Proteins that help sense sweet and spice also help mice detect iron, zinc, copper
Yukon Cornelius isn’t the only one with a taste for metals. While most people probably can’t find silver and gold by nibbling snow, as Cornelius seems to do in the Rudolph movie*, new research shows that taste buds can detect iron, zinc, copper, magnesium and other metals.
The source of metallic taste has long been elusive, but a study in the Feb. 25 Journal of Neuroscience traces the sensation to a combination of proteins used in detecting sweetness and the pain of red-hot chili peppers, and other as-of-yet unidentified proteins.
Scientists used to believe that there were only a handful of tastes the tongue could register — sweet, sour, bitter, salty and umami, a delicious, meaty taste found in monosodium glutamate, Parmesan cheese and portobello mushrooms. Scientists define a taste as something that is detected by a specific combination of proteins in taste buds, as distinct from a flavor that results from a combination of tastes and odors.
But it would be impossible to describe all the differences between chicken soup and lobster bisque using only the known tastes, says Johannes le Coutre of the Nestlé Research Center in Lausanne, Switzerland. So researchers think there are many other taste sensations. Le Coutre, Céline Riera and colleagues conducted the new study to find out if they could explain just one of them — metallic taste.
The researchers fed mice water containing varying concentrations of metal salts. Mice preferred water with low concentrations of iron and zinc over distilled water, but avoided high concentrations of the metals. The mice rejected copper or magnesium sulfate at any concentration.
The team found that mice lacking either the TRPM5 protein, which is involved in detecting sweet, umami and bitter tastes, or the T1R3 protein, another sweet and umami-detecting protein, shunned iron and zinc even at low concentrations, but lost some of their aversion to magnesium and copper.
Another protein, called TRPV1, is involved in the pain response to spicy foods. Mice that lack TRPV1 overcame some dislike of copper salts and high concentrations of iron. Those results indicate that the three proteins — involved in other tastes — are components of metallic taste.
But other proteins, independent of these three, must still be working as metal detectors because none of the mice completely lost their aversion to magnesium and copper or high concentrations of iron and zinc. The researchers don’t yet know which additional proteins are involved in the full response to metal.
“This is the most sophisticated work to date on metallic taste,” says Michael Tordoff of the Monell Chemical Senses Center in Philadelphia. People show a similar response to metallic tastes, preferring the low concentrations of metals found in mineral or tap water to the taste of soft or distilled water, he says.
Researchers are on the trail of other possible tastes, including sets of proteins that might detect fats or starches, Tordoff says. “The idea that there are four or five basic tastes is dying, and this is another nail in that coffin — probably a rusty nail given that it’s metallic taste.”
*Scenes deleted from some versions of the movie reveal that Cornelius was actually looking for peppermint, not silver and gold.
Riera, C.E. . . . and le Coutre, J. 2009. Sensory attributes of complex tasting divalent salts are mediated by TRPM5 and TRPV1 channels. Journal of Neuroscience 29(Feb.25):2654-2662. DOI:10.1523/JNEUROSCI.4694-08.2009