In the early 1900s, Japanese scientist Kikunae Ikeda first proposed umami as a basic taste in addition to sweet, salty, sour, and bitter. It was not until eight decades later that the scientific community finally agreed with him. Recently, a team of researchers has found evidence of a new basic taste.
New Flavor Frontier
The study was conducted by experts from the University of Southern California Dornsife College of Letters, Arts, and Sciences. Led by neuroscientist Emily Liman, they discovered that our tongue responds to ammonium chloride through the same proton receptor that signals a sour taste, suggesting the existence of a sixth taste.
For several decades, scientists have recognized the ability of our tongue to respond strongly to ammonium chloride. Despite extensive research, however, the specific tongue receptor that reacts to it remains unknown. Liman and her team believe that they might finally have an answer.
Their previous studies unlocked the protein responsible for detecting sour taste. This protein, called OTOP1, is located within cell membranes and creates a channel of hydrogen atoms that move into the cell.
Hydrogen ions are the key components of acids the tongue senses as sour. They move into taste receptor cells through the OTOP1 channel. Since ammonium chloride affects the concentration of acids within a cell, the researchers wondered if it can also trigger OTOP1.
To answer this, the OTOP1 gene was introduced into lab-grown human cells to enable them to produce the OTOP1 receptor protein. The cells are then exposed to acid or ammonium, and the responses are measured.
It was found that ammonium chloride is a strong activator of the OTOP1 channel, activating as well or better than acids. Since ammonium chloride gives off small amounts of ammonia that move inside the cell, the pH is more alkaline, meaning fewer hydrogen ions. The difference in pH level drives a proton influx through the OTOP1 channel.
Liman and her team also used a technique that measures electrical conductivity to simulate how nerves conduct a signal. They used taste bud cells from normal mice and from mice that were previously genetically engineered not to produce OTOP1. Then, they measured the ability of the taste cells to generate electrical responses called action potentials upon introducing ammonium chloride.
A sharp increase in action potentials was observed in the taste bud cells from wild-type mice after adding ammonium chloride, while taste bud cells from the mice without OTOP1 failed to respond to the salt. This confirms the experts' theory that OTOP1 responds to the salt and generates an electrical signal in taste bud cells.
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Evolutionary Advantage
The scientists speculate that the ability of animals to taste ammonium chloride might have evolved to help them avoid eating harmful substances that contain high concentrations of ammonium. This compound is found in waste products, such as fertilizer, and is somewhat toxic. It makes sense that humans evolved taste mechanisms to detect this flavor.
There are also variations in the sensitivity of animals in using OTOP1 channels to detect ammonium. For instance, chicken OTOP1 is more sensitive to ammonium zebrafish, suggesting that variations may reflect differences in the ecological niches of various animals. Fish may not encounter much ammonium in the water, while chicken coops are filled with ammonium, which must be avoided.
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