Modulation of Platelet Responses by 2-[3-(Bromo-2-oxopropylthio)]adenosine-5′-diphosphate Involves Its Binding to as Well as Covalent Modification of an ADP-Receptor, Aggregin

The 2-substituted ADP derivatives are known to activate human blood platelets with varying degrees of potency. For example, 2-(4-bromo-2,3-dioxobutylthio)adenosine-5′-diphosphate [2-BDB-TADP], an ADP-affinity analog, was previously shown by us to be 50% as potent as ADP in inducing human blood platelet responses [Puri, R. N., Colman, R. F., and Colman, R. W. (1996)Eur. J. Biochem.236, 862–870]. 2-Methylthio-ADP (2-MeS-ADP) has been known to be a far more potent agonist than ADP. However, the molecular basis for defining the rank order of potency of the 2-substituted ADP derivatives as agonists of platelet responses have been incompletely understood. We now report that 2-BOP-TADP (a one carbon atom lower homolog of 2-BDB-TADP) at equimolar concentration is as potent as ADP in inducing platelet responses. Prolonged incubation of platelets with 2-BOP-TADP abolished its ability to elicit cellular responses. An autoradiogram of the gel obtained by sodium dodecyl sulfate–polyacrylamide gel electrophoresis of solubilized platelets labeled by incubating the platelets with 2-BOP-TADP for 1 h followed by reduction by NaB[3H]4showed the presence of a single covalently radiolabeled protein band at 100 kDa. Preincubation of platelets with either ADP or ATP reduced the intensity of the band corresponding to the 100-kDa protein radiolabeled by 2-BOP-TADP and NaB[3H]4. The results show that (i) 2-BOP-TADP modulates ADP-induced platelet responses by interacting with aggregin and (ii) 2-BOP-TADP was twice as potent as 2-BDB-TADP, and (iii) the chain length of the substituent in a homologous series has an important bearing on the potency of a 2-substituted ADP analog.