On the Nature of the Interaction Between Serotonin and Serotonin Binding Protein: Effect of Nucleotides, Ions, and Sulfhydryl Reagents

Abstract: Rat brain serotonin binding protein (SBP) was found to have essential -S-S and -SH groups. Both reduction of the disulfide bond by dithiothreitol or mercaptoethanol and modification of -SH group(s) by Ellman reagent or alkylating agents caused loss of binding capacity. In contrast, formation of a mixed disulfide bond with sodium metabisulfite did not affect the binding capacity. Serotonin in the presence of Fe²⁺ and phosphate was found to bind to either an -SH group or to a site in very close proximity. Addition of serotonin protected -SH groups from modification by Ellman reagent and from denaturation of protein upon storage. Lipids that enhance binding of serotonin to SBP also protected -SH groups from modification. Nucleotides were found to be strong inhibitors of the binding of serotonin to SBP. The inhibitory effect of nucleotides was due to their chelating properties and not to their ability to phosphorylate the protein or to bind directly to it. Inhibition by nucleotides and other chelators was reversible. Binding capacity was fully restored after removal of the chelator by molecular sieve chromatography and addition of Fe²⁺. The ionic environment had a marked effect on the binding: intracellular ions such as K⁺ were found to enhance the binding, and extracellular ions such as Na⁺ and Ca²⁺ inhibited the binding. Based on these data and our previous studies, we suggest that SBP is an intracellular protein that acts as a storage protein. Consistent with our data is formation of a complex of SBP-S-Fe-S that in a hydrophobic surrounding could bind up to four molecules of serotonin in coordination bond with Fe²⁺ and thereby reduce the osmotic pressure within a storage vesicle. Extracellular ionic conditions that favor the dissociation of the complex would free the amine to interact with its receptor or the presynaptic reuptake carrier.