Abstract
The cutaneous photosynthesis of vitamin D3 represents the main source of vitamin D in humans. It is formed from 7-dehydrocholesterol (7DHC or provitamin D3), which is present in large amounts in cell membranes of keratinocytes of the basal and spinous epidermal layers (1). By the action of ultraviolet B light (UVB light with a wavelength of 290–315 nm) the B ring of 7DHC can be broken to form previtamin D3. Previtamin D3 has very low or no affinity for vitamin D binding protein, precluding its entrance into the circulation. In the lipid bilayer of membranes, the unstable previtamin D3 is further isomerized to vitamin D3 by thermal energy (1). The conformational change due to this isomerization can project vitamin D3 into the circulation, where it is caught by vitamin D binding protein and transported to the liver and kidney for further metabolization to 1α,25-dihydroxyvitamin D3 [l,25(OH)2D3] (1).
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