Adenosin

Adenosine, sometimes spelled Adenosin in non-English texts, is a purine nucleoside consisting of adenine attached to a ribose sugar via a beta-N9-glycosidic bond. It is a component of RNA and is also present free in cells and extracellular fluid. In energy metabolism it is closely related to ATP and ADP, being produced during ATP breakdown.

Free adenosine arises from the dephosphorylation of AMP by ectonucleotidases and from ATP degradation. In tissues, adenosine can be taken up by equilibrative nucleoside transporters and either recycled to AMP by adenosine kinase or deaminated to inosine by adenosine deaminase. Extracellular adenosine levels rise during metabolic stress, hypoxia, and inflammation, influencing tissue perfusion and immune responses.

Adenosine acts as a signaling molecule through four G protein–coupled receptors: A1, A2A, A2B, and A3. A1 receptors generally inhibit neuronal activity and slow AV nodal conduction; A2A and A2B receptors elevate cAMP, promoting vasodilation and anti-inflammatory effects; A3 receptors participate in protective cellular responses. In the central nervous system, adenosine promotes sleep and dampens excitability; in the heart, it can reduce heart rate and conduction. Caffeine and theophylline are nonselective antagonists of adenosine receptors, contributing to their stimulant effects.

Clinically, intravenous adenosine is used to acutely terminate certain supraventricular tachycardias by transiently blocking AV nodal conduction. Beyond therapy, adenosine levels reflect metabolic state and can influence inflammation and tissue perfusion. In nucleic acids, adenosine is the RNA nucleoside; in DNA, the analogous base is deoxyadenosine.

---