Specific binding by human polymorphonuclear leucocytes of the immunological mediator 1-O-hexadecyl/octadecyl-2-acetyl-sn-glycero-3-phosphorylcholine.

The binding of the platelet-activating factor 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphorylcholine (AGEPC) by human polymorphonuclear (PMN) leucocytes was complete within 20-30 min and optimal at 37 degrees. Scatchard plot analyses of the total binding of [3H]AGEPC by PMN leucocytes without and with an excess of unlabelled AGEPC revealed two distinct types of binding sites. One type of binding site exhibited a high affinity (KD = 0.11 +/- 0.02 nM, mean +/- SD), was saturable and had a maximal capacity of 5.2 +/- 2.1 x 10(6) (mean +/- SD) molecules of AGEPC per PMN leucocyte. The other binding site demonstrated a substantially lower binding affinity and a greater binding capacity consistent with nonreceptor uptake of AGEPC into cellular structures. The high affinity binding site of PMN leucocytes in suspension was saturated at 196 +/- 90 pmol (mean +/- SD) of AGEPC per ml, while 600 pmol of AGEPC per ml evoked maximal PMN leucocyte chemotaxis in modified Boyden chambers. The specificity of binding of AGEPC by PMN leucocytes was established by the capacity of analogues of AGEPC, but not structurally distinct chemotactic factors, to inhibit the binding of [3H]AGEPC. The high affinity PMN leucocyte binding site for AGEPC was specific for a phospholipid with an alpha-ether linkage and a beta-short chain fatty acid, but the binding site lacked stereospecificity. Similar structural requirements were stereospecificity. Similar structural requirements were observed for the elicitation of PMN leucocyte chemotaxis and the enhancement of the expression of PMN leucocyte C3b receptors by AGEPC.