Multiple receptor sites mediate sweetness: evidence from cross adaptation.

The method of cross adaptation was implemented to determine whether only one type of receptor site mediates the perception of sweetness, or whether more than one such type exists. Fourteen stimuli, seven artificial sweeteners varying widely in chemical structure as well as seven sugars, were cross adapted with one another. When a sugar was employed as the adapting stimulus, a consistent reduction in the intensity of the test solution's sweetness was found. However, the result of the cross adaptation when the adapting stimulus was an artificial sweetener was unpredictable; it led not only to a reduction but, in some cases, to an enhancement or no change in the test solution's intensity, depending on its identity. In previous investigations, enhancements have been explained through the existence of a water taste. Since this explanation is insufficient to account for the enhancement effects found in this study, it appears that cross adaptation does not always occur between sweet-tasting compounds. For this reason, it is concluded that more than one receptor mechanism may be responsible for the perception of the sweet quality.

Structural functions of the sweet pharmacophore.

The relative sweetness, onset times, and durations of response of D-glucose, D-xylose, D-quinovose, D-galactose, L-arabinose, and D-fucose were determined at four temperatures. The results can be interpreted by simple concepts of intramolecular hydrogen bonding which indicate that the so-called gamma-function of the tripartite AH,B, gamma sweet pharmacophore plays little or no part in sugar sweetness. Probably the Lemieux effect (intramolecular hydrogen bonding between the hydroxymethyl substituent and the 4-hydroxy group) is of overriding importance in determining sugar sweetness, and the separate features of intensity and time of response indicate distinct functions of chemoreception. The absence of a gamma-function in simple hydrophilic molecules such as glucose has already been emphasized. This function distinguishes them from the artificial sweetners such as saccharin, which may be 500 times or more sweeter than sucrose, depending on their stereostructure and lipophilicity.

Molecular mechanism of sweet taste: relationship of hydrogen bonding to taste sensitivity for both young and elderly.

Taste detection thresholds for 11 sweeteners varying widely in chemical structure were determined for young and elderly subjects. The rank orders of the thresholds for young and elderly were highly correlated (Spearman's rho=0.976) although the thresholds were 2.72 times higher on the average for the elderly when compared with the young. This decreased sensitivity was statistically highly significant, p=0.000266. Sweeteners with the lowest detection thresholds tended to have the greatest number of units (AH-B systems) capable of intermolecular hydrogen bonding. In addition, use of the method of magnitude estimation with 10 sweeteners revealed that the elderly perceived less growth in intensity with increasing concentration than young subjects. The slopes of the psychophysical functions relating concentration and perceived intensity were flatter in all cases for elderly subjects; the mean ratio, slope(young)/slope(elderly), was 2.06. The largest relative decline in slope was found for those stimuli with the greatest number of possible AH-B types, suggesting that the possibilities for concerted intermolecular hydrogen bonding may decline with age. If the elderly compensate for their diminished taste acuity with increased consumption, they may constitute a population with higher than average risk of any adverse dose-related biological effects of artificial sweeteners.