SPONTANEOUS TRANSFER OF GM3 GANGLIOSIDE BETWEEN VESICLES

The spontaneous transfer between membranes of GM3, a ganglioside prese nt in a vesicular form of aggregation instead of micellar form like th e majority of gangliosides in aqueous medium, has been studied. Upon i ncubation of GM3 in the presence of dipalmitoylphosphatidylcholine (DP PC) large unilamellar vesicles at 50 degrees C, mixed GM3/DPPC vesicle s are formed. The maximum amount of GM3 that can be inserted into vesi cles is about 8%. The temperature dependence of the kinetics has been followed by the excimer formation technique, using the fluorescent ana logue pyrenyldodecanoyl-GM3. The transfer of ganglioside from its vesi cles to DPPC vesicles depends on the physicochemical characteristics o f both the donor and of the acceptor vesicles and increases with the t emperature (k = 0.006 0.012, 0.037 at 30, 41 and 50 degrees C, respect ively), with a major break point at 41 degrees C and a minor one at 35 degrees C. These temperatures correspond to the gel- to liquid-crysta lline transition of DPPC (T-m=41.3 degrees C), and to a temperature tr ansition displayed by GM3 ganglioside. Similar experiments performed w ith erythrocyte ghosts yielded a rate constant of 0.04 at 37 degrees C . For the transfer of ganglioside from DPPC (donor) to DMPC (acceptor) the rate constants were 0 at 15 degrees C (both phospholipids in the gel phase), 0.005 at 37 degrees C (donor in the gel phase, acceptor in the fluid phase) and 0.04 at 50 degrees C (both phospholipids in the fluid phase). The fastest kinetics were observed when both donor and a cceptor membranes were in the fluid state. The kinetics was not affect ed by the physical state and by the lipid moiety of acceptor phosphati dylcholine when the donor was in the gel phase. The data obtained sugg est that GM3 transfer occurs via monomers through the aqueous medium a nd that the rate-limiting step is the off-rate of the ganglioside from the donor membrane.