Characterization of ectonucleotidases on vascular smooth-muscle cells

Abstract

We compared the properties of the ectonucleotidases (nucleoside triphosphatase, EC 3.6.1.15; nucleoside diphosphatase, EC 3.6.1.6; 5'-nucleotidase, EC 3.1.3.5) in intract pig aortic smooth-muscle cells in culture with the properties that we previously investigated for ectonucleotidases of aortic endothelial cells. In experiments with nucleotide phosphorothioate diastereoisomers, stereoselective catabolism of adenosine 5'-[β-thio]triphosphate, but not of adenosine 5'-[α-thio]triphoshate, by the triphosphatase and stereoselective catabolism of adenosine 5'-[α-thio]diphosphate by the diphosphatase were found, as occurs in endothelial cells. In contrast with endothelial ecto-5'-nucelotidase, the smooth-muscle-cell enzyme catabolized adenosine 5'-monophosphorothiote (AMPS) to adenosine: the affinity of the enzyme for AMPS was greater than for AMP, and V(max) for AMPS was about one-sixth that for AMP. In both cell types AMPS was an apparently competitive inhibitor of AMP catabolsim by 5'-nucleotidase. The relative rates of catabolism of nucleotide enantiomers in which the natural D-ribofuranosyl moiety is replaced by an L-ribofuranosyl moiety were similar to those in endothelial cells. No ectopyrophosphatase activity was detected in smooth-muscle cells, in contrast with endothelial cells, where modest activity is present.