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Figure 28.6 • Photoactivation of ergosterol to ergocalciferol.

Figure 28.6 • Photoactivation of ergosterol to ergocalciferol.

skin from 7-dehydrocholesterol by UV radiation (Fig. 28.7) in the range of 290 to 300 nm.66 7-Dehydrocholesterol is produced from cholesterol by the enzyme 7-dehydrocholes-terol reductase (EC 1.3.1.21). Only when exposure to sunlight is inadequate does cholecalciferol become a vitamin in the historical sense. As shown in Figure 28.7, upon exposure to UV irradiation, 7-dehydrocholesterol is converted rapidly to previtamin D3. Previtamin D3 undergoes slow thermal conversion to cholecalciferol and the biologically inactive lumisterol and tacalciol (formerly tachysterol3). Excess exposure increases production of the inactive compounds. The slow conversion of previtamin D3 to cholecal-ciferol ensures adequate supplies when the exposure is brief. Further, lumisterol and tacalciol can be converted back to previtamin D3 and thus serve as a reservoir.66 It has been estimated that a 10-minute exposure of just the uncovered hands and face will produce sufficient cholecalcif-erol.67 Inadequate photo conversion to cholecalciferol is most likely to occur in northern latitudes during the winter months, in people with dark-colored skin or in people who routinely use clothing or sunscreens to reduce their sun exposure, such as the elderly.

The mechanism responsible for the movement of chole-calciferol from the skin to the blood is not known. In the blood, cholecalciferol is bound primarily to GC. This protein selectively removes cholecalciferol from the skin because it has low affinity for 7-dehydrocholesterol, previ-tamin D3, lumisterol, and tacalciol.

Catabolism of vitamin D is initiated by the enzyme 1,25-dihydroxyvitamin D3 24-hydroxylase (EC 1.14.13.-), whose expression is stimulated by calcitriol, itself. 24-Hydroxylation is followed by oxidation to the ketone. Subsequent hydroxylation at C23 leads to cleavage of the side chain, resulting in the biologically inactive product calcitronic acid.68,69 The 24-hydroxy metabolites are excreted primarily in the bile.

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