Human taste sensor for amino acids found

[Guest guest]

http://www.hhmi.org/news/zuker3.html

Homing In On a Receptor for the Fifth Taste

" Since amino acids are essential building blocks of biologically important

molecules, it made evolutionary sense for

there to be a taste pathway that would make amino acids attractive to consume, "

said HHMI investigator Zuker.

February 25, 2002— Humans can recognize five tastes: bitter, salty, sour, sweet

and umami. Of the five, however, umami

is the most difficult to describe — it’s the flavor associated with monosodium

glutamate (MSG). Now, researchers have

identified a taste receptor that responds to amino acids, including umami, and

they hope to develop a more precise

description of the molecular events that allow the brain to perceive the five

different tastes.

With the discovery of the new receptor, scientists have now identified taste

receptors for amino acids, bitter and sweet

tastes. Given that many amino acids are essential components of our diet, this

work may also aid understanding of how

animals, including humans, regulate nutritional intake to achieve a balanced

diet. Better understanding of taste

receptors may permit scientists in the food industry to formulate new products

that have specific tastes.

A research team led by Medical Institute investigator S.

Zuker at the University of California,

San Diego, and J. P. Ryba of the National Institutes of Health reported

the identification of an amino-acid

taste receptor in an advanced online publication in Nature on February 25, 2002.

Zuker’s and Ryba’s groups previously collaborated in discovering sweet and

bitter taste receptors. After they had

identified those receptors, they set their sights on finding a taste receptor

for amino acids, reasoning that one must

exist because it had long been known that humans have the ability to taste umami

and other amino acids. " Since amino

acids are essential building blocks of biologically important molecules such as

proteins, it made evolutionary sense for

there to be a taste pathway that would make amino acids attractive to consume, "

said Zuker.

In their search for the amino acid receptor, the scientists focused on T1R

receptors, a family of proteins that are

distantly related to receptors in the brain that recognize the amino acid

glutamate and related chemicals. Different T1R

genes can be expressed in cells in different combinations to yields cells that

respond to a specific taste. For example,

T1R2 and T1R3, designated T1R2+3, combine to function as a sweet receptor.

To test various receptor possibilities, the scientists devised a human cell

culture method in which combinations of T1R

subunits were expressed in cells. This permitted the scientists to assess how

cells bearing different combinations of

T1R genes responded to particular amino acids.

Using the cell culture technique, Greg , a graduate student in Zuker’s

lab, discovered that the combination of

cells bearing T1R1 and T1R3 functioned as a " broadly tuned " receptor that was

stimulated by many amino acids. This

" T1R1+3 " receptor combination was highly selective for L-amino acids, which are

found in nature. D-amino acids, which

are mirror images of L-amino acids and do not occur naturally, did not activate

the receptor.

The scientist also tested whether their newly identified amino acid receptor

candidate behaved in a manner similar to

receptors that recognize glutamate. A signature of the umami taste is that it is

boosted by purine nucleotides, like

IMP. In the cell cultures, IMP dramatically enhanced the response of T1R1+3 to

amino acids.

The researchers next examined the effects of IMP in mice. They added the

chemical to the animals’ taste buds, then added

amino acids, and measured the specific response of nerve fibers connected to the

taste buds that expressed T1R1+3. The

response of these nerves was greatly enhanced by IMP.

In a final set of experiments, and his colleagues showed that mice do not

taste some artificial sweeteners such

as aspartame and cyclamate that humans can taste because of sequence differences

in the T1R receptors of the two

species.

" This last piece of the puzzle is worthy of note, " said Zuker. " Changes in the

sequence of taste receptors appear to be

responsible for some of the difference in tasting behavior between mice and

humans. "

According to Zuker, discovery of the amino acid taste receptor will have

important implications for understanding the

machinery of taste. " When Nick Ryba and I began this collaboration a bit over

four years ago, our ultimate goal was to

understand how the brain knows what you just tasted, " he said. " We wanted to

discover how taste receptor cells are

activated and how their signals travel to the brain to produce specific taste

perceptions.

" To do that, we first needed to define the different taste modalities at a

cellular level, so that we could then follow

their connectivity maps to the brain. The ‘Holy Grail’ in this field has been

the receptors, and now that we know the

receptors underlying three modalities – sweet, bitter and amino acid – we can

begin to work on our original goal, to map

this system to understand how taste is encoded, " Zuker said.

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Good health & long life,

Greg ,

http://optimalhealth.cia.com.au