The enantiomer and the positional isomer of platelet-activating factor.

We have compared for rabbit platelet aggregating and desensitizing activity two different preparations of platelet-activating factor (PAF-acether) (1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine) and of its enantiomer (3-O-alkyl-2-O-acetyl-sn-glycero-1-phosphocholine). After phospholipase A2 treatment, the unnatural enantiomers appeared about 3000-fold less active than PAF-acether, a result which definitively establishes the stereospecificity of the mode of action of this mediator. A new structural analog of PAF-acether, 1-O-hexadecyl-3-O-acetyl-sn-glycero-2-phosphocholine, was isolated and characterized. It was a weak platelet agonist, stressing further the importance for PAF-acether activity of the acetyl group at position 2 of the glycerol.