Fig. 1. Hydrolysis of various NPP3 substrates: ATP, AP₄A, and UDP-glucose are natural substrates of NPP3, p-Nph-5′-TMP is an artificial substrate used for colorimetric enzyme assays. AMP can subsequently be hydrolyzed to adenosine by ecto-5′-nucleotidase (CD73). NPP3 is expressed on immune cells, e.g. activated basophils and mast cells, suppressing immune activation by ATP thereby preventing chronic allergic inflammation.²² Immune cells such as leukocytes and T-lymphocytes can co-express other ATP-hydrolyzing ectonucleotidases such as NPP1 and CD39.^23,24 NPP3 is also highly expressed in the proximal region of the intestine and has been implicated in the hydrolysis of dietary nucleotides.^2,25.