Identification of TAS2R bitter taste receptor genes

Common knowledge about bitter taste shaped the search strategies of two groups who finally succeeded in cloning the long sought-for genes encoding putative bitter taste receptors (Adler et al. 2000; Matsunami et al. 2000). Firstly, based on the observation that bitter taste transduction is mediated by the G protein a-gustducin (McLaughlin et al. 1992; Wong et al. 1996a, b; Ming et al. 1998, 1999; Ruiz-Avila et al. 2000), bitter taste receptors must belong to the GPCR family and expressed in a-gustducin-positive TRCs. Secondly, in light of the numerous bitter tastants, not a single gene but rather a distinct gene family was expected to

Table 2 Compilation of various bitter phytonutrients together with their presumed beneficial and detrimental effects on health

Substances

Food source

Possible health benefits

Possible detrimental effects

Carotenoids, including beta-carotene, lycopene, lutein, xanthophylls

Limonoids, including, d-limonin, limonin glycoside

Phenol flavonoids, including catechins, anthocyanins, proan-thocyanidins, naringin, quercetin, tangeritin, gingko

Isoflavones, including, genistein, diadzein, gly-cyrrhizin, isocoumarin

Phenolic acids, including, gallic acid, ellagic acid, caffeic acid, isocoumarin

Glucosinoglates/isothio-cyanates, including allyl isothiocyanid

Polyphenols, including, tannins and resveratrol

Tomato, carrot, sweet potato, watermelon, spinach

Citrus fruit

Tea, berries, wine, citrus fruit, apple, endive, cranberry, onion, kale

Miso, soy milk, soy nut, tofu, licorice, carrot

Tea, strawberry, blueberry, apple, orange, grapefruit, white potato, grape juice, coffee, prune

Cabbage, kale, brussel sprout, broccoli

Wine

Quench singlet oxygen, increase cell-cell communication, decrease risk macular degeneration

Promote protective enzymes, antiseptic

Antioxidants, alter tyrosine kinase activity, decrease capillary fragility and permeability, anti-inflammatory

Metabolize to estrogenlike compounds, decrease tyrosine ki-nase activity, lower cholesterol

Increase phase II enzyme activity, antioxidant, inhibit N-nitrosation reactions

Increase phase II enzyme activity, decrease deoxyribonucleic acid (DNA) methylation

Inhibit tumor initiation, promotion, and progression, antiinflammatory, antioxidant

Interference with drug metabolism through inhibition of cytochrome P-450

Goitrogenic activity, estrogenic activity

Goitrogenic activity, contact dermatitis, carcinogenic activity

Interference with protein absorption, reduction of iron availability

Adapted from (Barratt-Fornell and Drewnowski 2002)

encode bitter taste receptors. Thirdly, genetic mapping of inherited bitter taste differences identified distinct loci determining the sensitivity to sucrose octaacetate, cycloheximide, raf-finose undecaacetate, and quinine on mouse chromosome 6 (Lush 1981, 1984, 1986; Lush and Holland 1988) as well as to propylthiouracil (PROP) on human chromosomes 5 and 7 (Reed et al. 1999). Searching the human genome database in intervals linked to bitter taste or in intervals syntenic to the mouse bitter taste loci the first genes encoding members of a new family of GPCRs were identified and called T2Rs or TRBs (Adler et al. 2000; Matsunami et al. 2000). With the available sequence information of these genes several groups cloned additional members of this family from rat, mouse, human, bonobo, chimpanzee, gorilla, orangutan, rhesus macaque, and baboon (Bufe et al. 2002; Conte et al. 2002, 2003; Shi et al. 2003; Parry et al. 2004; Fischer et al. 2005; Go et al. 2005). As many groups more or less simultaneously published their data confusion arose due to the non-consistent or overlapping nomenclature of the bitter taste receptor genes. In this review, the author follows

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