Definition: Sterol(s) formed by ultraviolet irradiation of any 5,7-diene-3ß-sterol, which breaks the 9,10 bond, but usually from either or both of ergosterol and lumisterol to produce tachysterol2 (ertacalciol, (6E,22E)-9,10-secoergosta-5(10),6,8,22-tetraen-3ß-ol) and from 7-dehydrocholesterol to produce tachysterol3 (tacalciol, (6E,3S)-9,10-secocholesta-5(10),6,8-trien-3ß-ol). When reduced to the 5,7-diene (or 5,7,22-triene) form, dihydrotachysterol3 (10,19-dihydrocalciol) or dihydrotachysterol2 (10,19-dihydroercalciol), antirachitic action appears. This property has been of therapeutic interest, but tachysterol is being replaced by the true vitamin D hormone (calcitriol) and its derivatives.
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