The specific interaction of S-100 protein with synaptosomal particulate fractions. Modulation of binding by fractional occupancy of sites and by the physical state of membranes.

The nonlinearity of single components of the Scatchard plot of S-100 binding to synaptosomal particulate fractions (SYN) and the observation that dilution of the 125I-labeled S-100 site complex results in a greater extent of dissociation of the tracer in the presence than in the absence of an excess of unlabeled S-100 suggest that sites change their binding behavior depending on fractional occupancy. To study this aspect of the interaction in more detail, 125I-labeled S-100 binding experiments were conducted in the presence of, or after preincubation of SYN with various concentrations of, unlabeled S-100. The results indicate that: (a) S-100 synaptosomal sites do change their binding behavior depending on fractional occupancy; and (b) the nonrapid equilibrium between bound S-100 and the medium, which has been referred to as the formation of a tight complex between S-100 and its binding sites, is related to the activation of high-affinity sites. However, no univocal interpretation of these data in terms of binding model can be offered at present, as the binding models currently employed in the analysis of ligand-site interactions can each account for only part of the results described in this report. In any case, data obtained by studying 125I-labeled S-100 binding to untreated SYN at 2 degrees C and to prefixed SYN at 37 degrees C indicate that the physical state of membranes influences both the extent of the interaction and the binding behavior of the sites.