STUDIES OF KINETICS AND EQUILIBRIUM MEMBRANE-BINDING OF CLASS-A AND CLASS-L MODEL AMPHIPATHIC PEPTIDES

We studied the kinetics and equilibrium membrane binding of two amphip athic cr-helical peptides: the Ist peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolip oprotein), according to classification of Segrest et al. ((1990) Prote ins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, t he presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane associ ation was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresp onded to monomer partitioning with saturation at high peptide/lipid ra tios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluo rescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membra ne equilibration was in the millisecond range. At low peptide/lipid ra tios it depended on lipid concentration and was independent of the pep tide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation ra te and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selectiv e peptide targeting and disruption of membranes with a specific lipid composition. (C) 1998 Elsevier Science B.V.