Sour is the taste of summer, a taste that evokes lemonade stands and vine-ripe tomatoes. Among the five basic tastes — the others being bitter, sweet, salty and umami — it is arguably the subtlest.
But what causes the sensation of sourness? USC scientists may have solved the first mystery.
Fruits and vegetables that taste sour have a low pH, meaning they are high in acids, including citric acid for lemons, tartaric acid for grapes and acetic acids in fermented foods like vinegar. It has been recognized for more than a century that the low pH — which translates to high concentration of hydrogen ions (H+) — in these foods generates a perception of sourness in humans. But how the tongue senses pH, and specifically what molecule constitutes the pH sensor, was not known.
A group led by Emily Liman, professor of biological sciences at USC Dornsife College of Letters, Arts and Sciences, have reported in Current Biology that otopetrin 1 (Otop1) is a sensor for pH on the tongue.
Otop1 is a member of a class of molecules called ion channels, which allow charged ions to cross cell membranes. In the case of Otop1, the charged ion carried across the membrane is H+.
Last year, Liman’s team published research in Science that closed in on the sour-taste sensor. While the scientists had identified Otop1 as forming a proton channel, they did not show that it was required for sour-taste responses in an intact animal. The new study makes that finding for the first time.
“Our results show that Otop1 is a bona fide sour-taste receptor,” Liman said. “This is the first definitive evidence for a protein that is both necessary and sufficient for sour-taste receptor cells to respond to acids and stimulate the nerves to enable sour-taste perception.”
Identification of the molecule responsible for taste opens up possibilities for wide application. This information may lead to an understanding of individual differences in food preferences and flavor perception, guide nutrition science and lead to novel approaches to pest control. Professional flavorists and chemists can use this information to manipulate flavors to make food or even medications more pleasing to the palate while also making household products containing toxic chemicals less pleasing.
Liman is the lead author of the study, joined by co-authors Teng and Tu at USC Dornsife; Wilson and Kinnamon of University of Colorado Medical School; and Narendra R. Joshi of Harvard Medical School.
This work was supported by NIH grants (#R01 DC013741, #R01 DC012555, #F31 DC015700 and #DP1 AT009497) and the Howard Hughes Medical Institute.