Evidence for human placental synthesis of 24,25-dihydroxyvitamin D3 and 23,25-dihydroxyvitamin D3

Abstract

The two principal dihydroxylated metabolites of the vitamin D prohormone 25-hydroxyvitamin D3 [25(OH)D3] are la,25-dihydroxyvitamin D3 [l,25(OH)2D3, the active hormone] and 24/?,25-dihydroxyvitamin D3]24,25(OH)2D3, a putative regulator of developmental bone formation]. Although several studies have demonstrated placental synthesis of l,25(OH)2D3 from 25(OH)D3, placental production of 24,25(OH)2D3 has not been thoroughly investigated. Therefore, we studied 25(OH)D3 metabolism in term human placenta using a villous explant model and cultures of isolated trophoblast and villous mesenchymal cells. We determined that both vitamin D-replete and vitamin D-deficient trophoblast metabolize 25(OH)D3 predominantly via 24-hydroxyIation. Placental 24,25(OH)2D3 was identified by cochromatography with authentic standard on four different HPLC systems, scanning UV spectrophotometry profile of the metabolite, sensitivity to periodate cleavage, and mass spectrometry of the putative placental 24,25(OH)2D3 and its periodate cleavage product. We also identified for the first time placental synthesis of 23,25(OH)2D3 using cochromatography with authentic standard on two different HPLC systems, scanning UV spectrophotometry, resistance to periodate cleavage, and mass spectrometry. When trophoblast was incubated for up to 4 h with physiologic concentrations of [3H]25(OH)D3 (6 nM) significant amounts of [3H]24,25(OH)2D3 were produced, but]3H]l,25(OH)2D3 could not be consistently detected. In contrast, when we incubated trophoblast with supraphysiologic concentrations of 25(OH)D3 (6-10 /uM), both 24,25(OH)2D3 and l,25(OH)2D3 were synthesized. These results provide unequivocal evidence for placental synthesis of both 24,25(OH)2D3 and 23,25(OH)2D3. These findings also suggest that supraphysiologic substrate concentrations saturate the placental 24-hydroxylase and may permit accumulation of placental l,25(OH)2D3 by preventing its further metabolism. Consequently, the identification of this high basal 24-hydroxylase activity in trophoblast may explain inconsistencies among previous reports regarding placental l,25(OH)2D3 production. We speculate that active placental 24-hydroxylation may serve important functions in perinatal vitamin D metabolism. © 1993 International Pediatric Research Foundation, Inc.