Interfacial enzyme activation, non-lamellar phase formation and membrane fusion. Is there a conducting thread?

Previous studies from this laboratory have shown that the enzymic generatio n of diacylglycerol in bilayers by phospholipase C may lead to membrane fus ion through the formation of transient non-lamellar lipidic intermediates. The present paper intends to explore the correlations existing among the th ree main processes involved, namely (a) the induction (or inhibition) of la mellar-to-non-lamellar phase transitions in lipid mixtures through the addi tion of small (< 5 mol%) proportions of other lipids, (b) the promotion, by the latter lipids, of fusion in otherwise stable phospholipid vesicles (la rge unilamellar liposomes) under conditions leading to inverted hexagonal/i nverted cubic phase formation in bulk lipid systems, and (c) the modulation , by the same small proportions of lipids, of phospholipase C hydrolysis of phosphatidylcholine in liposome bilayers. It is concluded that phospholipa se C may give rise to non-lamellar lipidic structures that in turn permit l iposomal fusion to occur, but neither enzyme activity is directly modulated by non-lamellar phase formation, nor will whatever kind of enzyme-induced non-lamellar structure give rise to fusion. Moreover, only under certain ki netic conditions will the enzyme give rise to the organization of non-lamel lar structures that are conducive to the fusion event.