Last month, Dr. Hiroaki Matsunami at Duke University Medical Center, reported the first study to define how humans perceive sour taste and identified the proteins, PKD1L3 and PKD2L1 (Proceedings of the National Academy of Sciences, Vol. 103, pp. 12569-12574).
The identification of these proteins, it was said, could lead to ways to manipulate the perception of taste in order to fool the mouth that something sour, such as some children's medicines or health foods, tastes sweet.
Matsunami said that further studies should focus on using animals with PKD1L3 and PKD2L1 'knocked out' (the genes will not code for the proteins of the same name) to further define the role of these proteins.
And such a study has come out, hot on the heels of the Duke reports. Charles Zuker from Howard Hughes Medical Institute and Nicholas Ryba from the National Institutes of Health used created genetically engineered mice that produced a toxin in cells that expressed PKD2L1, thus killing the cells.
They then recorded the nerve signals and tongue function coming from taste cells in the genetically-engineered mice. No matter what sour compounds they fed the mice, no nerve signals from the taste cells were detected showing that the animals were completely insensitive to the acids. The mice were still able to taste sweet, bitter, umami, and salt.
"Killing these cells and showing that the mice now are totally unable to detect sour proved that these cells are the sensors for sour taste, and that indeed no other taste cells detect sour," said Zuker of the research that was published in the journal Nature (Vol. 442, pp. 934-938).
Several intriguing questions can be pursued now that the sour-taste cells and candidate receptor have been found, said the researchers. One is how the PKD2L1 receptor is activated by acid stimuli.
"This work also proved that salt-sensing cells, just like those mediating sweet, bitter, umami and sour, must function as independent sensors because the "sourless mice" have perfectly normal salt perception," said Zuker.
"So this opens an exciting experimental platform to molecularly dissect the last of the five basic taste qualities: salt taste."
Bitter chemicals are said to be detected by 30 or so so-called T2R receptors. Sugars and sweet flavours are detected by T1R2 and T1R3 receptors, and umami tasting L-amino acids are reported to be detected by T1R1 and T1R3 receptors.
Contrary to popular understanding, taste is not experienced on different parts of the tongue. Though there are small differences in sensation, which can be measured with highly specific instruments, all taste buds, essentially clusters of 50 to 100 cells, can respond to all types of taste.