Figure 1. Schematic representation of extracellular pyrophosphate metabolism. Ectonucleotide pyrophosphatase phosphodiesterase (eNPP) hydrolyzes adenosine triphosphate (ATP) to release adenosine monophosphate (AMP) and pyrophosphate (PPi). By contrast, ectonucleoside triphosphate diphosphohydrolase (eNTPD), hydrolyzes ATP to release AMP and phosphate (Pi). Pyrophosphate is degraded to phosphate by tissue non-specific alkaline phosphatase (TNAP). Inhibition of eNTPD and TNAP activities may increase availability of both ATP and pyrophosphate.