained below.

Inducible NO synthase (iNOS) is present in macrophages and neutrophils. It produces NO to be used in the immune response. iNOS is activated in oxidative environments, causing the produces NO to rapidly react with superoxide to form large amounts of peroxynitrite. This is part of the oxidative burst, a process by which immune cells degrade phagocytosed pathogens.

Like the name implies iNOS is only induced when needed, in contrast to eNOS and nNOS, which are constitutively expressed.

iNOS and nNOS are soluble, which means they’re found in the cytosol, while eNOS is bound to the cell membrane.

NO as a vasodilator and protein kinase G

After NO has been synthesised in the endothelium it diffuses across the cell membrane and into the vascular smooth muscle. There NO will enter the cell and activate soluble guanylyl cyclase, an enzyme that catalyses the conversion of GTP to cGMP. cGMP is a second messenger which activates Protein Kinase G.

PKG is important in the vasodilatory response of NO. After being activated PKG activates multiple processes, all of which cause vasodilation.

PKG prevents outflow of calcium from the sarcoplasmic reticulum in vascular smooth muscle. This causes the smooth muscle to relax, causing vasodilation. PKG also activates myosin light chain phosphatase, which dephosphorylates myosin, causing vascular smooth muscle relaxation.

Breakdown of cGMP

The effect of endothelial NO is terminated by phosphodiesterase type 5 (PDE5), which breaks down cGMP to GMP.

Drugs which inhibit this enzyme are widely used. By inhibiting PDE5 the vasodilatory response of NO is increased. Drugs which inhibit PDE5 are used to treat erectile dysfunction (sildenafil/viagra) and pulmonary hypertension.

CO – carbon monoxide

Carbon monoxide is also toxic in high concentrations, but like NO it has physiological roles in low concentration.

CO is a gas like NO, allowing it to freely diffuse across cell membranes.

Synthesis and function

CO is synthesized by heme oxygenase (HO), which converts heme to biliverdin and CO. The enzyme requires NADPH as a cofactor. Two variations of HO exist, HO-1 and HO-2.

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

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

Toxicity

CO has much higher affinity to haemoglobin than oxygen, causing it to bind much easier to haemoglobin. This inhibits oxygen transport.

CO also inhibits complex IV of the oxidative phosphorylation.

Summary

• Why do gaseous hormones not require a cell surface receptor?
  • Because they freely diffuse across cell membranes
• Describe the half-life of NO
  • NO has a half-life of only a few seconds as it rapidly reacts with superoxide to form peroxynitrite
• Which enzyme synthesises NO?
  • NO synthase
• Describe the functions and differences of the various types of NOS
  • nNOS is present in neural tissue and produces NO to be used as a neurotransmitter
  • iNOS is present in macrophages and neutrophils and produces NO to be used in the oxidative burst
  • eNOS is present in endothelium and produces NO to induce vasodilation
• Describe the pathway of NO as a vasodilator
  • eNOS produces NO in the endothelium, which diffuses into the vascular smooth muscle
  • NO activates soluble guanylyl cyclase, increasing the level of cGMP
  • cGMP activates protein kinase G, which causes the vascular smooth muscle to relax
• Describe the breakdown of cGMP
  • cGMP is broken down by PDE5
• Which enzyme synthesises CO?
  • Haeme oxidase
• Describe the mechanism of toxicity of CO
  • CO binds to haemoglobin instead of oxygen, inhibiting oxygen transport
  • It also binds to and inhibits complex IV of the oxidative phosphorylation