TRANSBILAYER DIFFUSION OF DIVALENT-CATIONS INTO LIPOSOMES MEDIATED BYLIPIDIC PARTICLES OF PHOSPHATIDATE

Liposomes formed from egg-yolk phosphatidylcholine:egg-yolk phosphatid ate (molar ratio 2:1) containing pBR322 DNA and DNase I were induced t o form, with divalent cations, bilayer/nonbilayer phase transitions of phosphatidate which allowed cation diffusion into liposomes; then cat ion diffusion was measured by the activation of the hydrolysis of DNas e I on DNA. The formation of phosphatidate transitions on liposomes wa s demonstrated by freeze-fracture and P-31 NMR, and a direct correlati on between the formation of phosphatidate transitions and the transbil ayer diffusion of cations was found: only Ca2+ and Mn2+, which induce phase transitions, were able to penetrate liposomes and triggered the DNase I activity; in addition, Ca2+ at higher concentrations (10 mM) c aused fusion of liposomes, whereas Mn2+ did not, suggesting that trans itions induced by Mn2+ participated only in the diffusion of this ion; furthermore, Mg2+ neither formed phase transitions nor triggered the enzymatic activity. The liposomes studied represent more dynamic struc tures that can form phosphatidate structures involved in both (1) the interchange of divalent cations with the surroundings, thereby modulat ing encapsulated enzymes, and (2) the fusion of lipid vesicles probabl y implicated in the enrichment of liposomal content in the early Preca mbian Earth.