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(also provitamin D2), C28H44O, a natural compound of the sterol group with the structural formula
Ergosterol is formed in yeast, molds, and other fungi. The crystals have a melting point of 165°C. Insoluble in water, they are soluble in organic solvents. Ergosterol is optically active, with a specific rotation of [α]D = –129°. The compound is highly reactive because of conjugate double bonds in the B ring of the system. Like cholesterol, it can form molecular complexes and inclusion complexes in cells. When irradiated with ultraviolet light, the B ring breaks, and ergosterol forms precalciferol, which is easily isomerized by heating to yield vitamin D2 (ergocalciferol).
In the pharmaceutical industry, ergosterol is used in the preparation of vitamin D2 and the hormone progesterone. Ergosterol is extracted from the alkaline hydrolyzates of baker’s yeast or the mycelial by-products from the manufacture of antibiotics.