Inhibitory activity of a naturally occurring heterocyclic beta-substituted alanine, beta-(isoxazolin-5-on-4-yl)-L-alanine, on the L-glutamate/L-aspartate transporter (GLAST) expressed in Xenopus oocytes.
Excitatory amino acid (EAA) transporters are of physiological importance in the regulation of the extracellular concentration of excitatory amino acids and the neuroexcitation in CNS. Among four identified transporters, the Na+-dependent high-affinity L-glutamate/L-aspartate transporter (GLAST) is highly expressed in glial cells. Here, we report a naturally occurring inhibitor of GLAST, derived from bovine retina, using the Xenopus oocyte expression system. Beta-(isoxazolin-5-on-4-yl)-L-alanine (TAN), an antifungal antibiotic, inhibited [14C]L-glutamate (L-Glu) transport into GLAST-expressing oocytes. TAN also served as a substrate for this transporter in voltage-clamp experiments measuring the current coupled to the EAA transport. The maximum current of TAN itself was approximately 1/3 of that of L-glutamate, and its apparent affinity was almost the same as L-Glu. In combination with L-Glu, TAN antagonized L-glutamate transport. In radioisotope experiments, the inhibitory potency of this compound against [14C]L-Glu uptake into oocytes was approximately 1/6 of that of L-(-)-threo-3-hydroxyaspartate (THA). The glucoside of TAN (TANG), occurring in seedlings of the garden pea, the lentil and some Lathyrus species, did not show any electrophysiological activity nor was it transported into oocytes. It is proposed that TAN is a novel type antagonist of natural origin on GLAST. By affecting such transport system, naturally occurring compounds may affect the regulation of the extracellular level of endogenous EAA.