Adenosine is a naturally occurring purine nucleoside that forms from the breakdown of adenosine triphosphate (ATP). ATP is the primary energy source in cells for transport systems and many enzymes. Most ATP is hydrolyzed to ADP, which can be further dephosphorylated to AMP. Most ADP and AMP that form in the cell is rephosphorylated in the mitochondria by enzymatic reactions requiring oxygen. If there are large amounts of ATP hydrolyzed, and especially if there is insufficient oxygen available (i.e., hypoxia), then some of the AMP can be further dephosphorylated to adenosine by the cell membrane associated enzyme, 5'-nucleotidase.