24,25-Dihydroxyvitamin D3 and bone metabolism

The 1 alpha -hydroxylated metabolite of 25-hydroxyvitamin D-3, 1,25-dihydroxyvitamin D-3, is the biologically most active metabolite of vitamin D. The 24-hydroxylated metabolites were generally considered as degradation products 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 years various studies showed biologic effects of other metabolites than 1 alpha ,25(OH)(2)D-3. In particular the metabolite 24R,25(OH)(2)D-3 has been functionally analyzed, e.g. with respect to a role in normal chicken egg hatchability and effects on chondrocytes in the resting zone of cartilage. Numerous studies have shown the presence of the vitamin D receptor in bone cells and effects of 1 alpha ,25(OH)(2)D-3 on bone and bone cells. Also for 24R,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 24R,25(OH)(2)D-3 and bone and to evaluate its role in bone biology. (C) 2001 Elsevier Science Inc. All rights reserved.