title: 2-Methylthioadenosine[beta-32P]diphosphate. An agonist and radioligand for the receptor that inhibits the accumulation of cyclic AMP in intact blood platelets.
abstract: 2-Methylthio-ADP and its radioactive analogue [beta-32P]2-methylthio-ADP were synthesized and used to investigate platelet receptors for ADP. 2-Methylthio-ADP induced platelet aggregation and shape change, and inhibited cyclic AMP accumulation in platelets exposed to prostaglandin E1. Compared with ADP, 2-methylthio-ADP was 3-5 times as active as an aggregating agent and 150-200 times as active as an inhibitor of cyclic AMP accumulation. Binding of [beta-32P]2-methylthio-ADP to platelets was measured after centrifuging them through silicone oil to separate platelets from their suspension medium. Binding was reversible, saturable, and specific, with between 400 and 1,200 sites/cell in different platelet preparations. There was no evidence for a second class of binding sites with different affinity. The second order association rate constant was approximately 3.5 X 10(6) M-1 S-1, and the first order dissociation rate was 0.024 s-1, both measured at 23 degrees C. The dissociation equilibrium constant (approximately 15 nM) was about three times higher than the concentration giving half-maximal inhibition of prostaglandin E1-stimulated cyclic AMP accumulation in platelet-rich plasma. Binding was inhibited by ADP (Ki = 3.5 microM), ATP (7 microM), 2-azido-ADP (0.12 microM), inosine diphosphate (IDP, 150 microM), guanosine diphosphate (GDP, 350 microM), and AMP (800 microM). Binding of 2-methylthio-ADP was also blocked by the non-cell-penetrating thiol reagent, p-mercuribenzene sulphonate, a reagent that blocks the inhibition of adenylate cyclase by ADP, but which does not block the ability of ADP to induce aggregation or platelet shape change. The amount of 2-methylthio-ADP bound at saturation was independent of pH in the range 6-8, but the affinity was reduced at pH 6 compared with pH 6.5-8.0. The dissociation constant was not temperature dependent in the range 32 degrees -40 degrees C, whereas the rate of dissociation of 2-methylthio-ADP from platelets after the addition of an excess of ADP approximately doubled over this range. The activation energy for dissociation was approximately 15 kcal/mol. Our results support the conclusion that platelets have a receptor for ADP, which inhibits cyclic AMP accumulation, and which has a sulphydryl group in the binding pocket.
mesh_terms: D000244:Adenosine Diphosphate / Q000031:analogs & derivatives* / Q000097:blood / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism*; D000242:Cyclic AMP / Q000097:blood*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D011956:Receptors, Cell Surface / Q000187:drug effects* / Q000378:metabolism; D011983:Receptors, Purinergic; D013696:Temperature; D013873:Thionucleotides / Q000097:blood / Q000494:pharmacology*
chemical_list: D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D013873:Thionucleotides; C002480:methylthio-ADP; D000244:Adenosine Diphosphate; D000242:Cyclic AMP