BIOCHEMICAL-STUDIES OF TASTE SENSATION .9. ENHANCEMENT OF L-[H-3]LABELED GLUTAMATE BINDING TO BOVINE TASTE PAPILLAE BY 5'-RIBONUCLEOTIDES

  • 1 January 1980
    • journal article
    • research article
    • Vol. 627  (3) , 313-323
Abstract
The interaction of the taste substance monosodium L-glutamate with taste receptors was investigated. Binding of L-[3H]glutamate was measured to preparations of bovine circumvallate (taste) papillae (type I preparation) and to control tongue epithelial preparations (type II preparation) devoid of taste receptors. Binding is operationally defined using a membrane filtration assay. Substantially greater binding occurred to the type I preparation than to the type II preparation and the binding to the type I preparation showed evidence of saturation. The apparent Kd of L-glutamate was approximately 20-30 mM. The unique taste effect of L-glutamate apparently depended on its synergism in combination with certain 5''-ribonucleotides. A several-fold enhancement of binding of L-[3H]glutamate occurred in the presence of certain 5''-ribonucleotides. 5''-GMP, 5''-IMP and 5''-UMP each increased the binding of L-[3H]glutamate, while 5''-XMP, 5''-AMP and 5''-CMP did not. None of these nucleotides affected the lower level of binding to the type II preparation. The free bases, adenine and guanine, their nucleosides and their di- or triphosphonucleotides were not effective in increasing L-[3H]glutamate binding to the type I preparation. The nucleotide specificity of the glutamate binding enhancement shows a marked similarity with the nucleotide specificity in evoking the synergistic taste effect in humans. The mechanism of enhancement is not well defined. Using 5''-GMP stimulation as a model, the major effect apparently was to increase the maximal binding of L-[3H]glutamate, but no marked change in Kd was apparent. The 5''-ribonucleotide may act to increase the extent of L-glutamate binding by unmasking previously hidden or buried receptor sites for L-glutamate.