Further characterization of the binding of substance P to a fraction from rabbit brain enriched in synaptic membranes.

1. The present experiments were designed to further characterize the specific [3H]substance P (SP) binding to membrane fractions from rabbit brain. 2. The specific binding was resistant to treatment with proteolytic enzymes whereas phospholipase A(0.7 unit/ml), C(0.025 unit/ml) and D(6.2 unit/ml) reduced the specific binding without decreasing total binding. Neuraminidase (0.01 unit/ml) decreased total binding as well as the specific binding. 3. High concentration of Triton X-100 reduced the specific binding whereas deoxycholate, at 0.05%, reduced both total and the specific binding. 4. Na+ (50--200 mM) reduced the specific binding, depending on the concentrations employed, whereas K+ decreased the specific binding at 10 and 100 mM. The binding was reduced considerably by 100 mmol/l of CaCl2. 5. From these results it is suggested that the specific [3H]SP binding sites in membrane fractions from rabbit brain could be phospholipids.