CHARACTERIZATION OF SPECIFIC BINDING-SITES FOR [H-3] 2-MES-ADP ON MEGAKARYOCYTOBLASTIC CELL-LINES IN CULTURE

Binding of [H-3]2-methyl thio-adenosine 5' diphosphate ([H-3]2-MeS-ADP ), a stable analogue of adenosine 5' diphosphate (ADP) to DAMI and Meg -01, two megakaryocytoblastic cell lines, was time-dependent, reversib le and saturable. Scatchard analysis of the saturation binding data in dicated that [H-3]2-MeS-ADP bound to one class of specific binding sit es with high affinity (dissociation constants = 45.3 +/- 13.4 and 48.2 +/- 17.7 nM, and maximum binding capacities = 341.2 +/- 31.1 and 903 +/- 98 fmole/10(6) cells for DAMI and Meg-01, respectively) (N = 3). U nlabelled 2-MeS-ADP competitively and selectively inhibited the specif ic binding of [H-3]2-MeS-ADP on DAMI and Meg-01 with inhibitory consta nt values of 118 +/- 11 and 38 +/- 11 nM, respectively (N = 3). ADP wa s 3 to 10 times less potent than 2-Mes-ADP in displacing [H-3]2-MeS-AD P from its binding sites on DAMI and Meg-01, whereas other ADP analogu es, such as AMP, GDP, UDP, adenosine or FSBA, did not interfere with t he binding of [H-3]2-MeS-ADP, suggesting that DAMI and Meg-01 contain ADP-specific receptors.