Studies on 24R,25-dihydroxyvitamin D3:: Evidence for a nonnuclear membrane receptor in the chick tibial fracture-healing callus

The effect(s) of 24R,25-dihydroxyvitamin D-3 [24R,25(OH)(2)D-3] on fracture healing was studied in a vitamin D-depleted chick model. 24R,25(OH)(2)D-3, together with another hormonally active vitamin D metabolite, 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3], improved bone mechanical strength parameters (torsional strength, angular deformation, and stiffness) and the ash content. The synthetic epimer 24S,25-dihydroxyvitamin D-3 [24S,25(OH)(2)D-3] was not as potent as the natural 24R,25(OB),D,. In Light of the ability of the fracture-healing callus to discriminate between 24R,25(OH),D, and 24S,25(OH)(2)D-3, a search was initiated in fracture-healing callus tissue for the presence of a specific 24R,25(OH)(2)D-3 receptor. No evidence was obtained for a classical nuclear/ cytosol receptor for 24R,25(OH)(2)D-3 in the fracture-healing callus, A specific receptor/binding protein for 24R,25(OH)(2)D-3 was found in the callus membrane fraction, which showed different ligand binding affinities [K-D = 18.3 +/- 1.9 nmol/L, B-max = 43.9 +/- 6.0 fmol/mg; relative competitive index (RCI) for 24R,25(OH)(2)D-3/24S,25(OH)(2)D-3/25(OH)D-3/1 alpha,25 (OH)(2)D-3 = 100/37/401/2.0] compared with the ubiquitous serum vitamin D-binding protein (RCI = 100/99/219/5), Also, a callus membrane-binding protein/receptor for 1 alpha,25(OH)(2)D-3 was detected with a K-D = 0.83 +/- 0.35 nmol/L and a B-max = 35.5 +/- 5.2 fmol/mg. Thus, we have demonstrated a biological role for 24R,25(OH)(2)D-3 in fracture healing and described the presence of its receptor/binding protein in a callus membrane fraction. (Bone 23:141-146; 1998) (C) 1998 by Elsevier Science Inc. All rights reserved.