Placental ecto-ATP diphosphohydrolase: Its structural feature distinct from CD39, localization and inhibition on shear-induced platelet aggregation

Human placental ecto-ATP diphosphohydrolase (ATPDase), an 82 kDa single- chain glycoprotein, was purified to high specific activity using a specific murine monoclonal antibody MK33 (IgG1-κ). Structurally, protein-based analysis showed this enzyme to be almost identical to that of CD39 lymphoid cell activation antigen deduced by cDNA sequencing (Maliszewski CR, et al, J Immunol 1994;153:3574); but differing in the NH₂-terminal amino acid sequence, suggesting that placental ecto-ATPDase is most likely an isoform of CD39 generated by alternative splicing of the pre-mRNA. Functionally, placental ecto-ATPDase totally inhibits the secondary platelet aggregation induced by agonists at a final concentration (f.c.) of 1 μg/ml. The purified enzyme (1 μg/ml, final), pre-incubated with washed platelets prior to α- thrombin stimulation, completely inhibits the activation of platelet glycoprotein (GP) IIb/IIIa, thereby blocking the binding of fibrinogen or von Willebrand factor to platelets. Further, under different shear stresses, the enzyme modulates platelet aggregation differently. Low shear stress-induced platelet aggregation is blocked by this enzyme in a dose-dependent manner and is totally blocked at f.c. 0.5 μg/ml. Under high shear stress, however, this protein at a f.c. of 0.5 μg/ml mediates almost complete disaggregation of platelets without affecting the initial aggregation. Using immunohistochemical analysis, this enzyme was observed to be localized at the syncytiotrophoblasts of placental microvilli and the endothelial cells (ECs) of the umbilical vein obtained at full-term normal delivery, but scarcely at the ECs of the umbilical artery.