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Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in fungi, and named for ergot, the common name of members of the fungal genus Claviceps from which ergosterol was first isolated. Ergosterol does not occur in plant or animal cells; reports of its isolation from plants are generally ascribed to fungal contamination of the plant material from which steroids are being isolated. Ergosterol is a component of yeast and other fungalcell membranes, serving many of the same functions that cholesterol serves in animalcells. Ergosterol is provitamin form of vitamin D2; natural ultraviolet (UV) irradiation of ergosterol, isolated or in situ, results in vitamin D2 production that can contribute to satisfying human dietary vitamin D requirements.
Viosterol, the early name given to preparations of irradiated ergosterol, reflects some mixture of previtamin intermediate and formal isomerized product, but can be seen, practically, as being essentially synonymous with ergocalciferol.
Ergosterol is occasionally reported to occur in plants, e.g., grasses such as rye and perennial flowering plants such as alfalfa and hop plants.[non-primary source needed][better source needed] Consensus regarding these reports is that the ergosterol arises from fungal growth on or contamination of the plant, as fungi form an integral part of the systems responsible for plant decay. As such, production of vitamin D3 from fungal ergosterol via UV radiation of yeast, mushrooms, etc. is not true vitamin D production by the plant itself.
Because ergosterol is present in cell membranes of fungi, yet absent in those of animals, it is a useful target for antifungal drugs. Ergosterol is also present in the cell membranes of some protists, such as trypanosomes. This is the basis for the use of some antifungals against West African sleeping sickness.
Amphotericin B, an antifungal drug, targets ergosterol. It binds physically to ergosterol within the membrane, thus creating a polar pore in fungal membranes. This causes ions (predominantly potassium and protons) and other molecules to leak out, which will kill the cell. Amphotericin B has been replaced by safer agents in most circumstances, but is still used, despite its side effects, for life-threatening fungal or protozoan infections.
Miconazole, itraconazole, and clotrimazole work in a different way, inhibiting synthesis of ergosterol from lanosterol. Ergosterol is a smaller molecule than lanosterol; it is synthesized by combining two molecules of farnesyl pyrophosphate, a 15-carbon-long terpenoid, into lanosterol, which has 30 carbons. Then, two methyl groups are removed, making ergosterol. The "azole" class of antifungal agents inhibit the enzyme that performs these demethylation steps in the biosynthetic pathway between lanosterol and ergosterol.
Ergosterol powder is an irritant to skin, eyes, and the respiratory tract. Ingestion of large amounts can cause hypercalcemia, which (if prolonged) can lead to calcium salt deposits in the soft tissues and, in particular, the kidneys.
A related process of producing food vitamin D from fungi (though not a process acceptable to vegans) occurs when milk-cows are fed diets of UV-irradiated yeast (which contains D2 produced from the ergosterol in the yeast). This form of the vitamin is eventually excreted as D2 in cow milk. However, this process for increasing vitamin D in milk was never as common as "supplementing" milk directly, by adding vitamin D3 to the milk. It was also not as common as the earlier Steenbock process, in which milk was exposed directly to ultraviolet light, which converts milk's natural 7-dehydrocholesterol content to vitamin D3.
^Science Service (1930) Viosterol official name for irradiated ergosterol, J. Chem. Educ. 7(1) 166, DOI: 10.1021/ed007p166, see , accessed 10 July 2014.
^See "Viosterol" and "Calciferol" at Merriam-Webster Medical Dictionary, e.g.,  and , accessed 10 July 2014.
^Jäpelt, RB; Didion, T; Smedsgaard, J; Jakobsen, J (2011). "Seasonal variation of provitamin D2 and vitamin D2 in perennial ryegrass (Lolium perenne L.)". Journal of agricultural and food chemistry59 (20): 10907–12. doi:10.1021/jf202503c. PMID21919518.
^Magalhães, PJ; Carvalho, DO; Guido, LF; Barros, AA (2007). "Detection and quantification of provitamin D2 and vitamin D2 in hop (Humulus lupulus L.) by liquid chromatography-diode array detection-electrospray ionization tandem mass spectrometry". Journal of agricultural and food chemistry55 (20): 7995–8002. doi:10.1021/jf071308d. PMID17760411.
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^Roberts CW, McLeod R, Rice DW, Ginger M, Chance ML, Goad LJ (February 2003). "Fatty acid and sterol metabolism: potential antimicrobial targets in apicomplexan and trypanosomatid parasitic protozoa". Mol. Biochem. Parasitol.126 (2): 129–42. doi:10.1016/S0166-6851(02)00280-3. PMID12615312.
^Yazawa Y, Yokota M, Sugiyama K. (2000). "Antitumor promoting effect of an active component of Polyporus, ergosterol and related compounds on rat urinary bladder carcinogenesis in a short-term test with concanavalin A". Biol Pharm Bull11 (11): 1298–1302. doi:10.1248/bpb.23.1298. PMID11085355.
^Takaku, T; Kimura, Y; Okuda, H (2001). "Isolation of an antitumor compound from Agaricus blazei Murill and its mechanism of action". The Journal of nutrition131 (5): 1409–13. PMID11340091.