Platelet-activating factor: mediator of the third pathway of platelet aggregation? A study in three patients with deficient platelet-activating factor synthesis.

Thrombin, collagen, and Ca2+-ionophore A23187 aggregate platelets in the presence of inhibitors of the first (ADP-mediated) and second (cyclooxygenase-dependent) pathway of platelet activation. This aggregation, via a third pathway, was hypothesized to be mediated by the alkoxyether lipid platelet-activating factor (PAF). We recently demonstrated virtual absence of plasmalogen-type alkoxyether lipids and deficiency in key enzymes of their biosynthesis in Zellweger patients. We hypothesized that PAF synthesis might also be impaired. We report two Zellweger patients with an undetectable A23187-induced PAF synthesis of leukocytes (patients, less than 3 pmol PAF/10(8) granulocytes (PMN); four age-matched controls, 249-2,757 pmol PAF/10(8) PMN; five adult controls, 291-5,433 pmol PAF/10(8) PMN). In a third patient, residual PAF synthesis was detected. However in all patients the thrombin-induced third mechanism of platelet aggregation was present. We therefore conclude that PAF may not be the mediator of the third pathway.