Acylphosphatase possesses nucleoside triphosphatase and nucleoside diphosphatase activities

We have demonstrated that acylphosphatase possesses ATP-diphosphohydrolase (apyrase-like) activity. In fact, acylphosphatase first catalyses the hydro lysis of the gamma-phosphate group of nucleoside triphosphates, and then at tacks the beta-phosphate group of the initially produced nucleoside diphosp hates, generating nucleoside monophosphates. In constrast, it binds nucleos ide monophosphates but does not catalyse their hydrolyses. The calculated k (cat) values for the nucleoside triphosphatase activity of acylphosphatase are of the same order of magnitude as those displayed by certain G-proteins , An acidic environment enhances the apyrase-like activity of acylphosphata se. The true nucleotide substrates of acylphosphatase are free nucleoside d i- and tri-phosphates, as indicated by the Mg2+ ion inhibition of the activ ity. We have also demonstrated that, although nucleoside triphosphates are still hydrolysed at pH 7.2 and 37 degrees C, in the presence of millimolar Mg2+ concentrations this occurs at a lower rate. Taken together with the pr eviously observed strong increase of acylphosphatase levels during induced cell differentiation, our findings suggest that acylphosphatase plays an ac tive role in the differentiation process (as well as in other processes, su ch as apoptosis) by modulating the ratio between the cellular levels of nuc leoside diphosphates and nucleoside triphosphates.