Interaction of Immunoglobulins with Liposomes

test the effect of immunoglobulins on biomembranes. Heat-aggregated immunoglobulins (Ig) exceeded native immunoglobulins in their capacity to release anions and glucose from model liposomes (either lecithin-dicetyl-phosphate-cholesterol or lecithin-stearylamine-choles-terol in molar ratios of 7: 2: 1). This interaction was not dependent upon the presence of cholesterol in the membrane. Mild heat-aggregation (10 min at 61.50C) increased the membrane-perturbing activity of certain 1g. Activity varied among classes and subclasses: IgG> pooled IgG> IgG4> IgA,> IgGs. IgG2, IgA2 and IgM were inert. Fc fragments of IgG were as active as IgGi, whereas Fab fragments were inactive. Prolonging aggregation to 60 min destroyed the activity of 1g. Membrane-activity could not be induced in non-Ig molecules (such as bovine serum albumin) by 10 or 60 min heat-aggregation. Density gradient centrifugation of IgG, molecules indicated that membrane perturbing activity was associated with 15-20-s aggregates. Sepha-rose 4B chromatography demonstrated preferential inter-action between cationic membranes and aggregated Ig, whereas anionic membranes interacted nonselectively with both native and aggregated Ig via salt-like inter-actions. One explanation for these data is that heat aggregation induces a conformational change in the Fc regions of certain Ig permitting them to interact with liposomes, presumably by enhancing their hydrophobic associations with membrane phospholipids