Apyrase is a calcium-activated plasma membrane-bound enzyme (magnesium can also activate it) () 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.

It is the enzyme released by female mosquitoes to prevent blood clots when they suck blood.

The activity of apyrase at pH 7.5 in a Tris Buffer is approximately 80% of the activity at pH 6.5.

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