N‐acetylneuraminic Acid Molecules as Possible Serotonin Binding Sites1

Abstract 5‐Hydroxytryptamine (5‐HT), but not acetylcholine, carbamylcholine or L‐D‐noradrenaline, binds to ox brain ganglioside micelles, to phosphatidylcholine smectic mesophases (liposomes) containing gangliosides and to the glycoprotein fetuin, through the negatively charged N‐acctylneuraminic acid (NeuNAc) residues. The 5‐HT binding to NeuNAc is reversible, saturable, prodeeds in a 1: 1 fashion and can be specifically blocked by 7‐methyltryptamine. Thc affinity constant at equilibrium for the reaction is of the order of 102 1. mol‐1. No special ganglioside was identified as specifically associating with the amine. A terminal NeuNAc in the gangliosides is not a prerequisite for binding, although it seems important for binding 5‐HT in entire mcmbrane preparations (MARCHBANKS, 1966) or for bringing about the 5‐HT induced contraction of smooth muscle cells (WOOLLCY & GOMML 1964a). It is proposed that in 5‐HT target cells, NeuNAc residues, probably attached to membrane surface glycoprotein(s) are involved in thc mechanism of action of the drug.