Release of platelet-activating factor in systemic lupus erythematosus

The biologically active 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (platelet-activating factor; PAF) is inactivated in plasma mainly by a specific PAF acetylhydrolase (1-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine acetylhydrolase; EC 3.1.1.48). In the present study, PAF was released in detectable amounts (5.4 +/- 2.9 ng/ml; mean +/- 1 SD) in the plasma of 8 out of 10 patients with systemic lupus erythematosus (SLE) during the most active phases of the disease. PAF was never detectable in the plasma of patients with inactive SLE or of healthy subjects. PAF acetylhydrolase activity was markedly reduced in sera of 10 patients with active SLE as compared to 7 patients with inactive SLE, 16 patients with rheumatoid arthritis (RA), 5 patients with nephrotic syndrome (NS) and 15 healthy subjects. A kinetic study of the enzyme in patients with active SLE suggested an overall reduced activity rather than an intrinsic defect of the enzyme. PAF acetylhydrolase in sera of patients with active SLE shared with that of healthy subjects the same substrate specificity, sensitivity to enzymatic and chemicophysical treatments and association to low-density lipoprotein (LDL), acting as carrier of PAF acetylhydrolase in plasma. However, the protein concentration of LDL was significantly reduced in patients with active SLE as compared to patients with inactive SLE, RA and NS and to healthy subjects, thereby suggesting that the reduction of PAF acetylhydrolase activity in active SLE might be due at least in part to a carrier defect related to the activity of the disease. In addition, experiments in which serum of patients with active SLE and serum of healthy subjects were mixed in different combinations indicated the absence of factors inhibiting PAF acetylhydrolase activity in SLE patients.