Adenosine metabolism and its role in cell physiology

Adenosine is not just a major component of important molecules such as ATP, RNA or cAMP, but also has its own signaling function. Therefore, its extracellular level is strictly maintained by balance in its formation, degradation and transport. Both inside and outside the cell adenosine is formed mainly through degradation of ATP and is eliminated by two enzymes, adenosine kinase and adenosine deaminase. Transport of adenosine through the cell membrane is provided by nucleoside transporters, which are either equilibrative or concentrative according to the mechanism of transfer. All three processes described above contribute to maintaining adenosine level under normal conditions and its increase in pathological situations. Extracellular adenosine as a signal molecule binds to adenosine receptors (subtypes A1, A2A, A2B, A3) that affect many cellular signaling pathways via G-proteins. By these pathways adenosine regulates energy homeostasis, controls the function of various organs and also modulates the nervous and immune system and thus it may participate in a number of pathological processes. Pharmacological affecting of specific adenosine receptors or enzymes involved in its metabolism can serve as an effective therapy. Some drugs based on this system are already in use, others are being tested, and many..