Biochemical evidence for a cytoplasmic 1 alpha,25-dihydroxyvitamin d3 receptor-like protein in rat yolk sac.

The yolk sac in rats is an organ of exchanges between the mother and fetus. A vitamin D-dependent calcium-binding protein (CaBP) has been recently described in this organ. This led us to investigate the presence of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor-like proteins in the yolk sac cytosol. For this purpose we have utilized sucrose gradient centrifugation, Scatchard analysis, and DNA-cellulose chromatography. Our results show that cytosol prepared from rat yolk sacs contains a 3.3 S binding protein for 1,25-(OH)2D3. The binding is a highly specific, saturable process with high affinity(2 X 10(-10) M at 25 degrees C). The sterol-protein complex binds to DNA-cellulose. The 1,25-(OH)2D3 binding protein is present in the yolk sac from at least the 15th day until the 21st day of gestation. In contrast, such a binding protein is not found in the amnion, the other component of fetal membranes. The biochemical parameters of the 1,25-(OH)2D3 binding protein in the yolk sac are similar to those of 1,25-(OH)2D3 cytosolic receptors in vitamin D target organs. This strongly suggests that the 3.3 S protein in the yolk sac may function as a specific receptor, indicating that this organ may be a new target organ for vitamin D.