Molecular Gastronomy: Exploring the Science of Flavor (16 page)

seemed reasonable to scrutinize taste cells in the mouth for molecules analo-

gous to neuronal receptors.

Building on Faurion’s insight, Chaudhari and his colleagues looked for re-

ceptors paired with G-proteins—that is, with proteins embedded in the taste

cell membrane that transmit the message detected by the receptor, which, pro-

jecting from the cell surface, is exposed to the extracellular medium. They

made a surprising discovery: a truncated form of a neuronal protein known as

a metabotropic glutamate receptor, or mGluR4.

Searching for Receptors

In the brain this neuronal receptor is activated by very weak glutamate

concentrations. Unmodified, it would be completely saturated by the large

quantities of glutamate present in the mouth when we taste certain dishes.

Chaudhari and his colleagues concluded that the abbreviation of the protein’s

structure probably was an adaptation to the function of taste. Examining the

receptor that is synthesized in the gustatory tissues of the rat, they found that it

had lost the first 300 amino acids found in its neuronal counterpart and that in

this truncated form it binds very weakly to glutamate. Moreover, the glutamate

concentrations needed to activate it are similar to the perception thresholds

98 | t he physiology of f l a vor

measured in rats (a threshold perception being defined as the weakest concen-

tration to which an animal is sensitive).

Why does the shortening of the protein’s amino acid sequence reduce its

affinity for glutamate? The University of Miami physiologists observed that

the taste molecule strongly resembles a particular bacterial protein, extensively

studied by crystallography, that has two bonding sites for glutamate. Trunca-

tion seems to have eliminated the more sensitive one of the two.

Is the protein they discovered the receptor for the umami taste? After all,

the possibility could not automatically be excluded that the new protein trans-

mits neural (rather than sensory) information from the gustatory cells to the

brain. Confirmatory evidence is substantial. First of all, the mGluR4 protein is

activated by other sapid molecules that rats do not distinguish from glutamate.

This was not surprising because Faurion had observed that the tongue and

brain react similarly to these molecules. Moreover, the truncated mGluR4 pro-

tein is synthesized only in the gustatory papillae. Finally, neither the complete

nor truncated form of the mGluR4 protein has been detected elsewhere in the

mouth than in the gustatory papillae.

This discovery raises many questions. Is truncated mGluR4 the only recep-

tor for the umami taste? Is it involved in the perception of other tastes? Do

receptor cells each bear only a single type of receptor? The result obtained by

Chaudhari and his team seems to be the first of a series; physiologists have

found other molecules of the same class that also seem to be taste receptors.

Knowing the structure of the mGluR4 protein makes it possible to use mo-

lecular modeling systems to determine which molecules attach themselves to

it. The computer-assisted design of completely new sapid molecules, further

expanding the number of tastes, may not be far off.

Detecting Tastes
| 99

26

Bitter Tastes

Not only are there more than four tastes, but several types of bitterness

have been discovered.

i n 1995 t h e p h y s i o l o g y o f s m e l l took a great step forward with the

identification of the proteins that constitute the receptors of nasal olfactory

cells. However, the receptors of the papillary cells in the tongue and mouth

that are sensitive to taste remained unknown. These gaps are gradually being

filled. Alejandro Caicedo and Stephen Roper at the University of Miami have

shown that the human gustatory system is capable of distinguishing several

sorts of bitter taste.

In 2000, the same biologists who five years earlier had discovered families

of olfactory genes found a vast family of receptors for what was then thought

of as a single bitter taste. Other teams went on to show that individual recep-

tors react selectively to compounds of a particular sort of bitterness. In parallel

with this research it became apparent that each taste receptor cell expresses

several rna messengers, which in turn code for several receptors. It seemed

natural, then, to suppose that individual receptor cells react to two or more

taste compounds.

Neurological and behavioral studies conducted with rats, monkeys, and

human subjects indicated that these different species are able to distinguish

between several bitter stimuli. What is the cellular basis for this ability? Do

taste receptor cells react specifically to one stimulus or to several? Progress

in answering these questions was slow because ongoing neurophysiological

100 |

investigation pointed to a variety of sources of interference along the nerve

pathway that leads from the gustatory papillae to the brain.

Caicedo and Roper attacked the problem with a new imaging method that

showed, in situ, the activation of taste receptor cells by a stream of calcium

ions. A coloring agent sensitive to such ions was injected into the cells using

a micropipette, and the reactions of the cells to various stimuli were observed

while the distribution of these calcium ions was measured in real time with

the aid of a laser, which excites the coloring agent, and a confocal microscope,

which makes it possible to observe deep tissue cells. The University of Miami

biologists were thus able to examine the reaction of several hundred taste re-

ceptor cells to a series of bitter compounds.

Cycloheximide was found to trigger strong but transient variations in the

concentration of calcium ions in taste receptor cells. The four other molecules

tested—denatonium benzoate, sucrose octaacetate, phenylthiocarbamide, and

quinine—produced weaker but prolonged reactions lasting several minutes.

The intensity of the reaction in each case depended on the concentration of

the molecules to which the cell was exposed, which varied for each type of

molecule. What is more, the recorded results corresponded to the behavioral

response of rats fed with solutions of these bitter molecules (which act only

above a certain threshold concentration).

Five Types of Bitterness

After meticulous study the Miami researchers established that only 18% of

the 374 cells tested reacted to one or more of the five bitter compounds when

they were administered at moderate concentrations. Among the cells that were

sensitive to bitter compounds, reactions varied: 14% of the cells reacted to cy-

cloheximide, 4.5% to quinine, 3.7% to denatonium benzoate, 2.4% to phenyl-

thiocarbamide, and 1.6% to sucrose octaacetate.

The total proportion of taste receptor cells that register bitterness in the

gustatory papillae (which contain other kinds of cells as well) turned out to be

comparable to the proportion of cells sensitive to bitterness in the restricted

population of taste receptor cells tested. None of the papillae studied seems

specific to bitterness, and both the proportion and the distribution of cells sen-

sitive to bitterness are comparable to those of the rna messengers of receptors

Bitter Tastes
| 101

for bitter molecules. The Miami studies showed once again, only this time at

the cellular level, that the different parts of the tongue are not specific to par-

ticular tastes, contrary to a view widely held among cooks and gourmets.

Applying the five stimuli to each cell in turn, Caicedo and Roper observed

that a majority of the cells sensitive to bitterness reacted only to one of the five

compounds tested, with their neighbors reacting to different ones. One quarter

of these cells were activated by two bitter compounds, and 7% reacted to more

than two of the five compounds. Note that these reactions were independent

of one another: Stimuli were not simultaneously administered to a given taste

receptor cell, and higher concentrations of bitter molecules did not increase

the proportion of bitter-sensitive cells.

The sensitivity of individual cells to specific kinds of bitterness—obviously

it will no longer do to speak of bitterness as though it were a single thing—

would explain the observed behavioral reactions and, in particular, the capacity

to make sensory distinctions between different sorts of bitterness. The nerve

fibers that go out from cells specific to a given kind of bitterness appear to be

grouped in dedicated bundles that communicate with a particular area of the

brain.

It remains to come up with names for the various bitter tastes that are now

known to exist.

102 | t he physiology of f l a vor

27

Hot Up Front

Why spicy foods burn the mouth.

i n s e e k i n g t o c o m p o s e a perfectly balanced and flavorful dish, the cook

naturally looks to old recipes for ingredients whose combination has been test-

ed and validated over the course of centuries. But traditional ways are not al-

ways the best. The various elements assembled from culinary experience must

harmoniously stimulate not only the senses of taste and smell but also thermal

and mechanical sensors, to say nothing of the chemical sensors for spiciness.

Our chances of success will improve if we have a precise understanding of

the underlying molecular mechanisms. We know that sour and bitter tastes

are offset by sweet ones and that salt facilitates the perception of other tastes,

but the molecular basis of the physiology of taste remains incomplete. For ex-

ample, why do hot peppers of the
Capsicum
family set the mouth on fire? Why

are people who regularly eat hot peppers able to tolerate doses that novices

find intolerable? Why do we like to eat foods that cause us pain? David Julius

and his colleagues in the School of Medicine at the University of California,

San Francisco (ucsf), cast light on these questions by studying the receptor for

capsaicin, the active principle in chili peppers, paprika, and cayenne.

Pain and the Brain

An important advance in human physiology was made several decades

ago with the exploration of the effects of morphine on the brain and the

| 103

identification of the receptors for morphine and its derivatives. If an organ-

ism contains such receptors, researchers reasoned, molecules analogous to

morphine ought to be able to be found in the body. This assumption turned

out to be correct: Endogenous opioids were soon discovered, along with the

regulatory system for suppressing pain.

Julius and his colleagues reasoned in a similar fashion that if our species

consumes spicy foods whose molecules activate pain pathways, evolution

ought to have endowed the human organism with receptors for endogenous

molecules involved in signaling pain.

How to go about identifying these receptors? The ucsf biologists first iso-

lated the rna messengers present in the neurons that detect spiciness in the

mouth and synthesized a group of corresponding dna molecules. They then

introduced these molecules in various cell cultures to observe the bonding be-

tween capsaicin and the proteins produced by the inserted dna. Finally, they

identified the dna that codes for the receptor of capsaicin, known as vr1. Intro-

ducing this dna sample in frogs’ eggs yielded cell cultures whose membrane

contained the desired receptor. Further analysis established that vr1 is a mem-

brane channel protein, which regulates the passage of ions (above all calcium

ions) between the outside and inside of cells. It was also shown that vr1 has

four subunits and that binding with capsaicin opens the channel.

What value do these studies have for gastronomy? Earlier psychophysiologi-

cal research had led American chemist Wilbur Scoville in 1912 to devise a sort

of Richter scale for heat that now bears his name. The electrophysiological

recordings of frogs’ egg cells equipped with the vr1 receptor demonstrated the

soundness of this classification: The cellular response (and therefore, presum-

ably, the neuronal response in the brain) turned out to be proportional to the

concentration of capsaicin.

Julius and his colleagues showed that capsaicin, which is fat soluble, can

bind itself to the vr1 channel, either on the surface of nerve cells or inside

them. Its affinity for fatty substances explains why drinking water does not put

out the fire in the mouth, but eating bread does.

Acclimatization to Spices

Exposing the receptors to capsaicin yielded additional information about

acclimatization to spicy dishes. The opening of the vr1 channel triggers the

104 | t he physiology of f l a vor

inflow of calcium ions into the neuron, which emits a nerve impulse when