Apyrase (EC , ATP-diphosphatase, adenosine diphosphatase, ADPase, ATP diphosphohydrolase) is a calcium-activated plasma membrane-bound enzyme (magnesium can also activate it) (EC ) that catalyses the hydrolysis of ATP to yield AMP and inorganic phosphate. Two isoenzymes are found in commercial preparations from S. tuberosum. One with a higher ratio of substrate selectivity for ATP:ADP (approx 10) and another with no selectivity (ratio 1).

It can also act on ADP and other nucleoside triphosphates and diphosphates with the general reaction being NTP -> NDP + Pi -> NMP + 2Pi. This is the same activity that has been employed in the degradation of unincorporated nucleosides during pyrosequencing.

The salivary apyrases of blood-feeding arthropods are nucleotide hydrolysing enzymes that are implicated in the inhibition of host platelet aggregation through the hydrolysis of extracellular adenosine diphosphate.

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