N-ACETYLNEURAMINIC ACID MOLECULES AS POSSIBLE SEROTONIN BINDING-SITES

5-Hydroxytryptamine (5-HT), but not acetylcholine, carbamylcholine or DL-noradrenaline, bound to ox brain ganglioside micelles, to phosphatidylcholine smectic mesophases (liposomes) containing gangliosides and to the glycoprotein fetuin, through the negatively charged N-acetylneuraminic acid (NeuNAc) residues. The 5-HT binding to NeuNAc was reversible, saturable, proceded in a 1:1 fashion and could be specifically blocked by 7-methyltryptamine. The affinity constant at equilibrium for the reaction was of the order of 102 l mol-1. No special ganglioside was identified as specifically associating with the amine. A terminal NeuNAc in the gangliosides was not a prerequisite for binding, although it seemed important for binding 5-HT in entire membrane preparations or for bringing about the 5-HT induced contraction of smooth muscle cells. In 5-HT target cells, NeuNAc residues, probably attached to membrane surface glycoprotein(s) are involved in the mechanism of action of the drug.