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A TAS1R receptor-based explanation of sweet ‘water-taste’

Bitter sweeteners Paradoxically, artificial sweeteners such as sodium saccharin and acesulfame-K taken in high concentrations are not sweet: they can even seem bitter. And if the mouth is then rinsed out with water, it takes on a sweet taste. These observations have led to new insights into the acti...

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Published in:Nature 2006-05, Vol.441 (7091), p.354-357
Main Authors: Galindo-Cuspinera, Veronica, Winnig, Marcel, Bufe, Bernd, Meyerhof, Wolfgang, Breslin, Paul A. S.
Format: Article
Language:English
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Summary:Bitter sweeteners Paradoxically, artificial sweeteners such as sodium saccharin and acesulfame-K taken in high concentrations are not sweet: they can even seem bitter. And if the mouth is then rinsed out with water, it takes on a sweet taste. These observations have led to new insights into the action of the TAS1R taste receptor. As well as causing the ‘sweet water’ aftertaste, saccharin at high concentrations masks the effect of other sweeteners tasted at the same time. What emerges at the molecular level is a two-site system. Saccharin and acesulfame-K elicit a perception of sweetness when they bind to a high-affinity binding site; at high concentrations they bind to a second low-affinity inhibitory site. When the sweet taste inhibitors are washed away, the sweet receptor re-activates and the perception of sweetness returns. Sweet inhibitors are used in the food industry to offset the high sweetness that results from replacing fats with sweet carbohydrates in some reduced-fat products: the sweet water taste may be a useful predictor for sweet inhibitor activity. ‘Water-tastes’ are gustatory after-impressions elicited by water following the removal of a chemical solution from the mouth, akin to colour after-images appearing on ‘white’ paper after fixation on coloured images. Unlike colour after-images, gustatory after-effects are poorly understood 1 . One theory posits that ‘water-tastes’ are adaptation phenomena, in which adaptation to one taste solution causes the water presented subsequently to act as a taste stimulus 2 , 3 . An alternative hypothesis is that removal of the stimulus upon rinsing generates a receptor-based, positive, off-response in taste-receptor cells, ultimately inducing a gustatory perception 4 . Here we show that a sweet ‘water-taste’ is elicited when sweet-taste inhibitors are rinsed away. Responses of cultured cells expressing the human sweetener receptor directly parallel the psychophysical responses—water rinses remove the inhibitor from the heteromeric sweetener receptor TAS1R2–TAS1R3, which activates cells and results in the perception of strong sweetness from pure water. This ‘rebound’ activity occurs when equilibrium forces on the two-state allosteric sweet receptors result in their coordinated shift to the activated state upon being released from inhibition by rinsing 5 , 6 , 7 .
ISSN:0028-0836
1476-4687
1476-4679
DOI:10.1038/nature04765