An Ecto‐Nucleotide Pyrophosphatase Is One of the Main Enzymes Involved in the Extracellular Metabolism of ATP in Rat C6 Glioma

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Abstract
: The presence of a nucleotide pyrophosphatase (EC 3.6.1.9) on the plasma membrane of rat C6 glioma has been demonstrated by analysis of the hydrolysis of ATP labeled in the base and in the α‐and γ‐phosphates. The enzyme degraded ATP into AMP and PPi and, depending on the ATP concentration, accounted for ~50‐75% of the extracellular degradation of ATP. The association of the enzyme with the plasma membrane was confirmed by ATP hydrolysis in the presence of a varying concentration of pyridoxal phosphate‐6‐azophenyl‐2′,4′‐disulfonic acid (PPADS), a membrane‐impermeable inhibitor of the enzyme. PPADS concentration above 20 μM abolished the degradation of ATP into AMP and PPi. The nucleotide pyrophosphatase has an alkaline pH optimum and a Km for ATP of 17 ± 5 μM. The enzyme has a broad substrate specificity and hydrolyzes nucleoside triphosphates, nucleoside diphosphates, dinucleoside polyphosphates, and nucleoside monophosphate esters but is inhibited by nucleoside monophosphates, adenosine 3′,5′‐bisphosphate, and PPADS. The substrate specificity characterizes the enzyme as a nucleotide pyrophosphatase/phosphodiesterase I (PD‐I). Immunoblotting and autoadenylylation identified the enzyme as a plasma cell differentiation antigen‐related protein. Hydrolysis of ATP terminates the autophosphorylation of a nucleoside diphosphate kinase (NDPK/nm23) detected in the conditioned medium of C6 cultures. A function of the pyrophosphatase/PD‐I and NDPK in the purinergic and pyrimidinergic signal transduction in C6 is discussed.