NTPDase5 (CD39L4), encoded by the ENTPD5 gene, is an E-NTPDase family member that is secreted to the extracellular space where it hydrolyzes nucleoside diphosphates UDP, GDP, CDP and ADP (listed in the order of preference) to nucleoside monophosphates UMP, GMP, CMP and AMP, respectively. In vitro, NTPDase5 can hydrolyze nucleoside triphosphates GTP, CTP, UTP and ATP to corresponding nucleoside diphosphates but with very low efficiency. NTPDase5 requires Ca2+ or Mg2+ for catalytic activity (Mulero et al. 1999). NTPDase5 is most catalytically active as a monomer, although it can also form disulfide-linked dimers (Mulero et al. 2000).
NTPDase5 may function in the endoplasmic reticulum (ER), where its UDPase activity could contribute to protein glycosylation and folding. NTPDase5 may alleviate ER stress induced by protein overload caused by oncogenic PI3K/AKT signaling in cancer cells. NTPDase5 is over-expressed in tumors with activated AKT and is known as the PCPH oncogene. The underlying mechanism of NTPDase5 over-expression may be AKT-mediated inhibition of FOXO proteins, which are probable transcriptional repressors of the ENTPD5 gene (Fang et al. 2010, Shen et al. 2011).