Characterization of the binding of diadenosine 5′,5‴-P1,P4-tetraphosphate (Ap4A) to rat liver cell membranes

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Abstract
Diadenosine polyphosphates present in the extracellular environment can, through interaction with appropriate purinoceptors, influence a range of cellular activities. Here we have investigated the nature of the ligand:receptor interactions involved in diadenosine 5′,5″-P1,P4-tetraphosphate (Ap4A)-mediated stimulation of glycogen breakdown in isolated rat liver cells. [2-3H]Ap4A showed specific binding to both intact isolated liver cells and plasma membrane fractions prepared from isolated liver cells. HPLC analysis confirmed that binding was mediated by intact Ap4A and not by potential breakdown products (e.g. ATP, adenosine etc). Binding of [2-3H]Ap4A, to isolated liver cell plasma membrane preparations, was successfully displaced by a range of both naturally occurring and synthetic diadenosine polyphosphates with the rank order potency Ap4A ⩾Ap5A > Ap6A > Ap3A > Ap2A. [2-3H]Ap4A binding was not displaced by P1 effectors but was successfully displaced by a range of P2 effectors with the rank order potency 2-methylthio-ATP > ATP > ADP ⩾adenosine 5′-[αβ-methylene]triphosphate > adenosine 5′-[βγ-methylene]triphosphate. These findings are consistent with the interaction of Ap4A with a P2y-like subclass of purinoceptor and are discussed in relation to (1) the known purinoceptor populations in liver cell plasma membranes and (2) observations concerning the binding of diadenosine polyphosphates to purinoceptors in other tissues.