Reactions of ¹⁴C-ADP and ¹⁴C-ATP With Washed Platelets From Rabbits

Using suspensions of washed rabbit platelets that react to low concentrations of ADP in the same way as platelets in plasma, it has been found that the major change in added ¹⁴C-ADP is its conversion to ¹⁴C-ATP, catalyzed by nucleoside diphosphokinase at the platelet membrane. This enzyme is readily released from the platelets but the rate and extent of ¹⁴C-ADP to ¹⁴C-ATP conversion are greater in the presence of platelets than in the suspending fluid from which they have been removed. The platelets must provide the source of high-energy phosphate for the reaction, either from ATP released into the suspending fluid, or by the transfer of high-energy phosphate across the membrane. Thrombin stimulates the conversion, probably because it releases ATP from the platelets. Calcium and magnesium stimulate the conversion, whereas two of the inhibitors of ADP-induced aggregation (AMP and parachloromercuribenzenesulfonate) inhibit it. The ATPase activity of these platelets is low and it is not released into the suspending fluid. No adenylate kinase activity was demonstrable. The loss of high energy phosphate from the platelets that occurs during the conversion of ¹⁴C-ADP to ¹⁴C-ATP may be involved in the initiation of their response to ADP.