Parietin is the predominant cortical

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic comp ...

of lichens in the genus ''

Caloplaca ''Caloplaca'' is a lichen genus comprising a number of distinct species. Members of the genus are commonly called firedot lichen, jewel lichen.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014, gold lichens, "or ...

'', a secondary product of the lichen ''

Xanthoria parietina ''Xanthoria parietina'' is a foliose lichen in the family Teloschistaceae. It has wide distribution, and many common names such as common orange lichen, yellow scale, maritime sunburst lichen and shore lichen. It can be found near the shore on ...

'', and a pigment found in the roots of

Curled Dock ''Rumex crispus'', the curly dock, curled dock or yellow dock, is a perennial flowering plant in the family Polygonaceae, native to Europe and Western Asia. Description The plant produces an inflorescence or flower stalk that grows to high. I ...

(''Rumex crispus''). It has an orangy-yellow color and absorbs blue light. It is also known as physcion. It has also been shown to protect lichens against UV-B light, at high altitudes in Alpine regions. The UV-B light stimulates production of parietin and the parietin protects the lichens from damage. Lichens in arctic regions such as Svarlbard retain this capability though they do not encounter damaging levels of UV-B, a capability that could help protect the lichens in case of Ozone layer thinning. It has also shown anti-fungal activity against

barley powdery mildew Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pro ...

and

cucumber powdery mildew Cucumber (''Cucumis sativus'') is a widely-cultivated creeping vine plant in the Cucurbitaceae family that bears usually cylindrical fruits, which are used as culinary vegetables.fenarimol Fenarimol, sold under the tradenames Bloc, Rimidin and Rubigan, is a fungicide which acts against rusts, blackspot and mildew fungi. It is used on ornamental plants, trees, lawns, tomatoes, peppers, eggplants, cucumbers and melons. It is mainly ...

and polyoxin B. It reacts with KOH to form a deep, reddish-magenta compound.

Effect on human cancer cells

Also found in

rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick r ...

, the orange compound appears to have potential to suppress

6-phosphogluconate dehydrogenase 6-Phosphogluconate dehydrogenase (6PGD) is an enzyme in the pentose phosphate pathway. It forms ribulose 5-phosphate from 6-phosphogluconate: :6-phospho-D-gluconate + NAD(P)+ \rightleftharpoons D-Ribulose 5-phosphate + CO2 + NAD(P)H + H+ It ...

, or 6PGD. 6PGD is the third enzyme of the

pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-p ...

, or PPP, an oxidative process fueling growth in a still-relatively-unknown way. But it appears that arresting the chemical machinery at its third step could be promising for oncology. The parietin, identified from an FDA database of 2,000 known suppressors of 6PGD, killed half the human leukemia cells over two days in the laboratory. The pigment also slowed the growth of other human cancer cells in mouse models, according to the study. A more-potent derivative of the parietin called S3 may even cut the growth of lung cancer cells implanted in mice by two-thirds, over the course of 11 days. The compound also appears to be non-toxic to healthy cells.

References

''Caloplaca coralloides'' chemistry
* * {{Anthraquinone Anthraquinone dyes Antifungals Dihydroxyanthraquinones Phenol ethers Lichen products