Lipid monolayers: influence of lipid film and urea on the surface activity of staphylococcal alpha-toxin.

Abstract

AbstractUnder conditions in which neither ribonuclease nor lysozyme formed a film by either spreading or adsorption at the air‐water interface, the surface activity of staphylococcala‐toxin and streptolysin S in the absence of lipid was greatly enhanced by the presence of 6 M urea in the hypophase. The influence of urea on the anchoring of the protein was quantitatively similar to that of a lecithin film. Admixture of up to 50 mole % cholesterol to egg lecithin preserved the lecithin character of the lipid monolayer in the penetration ofa‐toxin, suggesting probably binding of cholesterol to phosphatidyl choline in the presence of excess lecithin. Penetration ofa‐toxin and streptolysin S into the air‐water interface was inhibited by lipid monolayers containing gangliosides. The inhibition, however, was removed when the subphase contained 6 M urea. The resulting penetration curve ofa‐toxin was identical with the penetration curve of the protein into the lecithin film in the absence or in the presence of urea. In general, the denaturing agent causes the protein to acquire a hydrated conformation, from which restructuring in the low dielectric constant of air or lipid is favored and brings about results comparable with those effected by the lipid. Pre‐incubation ofa‐toxin with gangliosides or sulfatides caused a marked decrease in film penetration of the toxin into lecithin monolayers. Unlikea‐toxin, streptolysin S was extremely sluggish in penetration of the air‐water interface and of several lipid monolayers. Protein concentration, a complement of either lipid or membrane protein, and specific molecular organizations of membranes must be taken into account in all cases; they, in particular, may be the basis for the enhanced cytolytic activity of streptolysin S in vivo.