Human neutrophil platelet-activating factor: molecular heterogeneity in unstimulated and ionophore-stimulated cells

The molecular heterogeneity of platelet-activating factor (PAF) in resting and ionophore (A23187) -stimulated human neutrophils was measured by a very sensitive gas chromatography-negative ion chemical ionization mass spectrometric method. The molecular species compositions of PAF, which are due to variations in the 1-O-alkyl chain length, were significantly different between resting and ionophore-stimulated polymorphonuclear leukocytes. The major species of PAF produced by unstimulated polymorphonuclear leukocytes were 16:0, 17:0, 18:1 and 18:0, representing 55, 14, 8 and 10%, respectively, of the total PAF; 16:0 was the predominant PAF (74%) in A23187-stimulated polymorphonuclear leukocytes. The PAF molecular species from unstimulated polymorphonuclear leukocytes was similar to compositions from those of the precursor 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine, whereas those from the ionophore-stimulated polymorphonuclear leukocytes differed from the precursor 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine, thus indicating a very high degree of substrate selectivity for PAF synthesis. Although the physiological implications of the variations in PAF composition are not known, these studies indicate that the PAF produced by resting polymorphonuclear leukocytes are significantly different from those produced in response to ionophore.