Phenylthiocarbamide

Phenylthiocarbamide (PTC), also known as phenylthiourea (PTU), is an organosulfur thiourea containing a phenyl ring.

It has the unusual property that it either tastes very bitter or is virtually tasteless, depending on the genetic makeup of the taster. The ability to taste PTC is a dominant genetic trait, and the test to determine PTC sensitivity is one of the most commonly used genetic tests on humans.

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 peoples of Australia and New Guinea to 98% for indigenous peoples of the Americas. One study has found that non-smokers and those not habituated to coffee or tea 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. There is conflicting evidence as to whether a higher percentage of women than men can taste PTC.

History
The genetic taste phenomenon of PTC was discovered in 1931 when a DuPont chemist named Arthur Fox accidentally released a cloud of a fine crystalline PTC. A nearby colleague complained about the bitter taste, while Dr. 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 correlation was so strong that it was used in paternity tests before the advent of DNA matching.

In 1976, a remarkable 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.

Some chimpanzees share the ability to taste PTC. However, chimpanzee PTC tasting relies on a different enzyme than human tasting, suggesting that the ability evolved independently in both species.

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).

Ability to taste PTC may be correlated with a dislike of plants in the Brassica genus, presumably due to chemical similarities.

Genetics
There are three SNPs (single nucleotide polymorphisms) along the gene that may render its proteins unresponsive. There is conflicting evidence as to whether the inheritance of this trait is dominant or incomplete 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 tasters experience a more intense bitterness than people that are heterozygous; other studies have indicated that another gene may determine taste sensitivity.

The frequency of PTC taster and non-taster alleles vary in different human populations.