杏吧原创

TASTE buds on rats鈥� tongues have shown how we sense monosodium glutamate, a
fundamental taste component together with bitter, sweet, salty and sour.
Researchers say that the new result highlights how evolution has fine-tuned our
mouths to be sensitive to the nutritional content of food.

Monosodium glutamate is a meaty flavouring often added to Chinese and far
eastern cuisine. Better known as umami, a Japanese word meaning
鈥渄elicious savoury taste鈥�, the monosodium glutamate taste applies equally to
glutamate alone, a raw amino acid found in meat. Despite its oriental
connections, umami is widespread in the Western diet, in foods such as
a ham-topped pizza, for example.

Nirupa Chaudhari and her colleagues at the University of Miami School of
Medicine in Miami, Florida, had a hunch that our taste buds have docking
points鈥攐r receptors鈥攆or glutamate similar to those found in the
brain. Researchers already know there are dozens of molecules on the surface of
brain cells that are receptors for glutamate. Glutamate also happens to be the
most common neurotransmitter ferrying electrical signals between brain
cells.

To confirm her hunch, Chaudhari鈥檚 team analysed genetic material in cells
from rat taste buds. And it turned out that the mGluR4 gene, which
makes the glutamate receptor in brain cells, is also active in taste bud
cells.

When they equipped hamster cells with the taste receptor gene from rats, the
cells reacted to glutamate in a similar way to brain cells. There was a crucial
difference, though鈥攊t took 1000 times as much glutamate to trigger a
response in the tongue cells as in brain cells.

The team later worked out why. The taste version of the receptor binds less
strongly to glutamate because it has a different structure from the one found in
the brain. In fact, single genes often make different variations of the same
protein as needed. The gene always makes the same RNA, but the final protein can
be folded up in different ways or have corners snipped off depending on the
enzymes present.

The differences between brain and taste receptors for glutamate make sense,
say the researchers. They point out that we鈥檇 be overwhelmed with umami
flavours if the taste receptors reacted as strongly to glutamate as brain
cells.

鈥淚t turns out that this taste receptor detects glutamate at concentrations
relevant for taste,鈥� says Chaudhari. 鈥淚t means that evolution tunes receptors to
ranges that are relevant, and taste exists to identify nutritionally relevant
concentrations of food ingredients.鈥�

She says that the receptor protein straddles the surface of the taste bud
cell, relaying signals inside as soon as glutamate binds. The well-known
receptors for the salty and sour components of food work differently. The ions
that trigger both these types of taste are small enough to zoom into molecular
channels in the surface of taste bud cells, creating electrical signals in their
own right.