For many years, people thought that there were four different tastes felt by the tongue -- sweet, sour, salty, and bitter. Then, over a century ago, Japanese scientist Kikunae Ikeda first proposed umami as a basic taste. About eight decades later, the scientific community officially agreed with him.
But now, University of California scientists maintain that there is a sixth basic taste and have published their claim in the prestigious journal Nature Communications under the title “The proton channel OTOP1 is a sensor for the taste of ammonium chloride.”
Neuroscientist and biological sciences Prof. Emily Liman and her team found that the tongue responds to ammonium chloride through the same protein receptor that signals sour taste. “If you live in a Scandinavian country, you will be familiar with and may like this taste,” she said. “In some northern European countries, salt licorice has been a popular candy at least since the early 20th century. The treat counts among its ingredients salmiak salt (ammonium chloride).
Ammonium, a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research.
Scientists have for decades recognized that the tongue responds strongly to ammonium chloride, but despite extensive research, the specific tongue receptors that react to it remained elusive. Liman and the research team thought they might have an answer.
Scientists find the 'sour' protein
In recent years, they uncovered the protein responsible for detecting sour taste. That protein, called OTOP1, sits within cell membranes and forms a channel for hydrogen ions moving into the cell. Hydrogen ions are the key component of acids, and as foodies everywhere know, the tongue senses acid as sour. That’s why lemonade, that is rich in citric and ascorbic acids, vinegar (acetic acid) and other acidic foods impart a zing of tartness when they hit the tongue. Hydrogen ions from these acidic substances move into taste receptor cells through the OTOP1 channel.
Because ammonium chloride can affect the concentration of acid – that is, hydrogen ions – within a cell, the team wondered if it could somehow trigger OTOP1. To answer this question, they introduced the Otop1 gene into lab-grown human cells so the cells produce the OTOP1 receptor protein. They then exposed the cells to acid or to ammonium chloride and measured the responses. “We saw that ammonium chloride is a really strong activator of the OTOP1 channel,” Liman said. “It activates as well or better than acids.”
Ammonium chloride gives off small amounts of ammonia, which moves inside the cell and raises the pH, making it more alkaline, which means fewer hydrogen ions.
They examined how mice react when given a choice to drink either plain water or water laced with ammonium chloride. Mice with a functional OTOP1 protein found the taste of ammonium chloride unappealing and did not drink the solution, while mice lacking the OTOP1 protein didn’t mind the alkaline salt, even at very high concentrations. This showed that the OTOP1 channel is essential for the behavioral response to ammonium.
Different animals vary in OTOP1 sensitivity
But the scientists weren’t done. They wondered if other animals would also be sensitive to and use their OTOP1 channels to detect ammonium. They found that the OTOP1 channel in some species seems to be more sensitive to ammonium chloride than in other species. And human OTOP1 channels were also sensitive to ammonium chloride.
Liman speculated that the ability to taste ammonium chloride might have evolved to help organisms avoid eating harmful biological substances that have high concentrations of ammonium.
“Ammonium is found in waste products like fertilizer and is somewhat toxic,” she explained, “so it makes sense we evolved taste mechanisms to detect it.
In the future, the researchers plan to extend these studies to understand whether sensitivity to ammonium is conserved among other members of the OTOP proton family, which are expressed in other parts of the body, including in the digestive tract.