The 1 alpha -hydroxylated metabolite of 25-hydroxyvitamin D-3, 1,25-dihydro
xyvitamin D-3, is the biologically most active metabolite of vitamin D. The
24-hydroxylated metabolites were generally considered as degradation produ
cts of a catabolic pathway finally leading to excretion of calcitroic acid.
Studies with analogues fluorinated at the C-24 position did not indicate a
physiological function for 24R,25(OH)(2)D-3. Nevertheless throughout the y
ears various studies showed biologic effects of other metabolites than 1 al
pha ,25(OH)(2)D-3. In particular the metabolite 24R,25(OH)(2)D-3 has been f
unctionally analyzed, e.g. with respect to a role in normal chicken egg hat
chability and effects on chondrocytes in the resting zone of cartilage. Num
erous studies have shown the presence of the vitamin D receptor in bone cel
ls and effects of 1 alpha ,25(OH)(2)D-3 on bone and bone cells. Also for 24
R,25(OH)(2)D-3 studies have been performed focusing on effects on bone and
bone cells. The purpose of this review is to summarize the data regarding 2
4R,25(OH)(2)D-3 and bone and to evaluate its role in bone biology. (C) 2001
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