Interaction of opioid peptides with model membranes. A carbon-13 nuclear magnetic study of enkephalin binding to phosphatidylserine

Abstract

The binding of enkephalin to phosphatidylserine was studied, by 13C NMR, as a model for interactions with components of biological receptors. Chemical shifts, line widths and spin-lattice relaxation times were measured for peptides enriched to 90% in 13C. The pKa [negative log of the Ka] values of the terminal amino and carboxyl groups were determined from the pH dependence of the 13C chemical shifts. Interaction of (2-[2-13C]glycine)methionine-enkephalin, (3-[2-13C]glycine)methionine-enkephalinamide with phosphatidylserine (PS) was studied as a function of pH. Salt and morphine antagonism to binding was manifest. Binding was pH dependent, exhibiting a maximum under slightly acidic conditions. The -NH3+ group of enkephalin is essential for binding; apparently the tyrosyl hydroxyl group or the COO- group is not involved. Binding affects the 13C spin-lattice relaxation times most strongly; the chemical shifts and line widths of the 13C-enriched material show little perturbation in the presence of PS. The internal flexibility of the peptides is described, on binding to model membranes, by 1 order of magnitude. Kd were measured as 4 .times. 10-1 M and 2.6 .times. 10-3 M for enkephalin and enkephalinamide, at pH 6.3 and 6.4, respectively.