CD39-L4 is a secreted human apyrase, specific for the hydrolysis of nucleoside diphosphates

The human ecto-apyrase gene family consists of five reported members (CD39, CD39-L1, CD39-L2, CD39-L3, and CD39-L4). The family can be subdivided into two groups by conservation of proposed structural domains. The CD39, CD39- L1, and CD39-L3 genes all encode hydrophobic portions in their carboxy and amino termini, serving as transmembrane domains for CD39 and potentially fo r the other two members. CD39-L2 and CD39-L4 genes encode hydrophobic porti ons in their amino termini, suggesting that they might encode secreted apyr ases. We demonstrate that the CD39-L4 gene encodes the first reported human secreted ecto-apyrase, COS-7 cells transfected with a CD39-L4 expression c onstruct utilizing the naturally occurring leader peptide express recombina nt protein outside of the cells. This expression can be blocked by brefeldi n A, a chemical that inhibits a step in mammalian secretory pathways. We al so demonstrate expression of CD39-L4 message in macrophages, suggesting tha t the protein is present in the circulation. Furthermore, we show that CD39 -L4 is an E-type apyrase, is dependent on calcium and magnesium cations, an d has high degree of specificity for NDPs over NTPs as enzymatic substrates . A potential physiological role in hemostasis and platelet aggregation is presented.