Bacterial expression and characterization of a novel, soluble, calcium-binding, and calcium-activated human nucleotidase.

A newly discovered human analogue of a bed bug apyrase, which we named hSCAN-1 for human soluble calcium-activated nucleotidase-1, was expressed in bacteria, refolded from inclusion bodies, purified, and characterized. This apyrase, which is distinct from the eNTPDases exemplified by the endothelial CD39 (NTPDase1) apyrase, is a 38 kDa monomeric enzyme capable of hydrolyzing a variety of nucleoside di- and triphosphates, but not monophosphates. Preferred substrates include GDP, UDP, and IDP, with a pH optimum for activity between 6 and 7. The specific activity and substrate preference of the bacterially expressed enzyme closely mimic those of the enzyme expressed in mammalian COS cells, as well as the enzyme synthesized in an in vitro bacterial expression system. This suggests that glycosylation and other posttranslational modifications that do not occur in bacteria are not necessary for nucleotidase activity or proper folding of this human apyrase. hSCAN-1 absolutely requires Ca(2+), but not Mg(2+) or other divalent cations analyzed, for enzymatic activity. Surprisingly, the activity does not increase in a quasi-linear fashion at sub-millimolar Ca(2+) concentrations, as would be expected if Ca(2+) were only used as a cosubstrate for the nucleotide substrate, but rather follows a sigmoidal curve. The intrinsic fluorescence and difference absorption studies of hSCAN-1 in the absence of nucleotides revealed Ca(2+)-induced changes in the environment of tryptophan and tyrosine residues with half-saturation at about 90 microM Ca(2+). NaCl increased the half-saturating Ca(2+) concentration needed for both structural changes detected by optical spectroscopy and enzymatic activation of hSCAN-1 detected by nucleotidase assay. These results suggest that Ca(2+) triggers a conformational change in hSCAN-1, converting the enzymatically inactive protein to the active enzyme, in addition to forming the metal-nucleotide substrate complex necessary for nucleotidase activity.