Apyrase (EC220.127.116.11, ATP-diphosphatase, adenosine diphosphatase, ADPase, ATP diphosphohydrolase) is a calcium-activated plasma membrane-bound enzyme (magnesium can also activate it) (EC18.104.22.168) 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).
^Smith TM, Hicks-Berger CA, Kim S, Kirley TL (October 2002). "Cloning, expression, and characterization of a soluble calcium-activated nucleotidase, a human enzyme belonging to a new family of extracellular nucleotidases". Arch. Biochem. Biophys.406 (1): 105–15. doi:10.1016/S0003-9861(02)00420-4. PMID12234496.