A soluble form of CD39 was expressed and purified from High-Five insect cells. The soluble CD39 is a monomer with a molecular weight of 54,000. The k(cat) and K-m of the purified soluble CD39 were 4.6 s(-1) and 12 muM for ATP and 1.3 s(-1) and 7 muM for ADP as substrates, respectively. One nucleotide binding site was detected on the monomer only in the presence of Ca2+. In contrast to the membrane bound CD39, soluble CD39 released ADP as an intermediate during ATP hydrolysis, as did the soluble potato apyrase,