Lipid-bound State Research Articles

Interaction of staphylococcal δ‐toxin and synthetic analogues with erythrocytes and phospholipid vesicles

Staphylococcal delta-toxin, a 26-residue amphiphilic peptide is lytic for cells and phospholipid vesicles and is assumed to insert as an amphipathic helix and oligomerize in membranes. For the first time, the relationship between these properties and toxin structure is investigated by means of eight synthetic peptides, one identical in sequence to the natural toxin, five 26-residue analogues and two shorter peptides corresponding to residues 1-11 and 11-26. These peptides were designed by the Edmundson wheel axial projection in order to maintain: (a) the hydrophilic/hydrophobic balance while rationalizing the sequence, (b) the alpha-helical configuration and (c) the common epitopic structure. The fluorescence of the single Trp residue was used to monitor the behaviour of the natural toxin and analogues. All 26-residue analogues were hemolytically active although to a lesser extent than natural toxin. The peptide of residues 11-26 bound lipids weakly and was hemolytic at high concentration. The peptide of residues 1-11 did not bind lipids and was hemolytically inactive. All peptides except the latter cross-reacted in immunoprecipitation tests with the natural toxin. The study of a 26-residue analogue by circular dichroism revealed an alpha-helical configuration in both the free and lipid-bound state. Changes in the fluorescence of the peptides in the presence of lipid micelles and bilayers varied according to the position of the reporter group. When bound to lipids, Trp5, Trp16 and the Fmoc-1 positions of the analogues became buried while Trp15 of the natural toxin and its synthetic replicate remained more exposed. All changes are rationalized by the proposal of an amphipathic helix whose hydrophobic face is embedded within the apolar core of bilayers while the hydrophilic and charged face remains more exposed to solvent.

Inhibition of de novo choline biosynthesis by 2-amino-2-methyl-1-propanol

Evidence has been obtained which supports the conclusion that the chronic administration of 2-amino-2-methyl-1-propanol to young rats produces an inhibition of the incorporation of the methyl group of methionine into lipid-bound choline of liver, kidney, and other tissues. The incorporation of formate-C 14, glycine-2-C 14, and serine-3-C 14, but not of betaine-CH 3-C 14, is also inhibited. The incorporation of choline-CH 3-C 14 or choline-1,2-C 14 is not affected under the same conditions. In vivo, the extent of inhibition increases with the duration of administration of aminomethylpropanol and reaches a maximal value after 5–6 days. No acute effect of the compound is seen in vivo, but an acute effect can be demonstrated in vitro. The inhibitory effect of chronically administered aminomethylpropanol in vivo is not affected by the acute administration of relatively large amounts of choline along with the l-methionine-CH 3-C 14. However, the chronic administration of sufficient quantities of choline together with the aminomethylpropanol prevents the inhibitory effect of the latter. These results, together with the lack of a demonstratable acute effect of aminomethylpropanol in vivo, have been interpreted as possibly indicating that 2-amino-2-methyl-1-propanol only becomes an inhibitor of de novo choline synthesis after it has become transformed, perhaps to a lipid-bound state.