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Glutamate news
Taste receptor for amino acids identified
NEW YORK, Feb 26 (Reuters Health) - Scientists have identified
the receptor we use to taste amino acids, the building blocks of
proteins. The receptor seems to be involved with umami, the
difficult-to-describe savory taste of meats and other foods that
are rich in amino acids called glutamates.
Since amino acids are essential nutrients, it makes sense that
humans have evolved to find amino acids tasty, according to one
of the study's lead authors.
The word umami is certainly less familiar than the names of other
tastes--sweet, salty, sour and bitter--but the taste itself is
ubiquitous. Glutamates abound in meat and other protein-rich
foods such as cheese and milk. The amino acids also contribute to
the distinctive tastes of other foods, such as tomatoes and
asparagus.
And perhaps the most famous glutamate of all is the flavor
enhancer monosodium glutamate, or MSG.
For several years, Dr. Charles S. Zuker of the Howard Hughes
Medical Institute and the University of California-San Diego in La
Jolla, and Dr. Nicholas J. P. Ryba of the National Institute of
Dental and Craniofacial Research in Bethesda, Maryland, have
been trying to identify the receptors in cells that allow people to
experience different tastes.
In an interview with Reuters Health, Zuker likened these
receptors to locks that can only be opened by a certain key, in
this case, a taste. Zuker and Ryba's team previously identified the
receptors for sweet and bitter tastes.
Now the researchers report that a receptor called T1R1+3 is the
lock that is opened by the taste of amino acids. The findings are
published in the advance online edition of the journal Nature.
There is a "wonderful logic" to the fact that humans have a
receptor for amino acids, according to Zuker. "We have a taste
receptor for what are essential nutrients," he said.
Scientists have identified candidates suspected of being receptors
for salty and sour tastes, but they have not been tested yet,
Zuker noted.
The good taste of amino acids may offer a reward for eating
them, the California scientist explained. In contrast, bitter taste
likely evolved as a mechanism to warn against eating toxic
substances, he said.
The researchers point out, however, that the T1R1+3 receptor
responds to most, but not all, amino acids. And only a handful of
amino acids have the flavor of umami for people. In fact, some
amino acids elicit a bitter taste.
Taste receptors do not seem to be uniform in all people. The
investigators found that genetic differences in the subunits of the
amino-acid receptor can influence taste. Such differences could
help explain individual variations in taste, the research suggests.
Taste receptors also differ between humans and other mammals,
Zuker added. Mice, for example, cannot taste aspartame and
some other sweeteners. "This study now shows that
such differences in taste preferences between mice and humans
are due to differences in the sequence of the receptors between
the two species," he said.
The researchers now are investigating whether placing human
taste receptors in mice allows them to taste aspartame.
SOURCE: Nature 2002;10.1038/nature726.
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