Platelet-activating factor (PAF) stimulates the PAF-synthesizing enzyme acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine O2-acetyltransferase and PAF synthesis in neutrophils.

Abstract

Platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) induced in isolated rat peritoneal and human peripheral neutrophils a rapid and potent activation of the PAF biosynthetic enzyme acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine O2-acetyltransferase (EC 2.3.1.67). The PAF-induced activation of the neutrophil acetyltransferase (8-10 times basal neutrophil activity) was maximal within 30 sec after PAF addition, as was the PAF-stimulated degranulation. After 1 min of PAF stimulation, the elevated acetyltransferase activity steadily decreased. Within 2 min of stimulation of neutrophils with 10(-6) M PAF, the 7-fold increase in acetyltransferase activity was coincident with substantial PAF synthesis (as measured by [3H]acetate incorporation into PAF), which was 14% of the PAF synthesis induced by the Ca2+ ionophore A23187 at 10(-5) M. PAF activation of the acetyltransferase and PAF synthesis required intact neutrophils as they did not occur in cells broken by sonication. The neutrophil acetyltransferase was 10-30 times more sensitive to activation by PAF than was degranulation as the acetyltransferase activation was evident with 10(-9) M PAF and was about maximal with 3 x 10(-8) M PAF. The unstimulated and PAF-induced acetyltransferase exhibited the same Km for acetyl-CoA (67 microM), but the Vmax for the PAF-induced enzyme (1667 pmol/min per 10(7) cells) was 10 times that of the unstimulated enzyme (175 pmol/min per 10(7) cells). The PAF induction of the acetyltransferase was less sensitive to inhibition by the specific PAF receptor antagonist L-652,731 than was PAF-induced degranulation. This, along with the differing sensitivities to PAF, suggests that acetyltransferase activation and degranulation induced by PAF either involve two different PAF receptors or involve one receptor type with different receptor occupancy requirements. Escherichia coli alkaline phosphatase, which greatly decreased the activity of the acetyltransferase in spleen microsomes, had little or no effect on the basal or PAF-induced neutrophil acetyltransferase. Thus, by stimulating the activity of acetyltransferase, PAF induces in neutrophils the synthesis of more PAF, thereby probably augmenting the neutrophil response to the initial PAF.