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

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Abstract
The interactions of dipyridamole with α1 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 α1 acid glycoprotein with a dissociation constant of 1.6 μM. 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 pattern with a dissociation constant of 0.04 μM. Approximately 2 × 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 μM. In the presence of α1 acid glycoprotein of plasma, the binding of dipyridamole to human platelets is inhibited. Correspondingly, the dipyridamole inhibition of adenosine uptake by platelets is reduced 1,000-fold by purified α1 acid glycoprotein. The binding of dipyridamole to human platelets was found to be essential for its inhibition of adenosine uptake by platelets. Dipyridamole decreases the incorporation of [14C]adenosine radioactivity in platelet nucleotides and reduces the [14C]-ATP to [14C]ADP ratio. Purified α1 acid glycoprotein reverses these effects of dipyridamole on adenosine metabolism of platelets in a concentration-dependent manner. An equilibrium of dipyridamole binding to α1 acid glycoprotein and to platelets is proposed.
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