Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 29

Vitamin D₃ (D₃) (cholecalciferol) is produced from 7-dehydrocholesterol through a 2 step process in which the B ring is broken by ultraviolet light (UVB spectrum 280–320 nm) in sunlight, forming pre-D₃ that isomerizes to D₃ in a thermo-sensitive but a noncatalytic process. Both UVB intensity and skin pigmentation level contribute to the rate of D₃ formation [4]. Melanin in the skin blocks UVB from reaching 7-dehydrocholesterol in the lower portions of the epidermis, thus limiting D₃ production, as do clothing and sunscreen. The intensity of UVB from sunlight varies according to season and latitude, so the further one lives from the equator, the less time of the year one can rely on solar exposure to produce D₃[5]. Vitamin D is also obtained from the food consumed. Most foods with the exception of fatty fish contain little vitamin D unless fortified. The vitamin D in fish is D₃, whereas that used for fortification is often D₂ (ergocalciferol). D₂ is produced by UVB irradiation of ergosterol in plants and fungi (e.g., mushrooms). It differs from D₃ in having a double bond between C22-C23 and a methyl group at C24 in the side chain. These differences from D₃ in the side chain lower its affinity for DBP resulting in faster clearance from the circulation, alter its conversion to 25-hydroxyvitamin D (25[OH]D) by at least some of the 25-hydroxylases to be described, and alter its catabolism by the 24-hydroxyase (CYP24A1) [6–8]. Moreover, a number of immunoassays do not recognize the D₂ metabolites as well as the D₃ metabolites. However, the biologic activities of D₂ and D₃ metabolites are comparable, and if no subscript is used, both forms are meant.