Demonstration of a novel ecto-enzyme on human erythrocytes, capable of degrading ADP and of inhibiting ADP-induced platelet aggregation

The role of ADP as an important inducer of platelet aggregation is generally accepted. Therefore it has been postulated by many authors that the enzymatic removal of extracellular ADP from the circulation is essential to avoid platelet aggregation and thrombus formation. Here we show that erythrocytes essentially contribute to the clearance of ADP. The removal of ADP from suspensions of washed human erythrocytes was due to at least two different activities. One activity, which had already been observed by earlier workers, was identified as adenylate kinase, on the basis of the reaction products and the inhibition by adenosine(5')pentaphospho(5')adenosine (Ap5A). This enzyme was not associated with the cells and was always detectable in cell-free supernatants, indicating that the enzyme had leaked from the cells into the extracellular medium. In contrast, the second activity, which is described here for the first time, was tightly bound to the cells. The activity was not inhibited by Ap5A. The main product of the reaction was AMP, and enzyme activity depended on the presence of divalent cations. The Michaelis constant was about 28 mumol/l. This activity seemed to be an ecto-ADPase. Studies with various inhibitors revealed that degradation of ADP was not due to a non-specific phosphatase. Besides the well known ADPase on the endothelium, the ecto-activity on erythrocytes may play an important part in destroying pro-aggregatory ADP.