Biochemistry · Molecule
Nitric oxide (NO) !
Nitric oxide is a short-lived gas made in the body that widens blood vessels, acts as a nerve messenger and is used by immune cells to kill microbes.
Updated 2026-11-01
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Key facts
- A free radical gas that diffuses across membranes; its half-life in blood is only seconds.
- Its main receptor is soluble guanylyl cyclase (binds its haem): cGMP rises and smooth muscle relaxes.
- It was identified as the endothelium-derived relaxing factor; it also inhibits platelet aggregation and leukocyte adhesion.
- Haemoglobin inactivates it quickly (forming nitrate and methaemoglobin); with superoxide it forms toxic peroxynitrite.
- Also called
- Endothelium-derived relaxing factor (EDRF)
- Formula
- NO
- PubChem
- CID 145068
Structure
- One nitrogen and one oxygen with an unpaired electron (a radical).
Clinical notes
- Inhaled NO dilates pulmonary vessels in persistent pulmonary hypertension of the newborn.
- Nitroglycerin and nitroprusside act by releasing NO; nitroprusside can also release cyanide.
- Macrophages use NO from iNOS against bacteria and parasites; excess NO drives the vasodilation of septic shock.
Related conditions
Related
- Nitric oxide synthase (NOS)
- Hemoglobin and 2,3-BPG
- Reactive oxygen species and antioxidants
- Superoxide dismutase (SOD)
Test yourself
Think of the answer, then tap the question to see it.
What enzyme is the main receptor of nitric oxide?
Soluble guanylyl cyclase, whose haem binds NO and raises cGMP.
Why does nitric oxide act only locally?
It is a reactive radical inactivated within seconds, especially by haemoglobin.
What toxic compound forms when NO meets superoxide?
Peroxynitrite (ONOO⁻), which damages proteins, lipids and DNA.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Nitric Oxide (CID 145068) (PubChem, NCBI, 2026); PDB-101: Molecule of the Month: Nitric Oxide Synthase (RCSB Protein Data Bank (PDB-101), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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