On the human tongue, there are numerous taste papillae. There are three types of papillae on the surface of the tongue that are distributed differently. On them, the so-called taste buds are located. Only some of the approximately 100 cells of a taste bud are in fact sensory cells, the so-called taste receptors. These are the special points where the flavors are received, which allow us to classify the food stuff in the appropriate “direction” (sweet, sour, salty, bitter, and umami). We taste something when the soluble food stuff stimulates the taste receptors, thereby an electronic impulse is sent to the brain. Previously, the parts of the tongue were incorrectly separated into taste zones. Now we know that each part of the tongue can perceive every taste, but with different sensitivity. The research of this mechanism has recently resulted in considerable advances in science.

Fig. 1 gustatory papillae, Source: Wolfgang Meyerhof, Deutsches Institut für Ernährungsforschung

Using taste receptors, humans are able to taste a wide range of bitter substances. The bitter substances cannot reach the taste receptors, but they activate the receptor molecules on the outside of the receptors, the so-called TAS2Rs (according to the new nomenclature hTAS2R, in which the “h” stands for the first letter in the English word “human”). A single receptor can be activated by a number of chemical substances. The bitter substances in turn are also able to activate not just one, but several receptors. In the formation of bitterness receptors, the human body uses “blueprints” that are saved on the 25 so-called TAS2R genes in our DNA. In our project we focused just on one gene from this family, namely the TAS2R38 (meaning, taste receptor, type 2, member 38).

The TAS2R38 is a gene that is responsible for the formation of the taste receptors that make the bitter substance PTC and PROP perceptible. The gene occurs in various forms, insofar as it can have three SNPs. This means that there are three positions where the order of DNA base pairs may slightly vary as a result of point mutation. Depending on the point mutation, the amino acids used in the synthesis change, resulting in differing receptor “building types.” The various building types are the reason why each person perceives bitter substances differently. Of the eight possible forms, the two primary types are called PAV and AVI. The PAV variant of TAS2R38 genes lead to an intense perception of bitterness. If a person possesses the other mutated AVI variant, it is very likely (80 percent) that this person will not enjoy the taste of PTC or PROP.

All substances that activate the TAS2R38 receptor - and which are therefore bitter tasting - contain the same set structure, particularly a N-C=S group (see inset). About 80 years ago, chemist Arthur Fox accidentally discovered that people can be classified with regard to their ability to perceive the chemical PTC as bitter. Attempting to fill up a bottle with PTC powder, he spilled some of it and it kicked up a lot of dust. While his assistant immediately complained of a bitter taste in his mouth, Fox did not taste anything, not even when he put one of the crystals directly in his mouth. He concluded that people have different sensitivities to tastes, and he defined these people as tasters and non-tasters. The resulting completed study of Albert Blakeslee shows that the ability to taste PTC is a dominant genetic characteristic. The first taste experiments took place over fifty years ago, long before modern molecular biology and the discovery of the TAS2R gene family. PTC and numerous additional chemicals that are used in such analyses, particularly to identify bitterness, include this N-C=S group. Earlier it was naturally impossible to explain the molecular basis for the development of these differences. No one knew that the TAS2R38 receptor in particular was responsible for this phenomenon.

The gene for the PTC taste receptor, TAS2R38, was identified in 2003 (see Drayna, Kim et al 2003). The first experiment about the sensation of taste linking bitter food stuffs such as red wine, cabbage, artichokes and also spinach followed one year later. Between 2004 and 2005 scientists found that there are three point mutations on this gene that effect different taste perceptions. In 2008, Hayes suggested eliminating the term supertaster, because more genes are responsible for taste perception.

Fig. 2 PROP, -N-C=S compound on the right side of the ring, C only as vertex

Fig. 3 PTC, -N-C=S compound in side chain, C only as vertex