Binding of Dipyridamole to Human Platelets and to α1 Acid Glycoprotein and its Significance for the Inhibition of Adenosine Uptake

Abstract

A B S T R A C T The interactions of dipyridamole with a, acid glycoprotein of plasma and with human platelets are related to inhibition of adenosine uptake by platelets. Binding studies by equilibrium gel filtration suggested that 1 mol of dipyridamole binds per mol of a1 acid glycoprotein with a dissociation constant of 1.6 ,M. Platelets contain two populations of binding sites, one with high and another with lower affinity for the drug. The binding of dipyridamole to the high-affinity sites follows a Michaelis-Menten binding pattem with a dissociation constant of 0.04 ,.M. Approximately 2 x 104 dipyridamole molecules are bound at the high-affinity sites of each platelet. The lower affinity sites bind the drug with a dissociation constant of 4 ,uM. In the presence of a, acid glycoprotein of plasma, the binding of dipyridamole to human platelets is inhibited. Correspondingly, the dipyridamole inhibition of adenosine uptake by plate- lets is reduced 1,000-fold by purified a1 acid glyco- protein. The binding of dipyridamole to human plate- lets was found to be essential for its inhibition of adenosine uptake by platelets. Dipyridamole de- creases the incorporation of ['4C]adenosine radioac- tivity in platelet nucleotides and reduces the ["4C]-ATP to [14C]ADP ratio. Purified a, acid glyco- protein reverses these effects of dipyridamole on adenosine metabolism of platelets in a concentration- dependent manner. An equilibrium of dipyridamole