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 importan ce in the regulation of the extracellular concentration of excitatory amino acids and the neuroexcitation in CNS. Among four identified tran sporters, the Na+-dependent high-affinity L-glutamate/L-aspartate tran sporter (GLAST) is highly expressed in glial cells. Here, we report a naturally occurring inhibitor of GLAST, derived from bovine retina, us ing the Xenopus oocyte expression system. beta-(Isoxazolin-5-on-4-yl)- L-alanine (TAN), an antifungal antibiotic, inhibited [C-14]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 approximate to 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 i nhibitory potency of this compound against [C-14]L-Glu uptake into ooc ytes was approximate to 1/6 of that of L-(-)-threo-3-hydroxyaspartate (THA). The glucoside of TAN (TANG), occurring in seedlings of the gard en pea, the lentil and some Lathyrus species, did not show any electro physiological activity nor was it transported into oocytes. It is prop osed that TAN is a novel type antagonist of natural origin on GLAST. B y affecting such transport system, naturally occurring compounds may a ffect the regulation of the extracellular level of endogenous EAA. (C) 1997 Elsevier Science B.V.