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Beyond Umami: Scientists Discover a Sixth Basic Taste [scitechdaily.com]:

ByBesides the well-known tastes of sweet, salty, sour, bitter, and umami, recent research proposes that the tongue might also detect ammonium chloride as a basic taste.

Japanese scientist Kikunae Ikeda first proposed umami as a basic taste in the early 1900s, in addition to the recognized tastes of sweet, sour, salty, and bitter. It took nearly eighty years for the scientific community to officially acknowledge his proposition.

Now, scientists led by researchers at the USC Dornsife College of Letters, Arts, and Sciences [scitechdaily.com] have evidence of a sixth basic taste.

In a study recently published in the journal Nature Communications, USC Dornsife neuroscientist 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,” says Liman, professor of biological sciences. 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, or ammonium chloride.In-Depth Research on the Tongue’s Reaction

Scientists have for decades recognized that the tongue responds strongly to ammonium chloride. However, despite extensive research, the specific tongue receptors that react to it remained elusive.

Liman and the research team thought they might have an answer.

In recent years, they uncovered the protein [doi.org] 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 (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.

Animal Reactions and the Role of 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. 

“This pH difference drives a proton influx through the OTOP1 channel,” explained Ziyu Liang, a Ph.D. student in Liman’s lab and the first author on the study.

To confirm that their result was more than a laboratory artifact, they turned to a technique that measures electrical conductivity, simulating how nerves conduct a signal. Using taste bud cells from normal mice and from mice the lab previously genetically engineered to not produce OTOP1 [cell.com], they measured how well the taste cells generated electrical responses called action potentials when ammonium chloride was introduced.

ByBesides the well-known tastes of sweet, salty, sour, bitter, and umami, recent research proposes that the tongue might also detect ammonium chloride as a basic taste.

Japanese scientist Kikunae Ikeda first proposed umami as a basic taste in the early 1900s, in addition to the recognized tastes of sweet, sour, salty, and bitter. It took nearly eighty years for the scientific community to officially acknowledge his proposition.

Now, scientists led by researchers at the USC Dornsife College of Letters, Arts, and Sciences [scitechdaily.com] have evidence of a sixth basic taste.

In a study recently published in the journal Nature Communications, USC Dornsife neuroscientist 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,” says Liman, professor of biological sciences. 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, or ammonium chloride.In-Depth Research on the Tongue’s Reaction

Scientists have for decades recognized that the tongue responds strongly to ammonium chloride. However, despite extensive research, the specific tongue receptors that react to it remained elusive.

Liman and the research team thought they might have an answer.

In recent years, they uncovered the protein [doi.org] 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 (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.

Animal Reactions and the Role of 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. 

“This pH difference drives a proton influx through the OTOP1 channel,” explained Ziyu Liang, a Ph.D. student in Liman’s lab and the first author on the study.

To confirm that their result was more than a laboratory artifact, they turned to a technique that measures electrical conductivity, simulating how nerves conduct a signal. Using taste bud cells from normal mice and from mice the lab previously genetically engineered to not produce OTOP1 [cell.com], they measured how well the taste cells generated electrical responses called action potentials when ammonium chloride was introduced.

Journal Reference:
Just a moment..., (DOI: 10.1126/science.aao3264 [doi.org])

Original Submission