Substance P: model studies of its binding to phospholipids.
1. The partition of substance P (SP) between buffer solutions (pH 1.6--7.8) and an organic, phospholipid (phosphatidyl serine, phosphatidyl ethanolamine, phosphatidyl inositol and phosphatidyl choline) containing phase (chloroform:methanol 2:1) was studied. 2. The binding of SP to phosphatidyl serine, phosphatidyl ethanolamine and phosphatidyl inositol was lowest at pH 2 and increased with pH. The binding to phosphatidyl choline was much smaller and less dependent on pH. 3. In contrast to the basic peptide SP (pI 10.5), physalaemin (pI 7.0) did not show any binding to phospholipids at any investigated pH value which underlines the importance of a basic group in the peptide for its binding. 4. The high affinity (KD = 0.1 microM) and capacity of 44 pmol SP/microgram phosphatidyl serine and 48 pmol SP/microgram phosphatidyl ethanolamine at pH 7.2 under conditions of saturation contrasted with the very low binding of SP to phosphatidyl inositol or phosphatidyl choline. Ionic bindings between the basic peptide and phosphatidyl serine or phosphatidyl ethanolamine are regarded to be predominant, although other binding forces cannot be excluded. 5. There was a concentration-dependent reduction in the binding of SP to phosphatidyl serine or phosphatidyl ethanolamine by Na+ and Ca2+, whereas K+ showed hardly any effect at physiological concentrations. 6. The model studies served to consider the possibilities of the binding of a basic peptide to lipid storage or receptor sites.