Japanese scientist Kikunae Ikeda first proposed umami as a primary taste (in addition to sweet, sour, salty, and bitter) in the early 1900s. Nearly eighty years later, the scientific community officially agreed with him. Now, scientists led by researchers from the Dornsife College of Letters, Arts and Sciences at the University of Southern California have obtained evidence for the existence of a sixth basic pleasure.
In a study published in the journal Nature Communications, USC Dornsife neuroscientist Emily Leeman 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’re familiar with the taste and you might like it,” says Leeman, a professor of biological sciences. In some Northern European countries, salted licorice has been a popular candy since at least the early 20th century. The delicacy contains salmiac salt or ammonium chloride.
Scientists have known for decades that the tongue responds strongly to ammonium chloride. However, despite extensive research, specific language receptors that respond have remained elusive.
Lyman and his research team believe they may find an answer. In recent years, they discovered the protein responsible for the sour taste. This protein, called OTOP1, is found inside cell membranes and creates a channel for hydrogen ions to enter the cell.
Hydrogen ions are an important component of acids, and foodies know that the tongue perceives acid as sour. Therefore, lemonade (rich in citric and ascorbic acid), vinegar (acetic acid) and other acidic products give a sharp astringency when they hit the tongue. Hydrogen ions from these acidic substances move towards taste receptor cells through the OTOP1 channel.
Since ammonium chloride can affect the concentration of acid, or hydrogen ions, within the cell, the team wondered whether this could somehow cause OTOP1.
To answer this question, they inserted the Otop1 gene into human cells grown in the laboratory so that the cells could produce the OTOP1 receptor protein. They then exposed the cells to acid or ammonium chloride and measured the response.
“We found that ammonium chloride is a really potent activator of the OTOP1 channel,” Leeman said. “It activates as well, if not better, than acids.”
Ammonium chloride releases a small amount of ammonia, which moves within the cell and raises the pH, making the cell more alkaline and therefore less hydrogen ion-rich.
Ph.D., a student in Lyman’s lab and the study’s first author. “This pH difference triggers proton flow through the OTOP1 channel,” explained Ziyu Liang.
To verify that their results were not just laboratory artifacts, they turned to a method that measures electrical conductivity, which mimics the way nerves transmit signals. Using taste bud cells from normal mice and mice that the lab had previously genetically engineered not to produce OTOP1, they measured how well the taste cells produced electrical responses called action potentials when injected with ammonium chloride.
While taste bud cells from wild-type mice showed a dramatic increase in action potentials after the addition of ammonium chloride, taste bud cells from OTOP1-null mice did not respond to salt. This supported the hypothesis that OTOP1 responds to salt by generating an electrical signal in taste receptor cells.
The same thing happened when another member of the research team, Courtney Wilson, recorded nerve signals innervating taste cells. He saw how nerves responded to the addition of ammonium chloride in normal mice, but not in mice lacking OTOP1.
The team then went one step further and examined how the mice responded when given the choice between drinking plain water or water with added ammonium chloride. In these experiments, they disabled the bitter cells that detect the taste of ammonium chloride. Mice with a functional OTOP1 protein found the taste of ammonium chloride appealing and drank the solution, whereas mice without OTOP1 protein did not object to alkaline salt even at very high concentrations.
“That was a really defining moment,” Leeman said. “This suggests that the OTOP1 channel is important for the behavioral response to ammonium.”
But the scientists are not done. They asked whether other animals would also be sensitive to OTOP1 channels and use them to detect ammonium. They found that the OTOP1 channel was more sensitive to ammonium chloride in some species than in others. Human OTOP1 channels were also sensitive to ammonium chloride.
So what is the advantage of tasting ammonium chloride, and why is it so evolutionarily conservative?
Lyman suggests that the ability to taste ammonium chloride may have evolved to help organisms avoid consuming high concentrations of harmful biological substances. Source
Source: Port Altele
