IS THE HIGH PROPENSITY OF ETHANOLAMINE PLASMALOGENS TO FORM NON-LAMELLAR LIPID STRUCTURES MANIFESTED IN THE PROPERTIES OF BIOMEMBRANES

Plasmalogens are glycerophospholipids characterized by an alk-1'-enyle ther bond in position sn-1 and an acyl bond in position sn-2. These ub iquitous etherlipids exhibit a different molecular structure as compar ed to diacyl phospholipids. The most peculiar change is a perpendicula r orientation of the sn-2 acyl chain at all segments to the membrane s urface, This extended conformation results in an effectively longer al iphatic chain in plasmalogen than in the diacyl analog. Moreover, the lack of the carbonyl oxygen in position sn-1 affects the hydrophilicit y of the headgroup and allows stronger intermolecular hydrogen-bonding between the headgroups of the lipid. These properties favour the form ation of non-lamellar structures which are expressed in the high affin ity of ethanolamine plasmalogen to adopt the inverse hexagonal phase, Such structures may be involved in membrane processes, either temporar ily, like in membrane fusion or locally, e.g. to affect the activity o f membrane-bound proteins. The predominant distribution of ethanolamin e plasmalogens in some cellular membranes like nerve tissues or plasma membranes and their distinctly different properties in model membrane s as compared to diacyl phospholipids impose the question, whether the se differences are also manifested in the heterogeneous environment of biological membranes. The integration of biophysical studies and bioc hemical findings clearly indicated that the high propensity of ethanol amine plasmalogen to form non-lamellar structures is reflected in seve ral physiological functions. So far it seems to by evident that ethano lamine plasmalogens play an important role in maintaining the balance between bilayer and non-lamellar phases which is crucial for proper ce ll function. Furthermore, they are the major phospholipid component of inverse hexagonal phase inclusions in native retina and are able to m ediate membrane fusion as demonstrated between neurotransmitter vesicl es and presynaptic membranes.