Transbilayer reorientation of platelet-activating factor in the erythrocyte membrane

Platelet-activating factor (PAF) is a phospholipid, intermediate in its structure between diacyl- and lysophospholipids. The reorientation of this highly bioactive compound to the inner membrane layer of human erythrocytes after its primary incorporation into the outer membrane layer has been studied. Reorientation was shown to be a slow process with an initial rate of only 0.012 h-1. It does not depend on energy supply. From the steady-state distribution of PAF between inner and outer membrane layers, after very long incubation times (40-50 h), a preference of PAF for the outer membrane layer analogous to that of endogenous lecithin is derived. Our data indicate that the process of simple transbilayer reorientation of PAF is probably too slow to account for very fast transmembrane signal transmission or for the fast uptake and metabolism observed in certain cell types.