Release of macromolecular markers (enzymes) from liposomes treated with antibody and complement: An attempt at correlation with electron microscopic observations

Antibody-complement dependent damage to liposomal model membranes has been previously investigated by measuring the release of low molecular weight markers such as glucose. To determine whether larger solutes are also released under these conditions, experiments have been performed using immunologically sensitive liposomes that contained not only trapped glucose, but also enzymes (hexokinase, glucose-6-phosphate dehydrogenase, β-galactosidase) as macromolecular markers. The largest of these enzymes (β-galactosidase) has dimensions which closely approximate the diameter of the lesions detected by negative staining in natural membranes after immune lysis. Liposomes prepared with lecithin, and either actively sensitized with globoside or passively sensitized with alkali-treated lipopolysaccharide, released the enzymes in parallel with glucose upon incubation with the appropriate antiserum and native guinea pig serum as source of complement. Immune damage to sphingomyelin liposomes was characterized by a significantly lower loss of the enzymes in comparison to the percentage of glucose released; a comparable response was manifested by liposomes prepared from sheep erythrocyte lipids. Electron microscopic examination of negatively stained lecithin liposomes, which had released the macromolecular markers, failed to reveal the characteristics lesions; these findings are consistent with evidence obtained by other laboratories suggesting that the lesions may not correspond to functional holes. Lesions were, however, consistently observed in liposome preparations that had been treated with the polyene antibiotics, filipin; this antibiotic causes appreciable loss of both glucose and enzymes from either lecithin or sphingomyelin liposomes.