BN 52021 displaces [3H]paf-acether from, and inhibits its binding to intact human platelets.

Intact platelets incubated at 20 degrees C in the presence of 0.25% (w/v) bovine serum albumin (BSA) bound [3H]paf-acether in a concentration-dependent (0-6.5 nM) manner. Specific [3H]paf-acether binding in the presence of unlabelled paf-acether or BN 52021, a chemically defined ginkgolide, reached a plateau of 14.5 +/- 5 or 17.5 +/- 7.0 fmol at concentrations higher than 0.65 nM [3H]paf-acether. Unlabelled paf-acether or BN 52021 inhibited and displaced [3H]paf-acether binding in a concentration- and time-dependent manner reaching a plateau at 5 nM or 5 microM. Unlabelled paf-acether inhibited [3H]paf-acether binding (75.1 +/- 8%) more strongly if labelled and unlabelled ligands were incubated together than after 15 min preincubation with [3H]paf-acether alone (32.2 +/- 8.6%). The enantiomer of paf-acether (50 nM) failed to displace platelet-bound [3H]paf-acether and the possibility of degradation of [3H]paf-acether by platelets or BN 52021 was excluded under binding conditions. The results demonstrate the presence of specific binding sites for paf-acether in human platelets and the direct competition by BN 52021 for these sites. Serum albumin inhibited total and enhanced specific [3H]paf-acether binding. Radiolabelled albumin itself did not bind to human platelets at 20 degrees C and preincubation of platelets with antibodies to human serum albumin did not inhibit specific paf-acether binding but increased total [3H]paf-acether binding slightly. Serum albumin appears as a necessary phospholipid carrier for specific [3H]paf-acether binding.