Characterization of diadenosine tetraphosphate (Ap4A) binding sites in cultured chromaffin cells: evidence for a P2y site

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Abstract
1 Diadenosine tetraphosphate (Ap4A) a dinucleotide, which is stored in secretory granules, presents two types of high affinity binding sites in chromaffin cells. A Kd value of 8 ± 0.65 × 10−11 m and Bmax value of 5420 ± 450 sites per cell were obtained for the high affinity binding site. A Kd value of 5.6 ± 0.53 × 10−9 m and a Bmax value close to 70,000 sites per cell were obtained for the second binding site with high affinity. 2 The diadenosine polyphosphates, Ap3A, Ap4A, Ap5A and Ap6A, displaced [3H]-Ap4A from the two binding sites, the Ki values being 1.0 nm, 0.013 nm, 0.013 nm and 0.013 nm for the very high affinity binding site and 0.5 μm, 0.13 μm, 0.062 μm and 0.75 μm for the second binding site. 3 The ATP analogues displaced [3H]-Ap4A with the potency order of the P2y receptors, adenosine 5′-O-(2 thiodiphosphate) (ADP-β-S) > 5′-adenylyl imidodiphosphate (AMP-PNP) > α,β-methylene ATP (α,β-MeATP), in both binding sites. The Ki, values were respectively 0.075 nm, 0.2 nm and 0.75 nm for the very high affinity binding site and 0.125 μm, 0.5 μm and 0.9 μm for the second binding site.