(ergosta-5,7,22-trien-3β-ol), a sterol, is the biological precursor to Vitamin D2. It is turned into viosterol by ultraviolet light, and is then converted into ergocalciferol, which is a form of Vitamin D.
Ergosterol is a component of fungal cell membranes, serving the same function that cholesterol serves in animal cells. The presence of ergosterol in fungal cell membranes coupled with its absence in animal cell membranes makes it a useful target for antifungal drugs. Ergosterol is also used as a fluidizer 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 is an antifungal drug that targets ergosterol. It binds to ergosterol and creates a polar pore in fungal membranes. This causes ions (predominantly K+ and H+) and other molecules to leak out of the cell, killing it. Amphotericin B has been replaced by safer agents in most circumstances but is still used, despite its side effects, for life-threatening fungal infections.
