Soluble apyrases release adp during ATP hydrolysis

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 microM for ATP and 1.3 s(-1) and 7 microM for ADP as substrates, respectively. One nucleotide binding site was detected on the monomer only in the presence of Ca(2+). In contrast to the membrane bound CD39, soluble CD39 released ADP as an intermediate during ATP hydrolysis, as did the soluble potato apyrase.