24,25-Dihydroxyvitamin D(3) and bone metabolism

The 1alpha-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 1alpha,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 1alpha,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.