Sweet taste

Sweet taste is generally perceived as pleasant and satisfying. The large spectrum of sweet-tasting molecules chemically diverse and includes carbohydrates, proteins, amino acids, peptides and complex proteins, heterocyclic compounds, terpenoids.

flavonoids. and steroidal compounds from plants. Their sweet taste has in some instances hundreds or e\en thousands of times greater intensity than sugar. Only a few of the intensely sweet compounds are approved in the United States for consumption (aspartame, acesulfame potassium, saccharin, sticralosc. amnion iated glycyrrhi/in. ihaumatin). a few others arc approved in other countries (Kinghom ci al.. !998).

Sv\cet-tasting carbohydrates include the oligosaccharides sucrosc and lactose, and ihe monosaccharides glucose, fructose, and galactose. The potency of carbohydrates differs considerably; fructose and sucrose taste sweeter than glucose i Stone and Oliver, 1969; SchilTman et al.. 2000). The sugar alcohols xylitol. sorbitol, lactitol. and maltitol are slightly less sweet than glucose.

Some amino acids, peptides, and proteins also have a sweet taste. Examples include D-amino acids and the artificial sweetener aspartame (N-L-a-aspanyl-L-phenytalanine I-methyl ester). Lysozyme (F.C.V2.1.17) from chicken and goose eggs tastes sv\ect. possibly due to an enzymic property that is independent of its ability to clea\e pepti-doglycan heteropolymers (Vlasuda et al.. 2001). Of greater practical importance may be the plant-derived complex proteins moneliin. ihaumatin, mabinlin. pentadin. brazzein. and curculin which are several thousand times sweeter than glucose (F-aus, 2000). Water and even sour substances still taste sweet after exposure to curculin has stopped (Kurihara. 1992), Miraculin is another taste-modifying glycoprotein that makes sour-tasting foods taste sweet (Thccrasilp and Kurihara. 1998).

Saccharin and other heterocyclic compounds also have an intense sweet taste, though with some lingering bitterness (Home er al.. 2002). This is an important example of overlapping taste qualities in a single compound. The nitrogen-containing sweetener cyclamate has similar characteristics. Concerns about the potential carcinogenicity of saccharine and cyclamate need to be mentioned.

The roots and rhizome of licorice contain 6—14% of the sweet-tasting terpenoid glycyrrhizin. Several related compounds also contribute to the sweet taste of the plant (Liu et at, 2000). Numerous plant glycosides have an intense sweet taste iKinghorn et al.. 1998), including mono-, di-. and tritcrpcnoids. saponins, proanthocyamdins. Another example of a plant-derived molecule with high potency is telosmoside A15. a polyoxypregnane glycoside I Huan at al.. 2001).

Sweet taste is inhibited, on the other hand, by the triterpene glycosides gvmnemic acid and ziziphin and the peptide gurmann (Kurihara, 1992).

Sweet taste is perceived throagh interaction of ingested molecules with one or more specific receptors. Sugars bind to G-protein-coupled reccptors that activate adenylate cyclase. The binding of synthetic sweeteners to G-protein-coupled receptors, in

Saccharin

Acesulfame

Sodium cyclamate

Figure 2.3 Artificial swrtrtmeri

Saccharin

Acesulfame

Sodium cyclamate

Figure 2.3 Artificial swrtrtmeri contrast, may I rigger the release of inositol triphosphate (IP3) by phospholipase C. Both IP3 and cAMP increase intracellular calcium ion concentration by promoting the release of calcium ions from intracellular stores and the flow of calcium ions across the basolatcral membrane into the cell The rise in calcium ion concentration can then trigger the secretion of the neurotransmitter serotonin into the synaptic cleft,

Heterodinters ofTlR3, a member of thcTIR family of taste receptors (Max ei at., 2001: Montmavcurw «/., 2001 ), with either T1KI or T1R2. respond to sweet-tasting substances, including sucrose and saccharin (Nelson et ai, 2001). The TIR1 Tl R3 dimer is present only in the front of tongue, the Tl R2:TI R3 dimer is present on the back of the tongue and on the palate. Intracellular signaling elicited by stimulation of these receptor dimers is mediated by the G-protein alpha-gustducin, a protein closely related to transducin of retinal rods.

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