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Metabolism of Adenine Nucleotides in Human Blood

Biologically active concentrations of potently vasoactivc and platelet-active adenine nucleotides are generated in plasma by a variety of pathophysiological mechanisms. Although there is evidence that ATP and ADP are inactivated by endothelial ectonucleotidases, there has been little attempt to study the metabolic routes of their catabolism in blood or to assess the contribution of this process to their clearance in vivo. Therefore, we have studied the rates and patterns of catabolism of ATP, ADP, and AMP in whole blood, plasma, and isolated blood cells. Rates of degradation of each nucleotide in cell-free plasma ranged from 0.07–032 nmol/min/ml with 1 μM substrates to 1.1–3.6 nmol/min/ml with 100 μM substrates. The pattern of catabolism indicated that sequential dephosphorylation from ATP → ADP → AMP → adenosine occurs. In whole blood, the pattern was similar although ATP and ADP (but not AMP) breakdown was more rapid. This was due to leukocyte ectonucleotidase activity. The use of selective inhibitors demonstrated that catabolism was not due to nonspecific phosphatase activity and that plasma 5-nucleotidase is distinct from ATPase or ADPase. In leukocytes, ATPase and ADPase activities were distinguishable, and each contributed substantially to the rates of catabolism in whole blood. Leukocyte 5-nucleotidase did not measurably contribute to AMP dephosphorylation in blood. By comparison, ecto-ATPase and ecto-ADPase activities on cultured human umbilical vein endothelial cells were similar to those on leukocytes while endothelial S'-nucleotidase per 10* cells was equivalent to the soluble activity in 1 ml of blood or plasma. We conclude that in microvascular beds, endothelial catabolism of circulating nucleotides predominates (as expected from the short half-lives of added nucleotides in vivo), but in larger blood vessels or at sites of impaired blood flow such as within a forming thrombus, leukocyte ecto-ATPase and ecto-ADPase and soluble 5'-nucleotidase may be important regulators of the concentrations of vasoactive nucleotides.