In the early 1900s, Japanese scientist Kikunae Ikeda proposed umami as a new basic taste alongside sweet, sour, salty, and bitter. Decades later, the scientific community accepted this, but now, researchers from USC Dornsife College of Letters, Arts, and Sciences suggest there might be a sixth basic taste.
Published in Nature Communications, the study led by USC Dornsife neuroscientist Emily Liman reveals that the tongue responds to ammonium chloride through the same protein receptor as sour taste. This phenomenon, familiar to those in Scandinavian countries where salty licorice is popular, has intrigued scientists for years.
Liman’s team discovered the protein responsible for detecting sour taste, known as OTOP1. It forms a channel for hydrogen ions to enter taste receptor cells. Since ammonium chloride can affect the concentration of acid in cells, the researchers wondered if it could trigger OTOP1.
To test this, they introduced the Otop1 gene into lab-grown human cells, which produced the OTOP1 receptor protein. When exposed to ammonium chloride, these cells showed strong OTOP1 channel activation, comparable to acids.
Ammonium chloride releases ammonia, raising the cell’s pH and reducing hydrogen ions. This pH difference causes proton influx through the OTOP1 channel, creating an electrical signal in taste bud cells.
In experiments with mice, those with functional OTOP1 proteins found ammonium chloride unappealing, while mice lacking OTOP1 didn’t mind it, confirming the importance of OTOP1 in the taste response.
The researchers also discovered that the OTOP1 channel’s sensitivity to ammonium chloride varies among species, suggesting an evolutionary advantage. Liman speculates it helps organisms avoid consuming harmful substances rich in ammonium.
This early research highlights a specific amino acid necessary for the OTOP1 channel’s ammonium response, conserved across species due to its importance in survival.
Future studies will explore whether ammonium sensitivity extends to other members of the OTOP proton family, found in various body parts, including the digestive tract. This research might expand our understanding of basic tastes, potentially adding ammonium chloride as the sixth.