Lipase-catalyzed reactions in vesicles as an approach to vesicle self-reproduction

As a continuation of our endeavor to find conditions under which bounded aggregate structures are able to self-reproduce, we have investigated the reactivity of lipase, both in free solution and vesicle-entrapped, against mixed oleic acid/oleate/ethyl oleate vesicles. Three types of vesicles have been prepared and characterized: (A) oleic acid/oleate vesicles; (B) oleic acid/oleate/ethyl oleate vesicles; and (C) lipase containing oleic acid/oleate vesicles. Long time stability studies by quasi elastic light scattering show that whereas (B) and (C) vesicles remain stable with a diameter of 110-130 nm and monodisperse for over a period of one month, vesicles (A) separated from an initial single population of 105 nm diameter into two populations, having respectively 70 nm diameter (more than 95% of the particles) and 180-210 nm diameter (less than 5% of the total population). In the case of vesicles (C), it could be shown that the enzyme remains localized inside the vesicles and it does not protrude into the water bulk phase. The enzymatic hydrolysis of ethyl oleate (which is water-insoluble) incorporated in the B-vesicles was studied under two configurations: (I) by adding lipase externally to the B-vesicles; (II) by mixing vesicles (B) and vesicles (C). In both cases, the reaction progressed to 100% hydrolysis. In the first case, the reaction was attended by an increase of the number of vesicles, and since this hydrolysis reaction takes place within the boundary of the parent vesicles, the criteria of autopoietic self-reproduction of vesicles are satisfied. In the case (II) instead, no increase of the population number of particles could be detected. The possible reasons for this difference are discussed.