Soluble purine-converting enzymes circulate in human blood and regulate extracellular ATP level via counteracting pyrophosphatase and phosphotransfer reactions

Extracellular ATP and other purines play a crucial role in the vasculature, and their turnover is selectively governed by a network of ectoenzymes expressed both on endothelial and hematopoietic cells. By studying the whole pattern of purine metabolism in human serum, we revealed the existence of soluble enzymes capable of both inactivating and transphosphorylating circulating purines. Evidence for this was obtained by using independent assays, including chromatographic analyses with H-3-labeled and unlabeled nucleotides and adenosine, direct transfer of gamma-terminal phosphate from [gamma-P-32]ATP to NDP/AMP, and bioluminescent measurement of ATP metabolism. Based on substrate-specificity and competitive studies, we identified three purine-inactivating enzymes in human serum, nucleotide pyrophosphatase (EC 3.6.1.9), 5'-nucleotidase (EC 3.1.3.5), and adenosine deaminase (EC 3.5.4.4), whereas an opposite ATP-generating pathway is represented by adenylate kinase (EC 2.7.4.3) and NDP kinase (EC 2.7.4.6). Comparative kinetic analysis revealed that the V-max values for soluble nucleotide kinases significantly exceed those of counteracting nucleotidases, whereas the apparent K-m values for serum enzymes were fairly comparable and varied within a range of 40-70 mumol/l. Identification of soluble enzymes contributing, along with membrane-bound ectoenzymes, to the active cycling between circulating ATP and other purines provides a novel insight into the regulatory mechanisms of purine homeostasis in the blood.