Scientists have discovered what
exactly leaves a sour taste in the mouth.
A US team have found a protein
in taste receptor cells on the tongue that detects
sourness - warning mammals the food might be spoiled
or unripe.
The protein is also present in
spinal fluid, and the researchers suggest it might
be useful for detecting acidity in other parts
of the body.
The research is published in
the journal Nature.
The researchers from Howard Hughes
Medical Institute and the University of California
San Diego found that the protein, PKD2L1, was
the key.
Tests showed that mice who were
genetically engineered to lack these cells cannot
respond to sour-tasting stimuli, such as citric
acid.
Responses to other tastes were
unaffected.
The team has already identified
cell types that mediate sweet, bitter and savoury
tastes.
The PKD2L1 receptor is also active
in certain neurons in the spinal cord and may,
the authors propose, be involved in detecting
the acidity of cerebrospinal fluid, suggesting
a common mechanism for detection of acidity in
different parts of the body.
Spinal cord
The sourness sensor was found
using a search of DNA and protein sequence databases
to find the PKD2L1 protein from a potential list
of 30,000.
Dr Charles Zuker, who led the
research, said: "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."
This finding that the protein
was present in the central nervous system could
help explain how the body monitors the quality
of critical body fluids, Dr Zuker said.
For example, the body controls
respiration in part by monitoring the acidity
of the blood, since an increase in carbon dioxide
dissolved in the blood increases acidity.
Dr Zuker said defects in these
blood, spinal or brain fluid-sensing systems might
underlie a wide range of disorders.
Professor Tim Jacob, of Cardiff
University, said the research contradicted the
idea that there are cells which can respond to
a variety of tastes.
"Neurophysiologists will have
to go back to the lab."
