Taste receptor site

Figure 3. Positioning of /3-d-glucose over the proposed taste receptor site to initiate sweet taste response...

Brand, J. G., Bryant, B. P., Cagan, R. H., and Kalinoski, D. L., Biochemical studies of taste sensation. XIII. Enantiomeric specificity of alanine taste receptor sites in catfish, Icatalurus punctatus, Brain Res., 416, 119, 1987. 

Temussi, P.A., Lelj, F., and Tancredi, T. (1978). Three-dimensional mapping of the sweet taste receptor site. ]. Med. Chem. 21,1154-1158. 

Section 7 8 Both enantiomers of the same substance are identical m most of then-physical properties The most prominent differences are biological ones such as taste and odor m which the substance interacts with a chiral receptor site m a living system Enantiomers also have important conse quences m medicine m which the two enantiomeric forms of a drug can have much different effects on a patient... 

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

The speed with which taste stimulation occurs, coupled with the fact that stimulation with toxic substances does no damage to the receptors, led Beidler to suggest that taste stimulus need not enter the interior of the taste cell in order to initiate excitation. Because a taste cell has been shown to be sensitive to a number of taste qualities, and to a large number of chemical stimuli, he and his coworkers concluded that a number of different sites of adsorption must exist on the surface of the cell. Therefore, they assumed that taste response results from adsorption of chemical stimuli to the surface of the receptor at given receptor sites. This adsorption is described by a monomolecular reaction similar to that assumed by Renqvist, Lasareff, and Hahn, but with a difference. From the fact that each type of chemical-stimulus compound has a unique level of saturation of the taste receptor, it was concluded that the magnitude of the response is dependent on the initial reaction with the receptor, and not on other, subsequent receptor-reactions that are common to all types of receptor stimulation. Therefore, it was assumed that the magnitude of neural response is directly proportional to the number of sites filled, the maximum response occurring when all of the sites are filled. Beidler derived a fundamental... 

Fig. 21.—Schematic Diagram of the Proposed Types of Bonding of Sweet Glycosides to the Taste-bud Receptor-sites. ...

That the sweet and bitter responses are intimately associated is clear from the results of gustatory studies of all of the conformationally defined sugars and of other organic compounds. If a carbohydrate has any taste at all, this is invariably sweet, bitter-sweet, or bitter. Chemical modification may alter the taste of a sweet compound so that the product is bitter-sweet or bitter, and it is now generally agreed that the two basic tastes may each be a feature of a single compound. It appears, therefore, that the interactions of these polyfunctional stimulants involve two different sets of receptor sites, representing sweet and bitter modalities. ... 

Taste and smell, unlike sight and sound, are not induced by any known waves in the air, but are caused by the presence of certain specific, chemical compounds by interaction with certain receptor-sites. The power of reaction to chemical stimuli has evolved from the diffused, and confused, chemotactic sensitivity of the primitive protozoa through to man, where specialized organs have been developed. 

It is interesting that the stimulus compounds used in the study differ widely in their molecular structures, and yet they all interact with antibodies to thaumatin. It is, therefore, probable that a single receptor-structure responds to all sweet stimuli,there being a variation in the relative effectiveness of sweet stimuli across individual nerve-fibers, and the characteristics of all receptor sites do not appear to be identical. Earlier elec-trophysiological studies of single primary, afferent taste-neurons uniformly agreed that individual fibers very often have multiple sensitivities, and that individual, gustatory receptors are part of the receptive field of more than one afferent fiber. " We have yet to learn how these interact, and the nature of their excitatory, or possible inhibitory, relations, or both. 

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