Meaty receptor helps tongue savor flavor

These days, monosodium glutamate (MSG) is doing more than spicing up Asian food. It’s giving scientists a taste of how the tongue recognizes flavors.

Scientists have struggled for many years to identify taste receptors—proteins on the surface of the tongue’s taste cells that recognize flavor-producing chemicals in food. “The effort . . . has not been very rewarding up to now,” says taste researcher Bernd Lindemann from Saar University in Homburg, Germany.

Researchers from the University of Miami School of Medicine report in the February Nature Neuroscience that they have identified the cell receptor specific for a taste called umami, the distinctive flavor of MSG.

Some scientists say that along with the four familiar tastes—salty, sweet, sour, and bitter—umami is a basic flavor recognized by the tongue. Loosely translated from Japanese, umami means delicious or yummy. Umami is the savory, meaty flavor of parmesan cheese, mushrooms, meat, and many Asian foods.

Last year, researchers reported that they had found two other possible taste receptors (SN: 2/27/99, p. 132: What tastes those molecules sense remains uncertain, says Lindemann, commenting in the same issue of Nature Neuroscience. The new study is the first to link a specific taste with a receptor protein that recognizes flavors. “I think this is a really important finding,” says neuroscientist Sue C. Kinnamon from Colorado State University in Fort Collins.

In 1996, Nirupa Chaudhari, who led the current study, and her coworkers at Miami identified a protein called mGluR4, which they thought corresponded to a receptor for umami. This molecule binds L-glutamate, the main food chemical that gives the umami flavor.

The candidate receptor, however, turned out to be far too sensitive, says Lindemann. High concentrations of glutamate in protein-rich foods would completely overwhelm any cells that used mGluR4 as a taste receptor.

In the new study, the researchers found a shortened version of mGluR4 in the taste buds. This receptor, which they call taste-mGluR4, lacks part of a pocket in the longer protein that recognizes and binds glutamate. So the receptor is much less sensitive to glutamate than mGluR4 is.

Cells producing the taste-mGluR4 receptor respond to glutamate at the same concentrations that produce the umami taste in rats and people, the researchers say.

“Everything is now consistent and appropriate. Before it was just inferences,” says Stephen D. Roper, who coauthored the study with Chaudhari and Ana Marie Landin.

Food and drug companies have a special interest in taste-receptor research. Knowing the structure of a taste receptor may enable manufacturers to design new sweeteners and other additives that could block bitter tastes or enhance the flavor of food.

Such flavor enhancers could benefit elderly people who have lost much of their sense of taste, says Lindemann. “They will be able to participate in the joy of eating again,” he says.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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