Further Studies on the Specific Interaction of S‐100 Protein With Synaptosomal Particulates 1

The specific interaction of S‐100 protein with disrupted synaptosomes was further investigated. The specific binding is a saturable and reversible process, and is time, temperature, and strictly Ca2+ ‐dependent. Two affinities affect the interaction (Kins= 7.04 × 10−9 M. 1.28 × 1012 binding sites/ mg protein; Kins2= 3.91 × 10−7M, 2.96 × 1013 binding sites/mg protein). The half‐saturation time is about 5.5 min at 37°C. The half‐life of the complex is 17 min at 37°C. At 0°C the binding is 75% slower than at 37° C, and only one‐third of the binding sites are involved. The binding capacity is decreased by high NaCl concentrations and by pretreating membranes at high temperatures. Digestion of membranes with trypsin practically abolishes the specific binding. Treatment of membranes with phospholipase C decreases the specific binding, while phospholipase D enhances it to some extent. Other lipid extractors decrease significantly the extent of the interaction. Synaptic plasma membranes seem to be the synaptosomal component involved in the high affinity binding. The S‐100 binding activity seems to undergo developmental changes, the adult values of kinetic parameters being reached around the 16th postnatal day in the rat. The results are discussed also in relation to the membrane‐bound fraction of S‐100.