TRANSBILAYER AND INTERBILAYER PHOSPHOLIPID EXCHANGE IN DIMYRISTOYLPHOSPHATIDYLCHOLINE DIMYRISTOYLPHOSPHATIDYLETHANOLAMINE LARGE UNILAMELLAR VESICLES

The rates of spontaneous interbilayer and transbilayer exchange of [H-3]dimyristoyl-phosphatidylcholine ([H-3]DMPC) were examined in DMPC and DMPC/dimyristoylphosphatidyl-ethanolamine (DMPE) large unilamellar vesicles in the liquid-crystalline-, gel-, and mixed-phase states. DMPC desorption rates from either gel or liquid-crystalline phases containing DMPE are very similar to the corresponding rates from pure DMPC gel or liquid-crystalline phases. This is not the case for DMPC desorption from distearoylphosphatidylcholine (DSPC)-containing gel phases, where the desorption rates are significantly faster than from a pure DMPC gel phase [Wimley, W. C., & Thompson, T. E. (1990) Biochemistry 29, 1296-1303]. We proposed that the DMPC/DSPC behavior results from packing defects in gel phases composed of both DMPC and DSPC molecules because of the four-carbon difference in the acyl chain lengths of the two species. The present results strongly support this hypothesis because no such anomalous behavior is observed in DMPC/DMPE, which is similar to DMPC/DSPC in phase behavior but does not have the chain length difference. The inclusion of 10-30 mol% DMPE in DMPC bilayers was also found to have a significant effect on the rate of transbilayer movement (flip-flop) of [H-3]DMPC in the liquid-crystalline phase. Between 10 and 30 mol% DMPE, flip-flop of DMPC is slowed by at least 10-fold relative to flip-flop in DMPC bilayers, and the entropy and enthalpy of flip-flop activation are both substantially decreased. These results are not consistent with lipid headgroup dehydration as the major energetic barrier to flip-flop but rather suggest that the spontaneous formation of "fluctuation defects" and the energetics of a lipid passing through or being part of such a defect may be the limiting factors. A mechanistic model is proposed. Finally, some predictions based on the model are made and experimentally tested for the related process of bilayer permeation by protons. The permeation results were found to be entirely consistent with the model.