EFFECTS OF HEXAGONAL PHASE INDUCTION BY DOLICHOL ON PHOSPHOLIPID MEMBRANE-PERMEABILITY AND MORPHOLOGY

The effect of H(II)-phase induction by dolichol (DOL) (C80-105, 0-2 wt %) within the bilayers of multilamellar liposomes (MLV, DOPE:DOPC 2:1 or 3:1 w/w) on their permeability, lipid mixing and morphology was det ermined. Low-angle X-ray diffraction patterns were consistent with mix tures of bilayer and H(II) phases, the latter increasing with increasi ng DOL or DOPE content and temperature. Efflux rate constants for 6-ca rboxyfluorescein (6-CF) from 2:1 DOPE:DOPC vesicles depended on temper ature and DOPE/DOL content, increasing as much as 200-fold over DOL-fr ee controls at 2% w/w DOL. Fluorescence resonance energy transfer assa ys detected lipid mixing with unlabeled target MLV. It was appreciable only when target MLV contained DOPE and increased with DOL content. C onfocal scanning fluorescence microscopy was applied to study the morp hological structure of fully-hydrated samples and field scanning elect ron microscopy the ultrastructure of cryo-stabilized samples. 3:1 DOPE /DOPC MLV, stable at pH 9.5, underwent rapid morphological changes at pH 7.4. Within minutes filaments formed and large areas of membrane su rface became studded with 10-15 nm bumps and 5 nm holes, resembling in size and shape unilamellarly covered interlamellar micellar intermedi ates and interlamellar attachments (ILA) previously associated with H( II)-phase transitions. The filaments, seen in MLV with and without DOL , may represent extensions of IMI into coaxial assemblages of rod mice llar intermediates (RMI). These phenomena may have implications for li posomal delivery of therapeutic peptides/proteins if they can be made to trigger the convective release of liposomal contents via controlled formation of ILA between adjacent lamellae of MLV.