LACK OF IN-VITRO EVIDENCE FOR STORAGE OF 1,25-DIHYDROXYCHOLECALCIFEROL (1,25(OH)(2)D-3) AND 1,25(OH)(2)D-3 BINDING-PROTEIN IN SKELETAL MATRIX

A few studies have reported on the measurement of 1,25-dihydroxycholec alciferol (1,25(OH)(2)D-3) in bone, using chloroform/methanol extracti on and radioreceptor assay. As the significance of bone 1,25(OH)(2)D-3 content was not defined in any of these reports, the objective of the current investigation was to determine whether 1,25(OH)(2)D-3 may be stored in skeletal matrix. Bone powder samples from the iliac crest we re extracted in ethylacetate/cyclohexane and 1,25(OH)(2)D-3 isolated f rom the extract by means of Sephadex LH-20 and high pressure liquid ch romatographic separation and subsequently measured by radioimmunoassay (RIA). Within the detection range of the RIA, no 1,25(OH)(2)D-3 could be measured, suggesting that 1,25(OH)(2)D-3 is not stored in skeletal matrix. Vitamin D bone concentrations previously measured may therefo re have reflected plasma contamination. Consistent with this hypothesi s, only traces of skeletal 1,25(OH)(2)D-3 binding protein were measure d when compared with serum values. Although 1,25(OH)(2)D-3 may act as a potential local determinant of bone remodeling, there is no evidence supporting a delayed paracrine function by matrix-derived 1,25(OH)(2) D-3.