Researchers have identified a huge family of receptors that help us taste bitter compounds. The finding, published in the 17 March Cell, solves a major riddle in taste research and may help researchers find antidotes for bitter-tasting medicines and foods.
Scientists have struggled for years without much success to identify receptors for the five different tastes--sweet, bitter, sour, salty, and umami (MSG)--that the taste cells in our taste buds detect. Only in January did umami reveal its secret (ScienceNOW, 26 January). Bitter has been an especially perplexing flavor, because a wide range of unrelated chemicals all taste similarly bitter even though their diverse structures suggest that they must trigger different receptor molecules.
To tackle the problem, a team led by Nicholas Ryba of the National Institute of Dental and Craniofacial Research and Charles Zuker of the University of California, San Diego, started out with PROP, a bitter compound that only some people can taste. Last year, researchers narrowed down the chromosomal location of the gene responsible for that difference. The team also knew that bitter receptors interact with so-called G proteins, which are involved in intracellular signaling in taste and other responses. So they looked near the PROP-tasting mutation for genes that might encode receptors able to interact with G proteins. They found one, which later turned out to be part of a family of at least 50 genes.
Next, they isolated the mouse counterparts of the human genes and investigated which taste cells in mice express them. They discovered that individual bitter-tasting cells generally express not just one or two of the 50 or so receptor genes, but most of them. As a result, each cell should be able to detect a wide variety of compounds. This may explain why the brain can't distinguish among bitter chemicals: No matter which receptor type is activated, the cell will send the same signal to the brain.
Experts say the findings unequivocally nail the genes as bitter receptors. "This is clearly a major breakthrough for taste research," says Gary Beauchamp, director of the Monell Chemical Senses Center in Philadelphia. "It all fits together in a very nice story."