Binding of [35S]saccharin to a protein fraction of rat tongue epithelia

Interactions between the human sweet-sensing T1R2-T1R3 receptor and sweeteners detected by saturation transfer difference NMR spectroscopy

The sweet receptor is a member of the G-protein coupled receptor family C that detects a wide variety of chemically and structurally diverse sweet-tasting molecules. We recently used saturation transfer difference spectroscopy (STD) to monitor the direct binding of a set of sweet agonists and antagonists to the human taste receptor in membranes prepared from human embryonic kidney (HEK293) cells transfected with and expressing the sweet receptor [F.M. Assadi-Porter, M. Tonelli, E. Maillet, K. Hallenga, O. Benard, M. Max, J.L. Markley, J. Am. Chem. Soc. 130 (2008) 7212-7213]. Here we review this work and related studies, discuss the procedures involved, and expand on their potential for identifying specific binding interactions of ligands to the membrane spanning and extracellular regions of the full heterodimeric sweet taste receptor. Whereas activity assays are unable to distinguish mutations that alter ligand-binding sites from those that alter signal transduction downstream of the binding site, STD NMR now allows us to make this distinction.

Direct NMR detection of the binding of functional ligands to the human sweet receptor, a heterodimeric family 3 GPCR

We present a robust method for monitoring the binding of ligands to the heterodimeric (T1R2+T1R3) human sweet receptor (a family 3 GPCR receptor). The approach utilizes saturation transfer difference (STD) NMR spectroscopy with receptor proteins expressed on the surface of human epithelial kidney cells. The preparation investigated by NMR can contain either live cells or membranes isolated from these cells containing the receptor. We have used this approach to confirm the noncompetitive binding of alitame and cyclamate to the receptor and to determine that greatly reduced receptor binding affinity compared to wild-type brazzein explains the lack of sweetness of brazzein mutant A16C17. This approach opens new avenues for research on the mechanism of action of the sweet receptor and for the design of new noncalorigenic sweeteners.

Sweeteners: state of knowledge review

Sweeteners are widely used in the food and pharmaceutical industry. The purpose of this paper is to review our current knowledge of sweet taste from chemical, biochemical, electrophysiological, psychophysical, and psychological points of view. The most common sweetners likely to be used in food and pharmaceuticals will be examined in detail. First, the chemical structures of sweet compounds including saccharides, diterpene glycosides, polyols, amino acids, dipeptides, and other nonsugars will be discussed. Second, biochemical approaches to understanding sweetner receptors will be reviewed. Third, electrophysiological and behavioral approaches to understanding sweetner receptors will be discussed. Fourth, psychophysical studies in humans will be shown to be consistent with biochemical and neurophysiological data. In addition, the basic mechanisms of sweet taste revealed by psychophysical studies will be given, including the role of multiple receptor sites, hydrogen bonding, and sodium transport. Finally, the factors that affect preference for sweet taste including the psychological and physiological variables associated with sweet preference will be explored.

Abnormal taste preference for saccharin in hypothyroid rats

Taste preferences for saccharin in concentrations ranging from 0.16 mM to 50 mM were determined in rats made hypothyroid with radioactive iodine and in their littermate controls. Hypothyroid rats demonstrated taste preferences for saccharin which were similar to those of controls only at very low (0.016 mM) or very high (49.0 mM) saccharin concentrations. At these concentrations of tastant, the preferences for tastant and water were similar to one another. At a concentration of 5.1 mM, preferences were also very similar in both groups but were very high. At intermediate saccharin concentrations of 1.1 and 3.0 mM, hypothyroid animals showed significantly lower percent preferences for the sweet tastant than did controls, mean +/- SEM (62.48 +/- 5.97 vs. 82.92 +/- 4.60, p = 0.0002) for the 1.1 mM concentration and (74.98 +/- 5.12 vs. 89.40 +/- 2.54, p = 0.0029) for the 3.0 mM concentration. These changes in taste preference for saccharin in hypothyroid rats were similar in direction and magnitude to those previously published by this laboratory using sucrose as the tastant. Thus, hypothyroid rats demonstrate abnormalities in taste preference for both the nonnutritive sweetener, sodium saccharin, as well as for the nutritive sweetener, sucrose.

Photoaffinity labeling of thaumatin-binding protein in monkey circumvallate papillae

Thaumatin I is an intensely sweet-tasting protein. It was photo-crosslinked with taste papillae of crab-eating monkey by using a conjugated photo-affinity reagent [3H]azidobenzoylthaumatin I. Serial sections of SDS-polyacrylamide gel electrophoresis of the 0.1 M sodium phosphate buffer-soluble fraction from taste papillae had a large peak of radioactivity at the Mr region of approx. 70,000; fractions from non-taste papillae did not. Excess unlabeled thaumatin I reduced the photo-crosslinking at the 70 kDa region; acetylated thaumatin I (which is not sweet) did not. The results show that taste papillae of the monkey contain a protein of Mr approx. 50,000, which binds to thaumatin I (Mr 22,209) but not to completely acetylated thaumatin I. The possibility that the thaumatin-binding protein is a sweet receptor protein is discussed.

Binding of [14C]inosine to gustatory tissues of the turbot, Scophthalmus maximus

Reversible specific binding of [14C]inosine has been demonstrated in turbot (Scophthalmus maximus) buccal tissues rich in taste buds. A highly significant correlation was found for a series of nucleosides and nucleotides between their feeding stimulant activity and their ability to displace bound [14C]inosine. It is concluded that the binding of inosine is the first step in the gustatory process.