Bitter-sensitive protein

Taste Threshold of Bitter Compounds, and Equilibrium Constants of Binding of Bitter Compounds by a Bitter-sensitive Protein ... 

Several aspects affect the extent and character of taste and smell. People differ considerably in sensitivity and appreciation of smell and taste, and there is lack of a common language to describe smell and taste experiences. A hereditary or genetic factor may cause a variation between individual reactions, eg, phenylthiourea causes a bitter taste sensation which may not be perceptible to certain people whose general abiUty to distinguish other tastes is not noticeably impaired (17). The variation of pH in saUva, which acts as a buffer and the charge carrier for the depolarization of the taste cell, may influence the perception of acidity differently in people (15,18). Enzymes in saUva can cause rapid chemical changes in basic food ingredients, such as proteins and carbohydrates, with variable effects on the individual. 

Wine is one of the most complex and interesting matrices for a number of reasons. It is composed of volatile compounds, some of them responsible for the odor, and nonvolatile compounds which cause taste sensations, such as sweetness (sugars), sourness (organic acids), bitterness (polyphenols), and saltiness (mineral substances Rapp and Mandary, 1986). With a few exceptions, those compounds need to be present in levels of 1%, or even more, to influence taste. Generally, the volatile components can be perceived in much lower concentrations, since our organs are extremely sensitive to certain aroma substances (Rapp et ah, 1986). Carbohydrates (monosaccharides, disaccharides, and polysaccharides), peptides, proteins, vitamins, and mineral substances are among the other wine constituents. 

Just as in olfaction, a number of clues pointed to the involvement of G proteins and, hence, 7TM receptors in the detection of bitter and sweet tastes. The evidence included the isolation of a specific G protein a subunit termed gustducin, which is expressed primarily in taste buds (Figure 32.13). How could the 7TM receptors be identified The ability to detect some compounds depends on specific genetic loci in both human beings and mice. For instance, the ability to taste the bitter compound 6- -propyl-2-thiouracil (PROP) was mapped to a region on human chromosome 5 by comparing DNA markers of persons who vary in sensitivity to this compound. 

Arc these proteins, in fact, bitter receptors Several lines of evidence suggest that they are. First, their genes are expressed in taste-sensitive cells — in fact, in many of the same cells that express gustducin. Second, cells that express individual members of this family respond to specific bitter compounds. For example, cells that express a specific mouse receptor (mT2R5) responded when exposed specifically to cycloheximide. Third, mice that had been found unresponsive to cycloheximide were found to have point mutations in the gene encoding mT2R5. Finally, cycloheximide... 

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