MOLECULAR STRUCTURAL REQUIREMENTS FOR BINDING AND ACTIVATION OF L-ALANINE TASTE RECEPTORS

L-Alanine binds to and activates specific taste receptors of Ictalurus punctatus, the channel catfish. In order to determine the structural requirements for receptor binding and activation in this model system, a number of analogues of L-alanine were tested using a neurophysiolog ical assay and a competitive ligand binding assay. These assays measur ed the ability of analogues to activate taste receptors and to displac e L-[H-3]alanine from L-alanine binding sites. Of those derivatives wi th modifications of the sidechain, L-serine, glycine, beta-chloroLalan ine and 1-amino-cyclopropane-1-carboxylic acid were the most potent an alogues with IC50s similar to and neural responses slightly decremente d from that of L-alanine. Derivatives containing branched sidechains o r sidechains of otherwise increased volume were considerably less acti ve. All modifications of the alpha-carboxylic acid and the alpha-amine , including amides, esters and various isosteres, led to substantial r eduction in the analogues' ability to displace L-[H-3]alanine and, in most cases, very weak stimulatory capability. However, L-lactic acid w as a reasonably strong stimulus, but a poor competitor, suggesting tha t it acts at a different receptor site. Overall, these results indicat e the importance of the charged amine and carboxylic acid groups for b inding to and activation of the receptor for L-alanine. Moreover, modi fications around the chiral center of L-alanine support the hypothesis that receptor binding and activation are separate processes in this m odel taste system.