Interaction of Human Platelets with the 2′, 3′-Dialdehyde Derivatives of ADP (o-ADP) and ATP (o-ATP) as Potential Affinity Labels for the ADP Receptor

In order to characterise the receptor responsible for mediating platelet aggregation and secretion induced by ADP we have synthesised oADP and oATP by periodate oxidation of ADP and ATP. oADP induces platelet shape change but inhibits aggregation competitively with respect to ADP. The properties of inhibition of aggregation induced by adrenaline, collagen or thrombin are also consistent with the conclusion that the effects of oADP result from interaction with the ADP receptor(s). The effects of oATP generally resemble those of oADP except that oATP inhibits, rather than induces, shape change and causes this effect at a concentration in excess of that required for inhibition of aggregation. ,

Brief exposure of intact platelets to [³H]-oADP or oATP causes stable incorporation of [³H] into the membrane proteins. Analysis of [³H] distribution by SDS gel electrophoresis after prolonged dialysis against SDS/mercaptoethanol shows predominant incorporation of [³H]-OATP into a fraction at 28000 dal tons over a concentration range consistent with that required for inhibition of aggregation. When [³H]-OADP is used, incorporation occurs predominantly into 4 fractions at 28000, 45000 (probably actin), 65000 and 220000 daltons. The [³H] incorporation/ [oADP] relationship and the time course of incorporation suggest that the 28000 and 65000 dalton fractions are likely candidates for identification as ADP receptors. Our data suggest a tentative identification of the 28000 dalton fraction as the receptor responsible for mediating aggregation induced by ADP and also indicate the possibility that different receptors may be involved in induction of shape change and aggregation by this agonist.