Catatan

The biologically active form of vitamin D, la,25-dihydroxyvitamin D3 (VD), is
an important nuclear hormone with pleiotropic physiological function, such as
maintenance of calcium and phosphate homeostasis, bone formation, and cellular
growth [l]. Moreover, VD can induce cellular differentiation and apoptosis [2] in

addition to immune-suppressive effects [3]. VD is a lipophilic molecule that eas-
ily passes cellular membranes and enters the nucleus, where it binds with high

affinity (& approximately 0.1 nM) to the nuclear VD receptor (VDR). VDR is
rather ubiquitously expressed and also found in the skin [4,5], i.e., skin is a VD

target tissue. Vitamin D, the precursor of VD, is either taken up systemically di-
rectly through diet or, interestingly, can be synthesized in the skin from 7-dehy-
drocholesterol, indicating that the term vitamin was not ideally chosen. Synthesis

of vitamin D requires energy from ultraviolet (W) radiation at an approximate
wavelength of 300 nm, therefore restricting endogenous production of vitamin D

to daylight hours. Two subsequent enzymatically controlled hydroxylation reac-
tions at carbon atoms 25 and 1, mainly performed in the liver and the kidney, re-
spectively, lead to the hormonally active form, VD.