Studies on the role of 24-hydroxylation of vitamin D in the mineralization of cartilage and bone of vitamin D-deficient rats

Abstract

Summary. To test the importance of 24-hydroxylation of vitamin D 3 on bone mineralization, rat pups born to vitamin D-deficient females were given either 25-hydroxyvitamin D3 or 24,24-difluoro-25-hydroxyvitamin Da for 16 days beginning at the time of weaning. Following such treatment analysis of blood samples revealed no detectable 24R,25-(OH)2Dz and 1,25-(OH)2Dz in the rats given the difluoro compound while revealing the expected 24,24-difluoro-25-hydroxyvitamin D3 and 24,24-difluoro-l,25-dihydroxyvitamin Da. The rats given 25-hydroxyvitamin D3 had the expected levels of 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, and 1,25-dihydroxyvitamin D 3. Following sacrifice at day 17, postweaning bone mineralization and modeling were studied in long bones using histological methods. Bones taken from vitamin Ddeficient rats at the beginning and end of the experimental period had lesions typical of rickets. These included wide growth plates, excessive amounts of osteoid, and metaphyseal fibrosis. Following treatment with either 25-hydroxyvitamin D3 or 24,24-difluoro-25-hydroxyvitamin D3, bone mineralization returned to normal. Growth plate widths and the amount of osteoid on bone surfaces were both substantially reduced and to a similar degree in both treatment groups. Normal cartilage core formation and trabecularization of the metaphyseal primary spongiosa were also restored to a similar degree in both groups. In effect, no difference was observed in any bone parameter studied between the 25-hydroxyvitamin D3-and the 24,24-difluoro-25-hydroxyvitamin Dz-treated animals. These results provide strong evidence that 24-hydroxylation of the vitamin D molecule plays little or no role in the modeling and mineralization of bone.Send offprint requests to H. F. DeLuca at the above address.Key words: Vitamin D-Vitamin D deficiencyCartilage --Bone.The importance of the vitamin D endocrine system in the maintenance of serum calcium and phosphorus levels and the promotion of bone mineralization is well established [1-3]. Vitamin D3, or cholecalciferol, is metabolically activated by a series of hydroxylations prior to exerting its biological activity. Vitamin D a is first hydroxylated at the 25 position in the liver to form 25-hydroxyvitamin D3 (25-OH-D3) and then further hydroxylated in the kidney to form 1,25-dihydroxyvitamin D z (1,25-(OH)2D3) or 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3) [1,2]. 24-Hydroxylation also takes place elsewhere [1,2]. The formation of 24,25-(OH)2D3 is related in a reciprocal fashion to the formation of 1,25-(OH)2Dz in the kidney [4]. It has been demonstrated that the synthesis of 1,25-(OH)2Dz from 25-OH-Dz decreases with increasing serum calcium levels while the synthesis of 24,25-(OH)2Dz increases [5]. Thus the circulating levels of these metabolites are influenced indirectly by plasma calcium concentrations via the parathyroid hormone [6][7][8]. Although 25-OH-D z and 24,25-(OH)2Dz are, under normal circumstances, the dominant metabolites found in the serum [9], it is well established that 1,25-(OH)2D...

The Functional Metabolism and Molecular Biology of Vitamin D Action

“…The fluoro groups on the 24-position preclude metabolism of 1,25-(OH) 2 D 3 to 24-hydroxylated metabolites [101]. When vitamin D-deficient rats were maintained on 24,24 difluoro-25-OH-D 3 as their sole source of vitamin D 3 , growth, reproduction, and skeletal mineralization remained normal when compared to rats maintained on 25-OH-D 3 or 1,25-(OH) 2 D 3 [19,100,102]. Thus, it is clear that 24-hydroxylation of vitamin D plays no significant role in the functions of vitamin D. The CYP-24 null mouse has been produced with a phenotype of high plasma levels of 1,25-(OH) 2 D 3 and defective mineralization [103].…”