Transbilayer reorientation of phospholipid probes in the human erythrocyte membrane. Lessons from studies on electroporated and resealed cells.

In order to characterize in more detail the previously observed (Dressler et al. (1983) Biochim. Biophys. Acta 732, 304-307) increases in transbilayer mobility of phospholipids in the erythrocyte membrane following electroporation at 0 degrees C and subsequent resealing at 37 degrees C of the cells, we have studied rates of flip and flop as well as steady state distributions of the fluorescent N-(NBD)-aminohexanoyl-analogues of the four major membrane phospholipids. Measurements comprised the passive non-mediated components as well as those mediated by specific translocases (flippase and floppase). The major new findings and insights can be summarized as follows. (1) The enhancement of passive transbilayer mobility which increases with the strength, duration, and number of field pulses at 0 degrees C, cannot be fully reversed by subsequent resealing at 37 degrees C. Flip-flop remains considerably elevated relative to the original values.(2) Enhanced mobilities induced by electroporation differ for the probes studied in the sequence SM <<< PS << PC < PE. Other membrane perturbations going along with enhanced flip-flop share only in part this pattern. (3) Mediated, ATP-dependent components of flip and flop of the probes are suppressed in electroporated/resealed cells, partly due to loss of cellular Mg2+, partly - in case of flippase - due to competition by externalized endogenous PS. (4) Electroporated/resealed cells provide an elegant means to demonstrate the contribution of various components of flip and flop to the steady state transbilayer distribution of phospholipids, in particular the role of passive mobility. The new, detailed information on the displacements of phospholipid between the two leaflets of the membrane bilayer in porated/resealed cells will help to understand erythrocyte shape changes following poration and during resealing (Henszen et al. (1993) Biol. Chem. Hoppe-Seyler 374, 114).