Phenylthiocarbamide (PTC), also known as phenylthiourea (PTU), is an

organosulfur Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur ...

thiourea Thiourea () is an organosulfur compound with the formula and the structure . It is structurally similar to urea (), except that the oxygen atom is replaced by a sulfur atom (as implied by the ''thio-'' prefix); however, the properties of urea a ...

containing a

phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...

ring. It has the unusual property that it either

taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...

s very bitter or is virtually tasteless, depending on the genetic makeup of the taster. The ability to taste PTC is often treated as a dominant genetic trait, although inheritance and expression of this trait are somewhat more complex. PTC also inhibits melanogenesis and is used to grow transparent fish. About 70% of people can taste PTC, varying from a low of 58% for

Indigenous Australians Indigenous Australians or Australian First Nations are people with familial heritage from, and membership in, the ethnic groups that lived in Australia before British colonisation. They consist of two distinct groups: the Aboriginal peoples ...

and indigenous peoples of New Guinea to 98% for

indigenous peoples of the Americas The Indigenous peoples of the Americas are the inhabitants of the Americas before the arrival of the European settlers in the 15th century, and the ethnic groups who now identify themselves with those peoples. Many Indigenous peoples of the A ...

. One study has found that non-smokers and those not habituated to

coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. Seeds of ...

tea Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of ''Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of southwestern China and north ...

have a statistically higher percentage of tasting PTC than the general population. PTC does not occur in food, but related chemicals do, and food choice is related to a person's ability to taste PTC.

History

The tested genetic taste phenomenon of PTC was discovered in 1931 when DuPont chemist Arthur Fox accidentally released a cloud of fine crystalline PTC. A nearby colleague complained about the bitter taste, while Fox, who was closer and should have received a strong dose, tasted nothing. Fox then continued to test the taste buds of assorted family and friends, setting the groundwork for future genetic studies. The genetic

penetrance Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease-causing mutation is t ...

was so strong that it was used in paternity tests before the advent of DNA matching. The PTC taste test has been widely used in school and college practical teaching as an example of

Mendelian Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...

polymorphism in human populations. Based on a taste test, usually of a piece of paper soaked in PTC (or the less toxic propylthiouracil (PROP)), students are divided into taster and non-taster groups. By assuming that PTC tasting is determined by a dominant

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

at a single

autosomal An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosom ...

gene, and that the class is an unbiased sample from a population in Hardy–Weinberg equilibrium, students then estimate allele and genotype frequencies within the larger population. While this interpretation is broadly consistent with numerous studies of this trait, it is worth noting that other genes, sex, age and environmental factors influence sensitivity to PTC. Also, there are several alleles segregating at the major gene determining the taste of PTC, particularly in African populations, and the common "taster" allele is incompletely dominant (

homozygote Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

s for this allele are more sensitive to PTC than are heterozygotes). Additionally, PTC is toxic and sensitivity to the substitute, PROP, does not show a strong association with the gene controlling ability to taste PTC.

Role in taste

There is a large body of evidence linking the ability to taste thiourea compounds and dietary habits. Much of this work has focused on 6-propyl-2-thiouracil (PROP), a compound related to PTC that has lower toxicity. A supertaster has more of an ability to taste PTC. On the other hand, heavy cigarette smokers are more likely to have high PTC and PROP thresholds (i.e. are relatively insensitive). In 1976, an inverse relationship between taster status for PTC and for a bitter component of the fruit of the tree '' Antidesma bunius'' was discovered. Research on the implications still continues. Ability to taste PTC may be correlated with a dislike of plants in the genus ''

Brassica ''Brassica'' () is a genus of plants in the cabbage and mustard family ( Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole c ...

'', presumably due to chemical similarities. However, studies in Africa show a poor correlation between PTC tasting and dietary differences.

Genetics

Much of the variation in tasting of PTC is associated with polymorphism at the TAS2R38 taste receptor gene. In humans, there are three SNPs ( single nucleotide polymorphisms) along the

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

that may render its proteins unresponsive. There is conflicting evidence as to whether the inheritance of this trait is dominant or incompletely dominant. Any person with a single functional copy of this gene can make the protein and is sensitive to PTC. Some studies have shown that

homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

tasters experience a more intense bitterness than people that are

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

; other studies have indicated that another gene may determine taste sensitivity. The frequency of PTC taster and non-taster

s vary in different human populations. The widespread occurrence of non-taster alleles at intermediate frequencies, much more common than recessive alleles conferring genetic disease, across many isolated populations, suggests that this polymorphism may have been maintained through balancing selection.

Chimpanzees The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...

and

orangutans Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus ' ...

also vary in their ability to taste PTC, with the proportions of tasters and non-tasters similar to that in humans. The ability to taste PTC is an ancestral trait of hominids that has been independently lost in humans and chimpanzees, through distinct mutations at TAS2R38.

Non-taster phenotype distribution in selected populations

References

External links