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

organosulfur Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

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

containing a

phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...

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. Taste is the perception stimulated when a substance in the mouth biochemistry, reacts chemically with taste receptor cells l ...

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 Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart foun ...

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 are people with familial heritage from, or recognised membership of, the various ethnic groups living within the territory of contemporary Australia prior to History of Australia (1788–1850), British colonisation. The ...

and indigenous peoples of New Guinea to 98% for

indigenous peoples of the Americas In the Americas, Indigenous peoples comprise the two continents' pre-Columbian inhabitants, as well as the ethnic groups that identify with them in the 15th century, as well as the ethnic groups that identify with the pre-Columbian population of ...

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

coffee Coffee is a beverage brewed from roasted, ground coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content, but decaffeinated coffee is also commercially a ...

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 south-western China and nor ...

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 can be related to a person's ability to taste PTC.

History

The tested genetic taste phenomenon of PTC was discovered in 1931 when

DuPont Dupont, DuPont, Du Pont, duPont, or du Pont may refer to: People * Dupont (surname) Dupont, also spelled as DuPont, duPont, Du Pont, or du Pont is a French surname meaning "of the bridge", historically indicating that the holder of the surname re ...

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 (genotype) that also expresses an associated trait (phenotype). In medical genetics, the penetrance of a disease-causing mutation is the pr ...

was so strong that it was used in paternity tests before the advent of

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

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 popularized ...

polymorphism in human populations. Based on a taste test, usually of a piece of paper soaked in PTC (or the less toxic

propylthiouracil Propylthiouracil (PTU) is a medication used to treat hyperthyroidism. This includes hyperthyroidism due to Graves' disease and toxic multinodular goiter. In a thyrotoxic crisis it is generally more effective than methimazole. Otherwise it is ...

(PROP)), students are divided into taster and non-taster groups. By assuming that PTC tasting is determined by a dominant

allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

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 autosome ...

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. Mos ...

s for this allele are more sensitive to PTC than are

heterozygote 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. Mos ...

s). 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 Supertasters are individuals whose sense of taste for certain flavors and foods, such as chocolate, is far more sensitive than the average person. The term originated with experimental psychologist Linda Bartoshuk and is not the result of respo ...

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 ''Antidesma bunius'' is a species of fruit tree in the family Phyllanthaceae. It is native to South Asia, Southeast Asia, Melanesia, and northern Australia. It is commonly known as bignay, after its native name in the Philippines, where the fruit ...

'' 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, mustard plants, or simply brassicas. Crops from this genus are sometim ...

'', 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 2 member 38 is a protein that in humans is encoded by the ''TAS2R38'' gene. TAS2R38 is a bitter (taste), bitter taste receptor; varying genotypes of ''TAS2R38'' influence the ability to taste both propylthiouracil, 6-''n''-propylt ...

taste receptor gene. In humans, there are three SNPs (

single nucleotide polymorphisms In genetics and bioinformatics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in ...

) along the

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

that may render its proteins unresponsive. There is conflicting evidence as to whether the inheritance of this trait is dominant or

incompletely dominant In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...

. 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. Mos ...

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. Mos ...

; 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 simply known as the chimp, is a species of great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close relative the ...

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

Non-taster phenotype distribution in selected populations

Notes

References

External links

Dennis Drayna's home page.
Drayna has done extensive studies of PTC in various populations

* Thiourea tasting {{Cruciferous Biochemistry Phenyl compounds Human genetics Thioureas Bitter compounds