An ecto-nucleoside triphosphate diphosphohydrolase (ecto-NTPDase) has been cloned from human liver RNA by RT-PCR. The 1.5 kb cDNA codes for a protein of 495 amino acids. Sequence analysis indicated that it is most closely related to a chicken ecto-ATPDase previously cloned in our laboratory [Knowles et al. (2002) Eur. J. Biochem. 269, 2373-2382] and a mouse homologue that has been designated as E-NTPDase 8 [Bigonnesses et al. (2004) Biochemistry 43, 5511-5519]. The human E-NTPDase 8 has similar topology as the avian and mouse E-NTPDase 8 but has fewer potential N-glycosylation sites and only two amino acid residues in the cytoplasm at its C-terminus. Despite 52% identity in primary structures, enzymatic properties of human E-NTPDase 8 expressed in HEK293 cells differ from that of the chicken E-NTPDase 8. In contrast to the chicken E-NTPDase 8, the human E-NTPDase 8 hydrolyzes MgADP poorly and is inhibited by several detergents as well as benzyl alcohol; the latter attribute may be related to weaker interaction of the transmembranous domains of the human E-NTPDase 8. To demonstrate that inhibition by detergents is mediated by the transmembranous domains, a recombinant pSecTag2 plasmid containing the extracellular domain (ECD) of the human E-NTPDase 8 was constructed. The soluble human E-NTPDase 8 which was secreted into the culture media of transfected HEK293 cells was purified by ammonium sulfate fractionation and nickel affinity chromatography. Besides becoming resistant to detergent inhibition, the soluble human E-NTPDase 8 ECD displays greater activity with Ca nucleotide substrates, an increased affinity for ATP, different pH dependence, and a decreased sensitivity to azide inhibition when compared to the membrane-bound enzyme. These differences may result from the different conformations that the ECD assume without or with constraints exerted by the transmembranous domains. These results indicate that the transmembranous domains are important in regulating enzyme activity as well as in determining the structure of human E-NTPDase 8.