Presence of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 24-hydroxylase in vitamin D target cells of rat yolk sac.

In the pregnant rat, the yolk sac, which possesses true placental functions, is a vitamin D target organ. We tested its ability to hydroxylate 25-hydroxy- and 1,25-dihydroxyvitamin D3 (25-OHD3 and 1,25-(OH)2D3). 24,25-Dihydroxy- and 1,24,25-trihydroxyvitamin D3 were produced by rat yolk sac homogenates incubated with tritiated 25-OHD3 and 1,25-(OH)2D3. Rat yolk sac homogenates also formed small amounts of 25,26-dihydroxyvitamin D3. These newly synthesized metabolites were isolated and identified by Sephadex LH-20 chromatography, high performance liquid chromatography, and periodate cleavage. Yolk sac 25-OHD3- and 1,25-(OH)2D3-24-hydroxylases were present in mitochondria and were of a mixed function oxidase nature. They were detected in the yolk sac as early as day 12 in the embryonic period and until the end of gestation. No hydroxylation occurred in maternal liver, amnion, fetal brain, or skin homogenates. Both 24-hydroxylases were detected in pure isolated rat yolk sac endodermal cells. This may be of physiological importance, since they are the 1,25-(OH)2D3 target cells in the yolk sac. Injection of 1,25-(OH)2[3H]D3 into rat yolk sac vitelline veins strongly suggested that the yolk sac vitelline veins strongly suggested that the yolk sac produced 1,24,25-(OH)3D3 in vivo. We conclude that the yolk sac and more precisely its endodermal cells may help to control vitamin D metabolism within the fetoplacental unit.