Stimulation of monocytes and platelets by short-chain phosphatidylcholines with and without terminal carboxyl group

Oxidation of unsaturated phosphatidylcholine (PC) produces fragmented phospholipids which have similar bioactivities as the platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-PC). Since a large number of molecular species are produced upon PC oxidation, the active ingredients have not been identified. We synthesized several short-chain PCs which are known to be characteristic PC oxidation products to test their PAF-like activity. The synthetic PCs contained palmitoyl or hexadecyl residues (both C₁₆) in sn-1 position, and propionyl (C₃), valeroyl (C₅), succinyl (C₄ with ω-carboxyl), glutaroyl (C₅ with ω-carboxyl), or suberoyl (C₈ with ω-carboxyl) residues in sn-2 position. Biological activity was measured by: (1) increase of intracellular calcium in human monocytes; (2) [³H]serotonin release from rabbit platelets; and (3) aggregation of human platelets. Specificity of the cellular response was tested by inhibition with the PAF-receptor antagonists BN 52021 and WEB 2086. Synthetic PC oxidation products activated both monocytes and platelets in a PAF-specific manner. The effective concentration varied with respect to assay system and chemical structure. In general, 1-hexadecyl-PCs were more effective than 1-palmitoyl-PCs, while increasing chain length in sn-2 position lowered biological activity. However, several 1-palmitoyl-PCs activated monocytes in concentrations between 10⁻⁸ and 10⁻⁶ M. In contrast, platelets were less susceptible to 1-palmitoyl-PCs. No significant difference was found between 2-valeroyl-PC (C₅ with ω-methyl) and 2-glutaroyl-PC (C₅ with ω-carboxyl). The data suggest that typical products of PC oxidation, containing propionyl, succinyl, or glutaroyl residues in sn-2 position, display PAF-like activity at micromolar concentrations.