Human serum binding capacity and affinity for 25-hydroxyergocalciferol and 25-hydroxycholecalciferol

Abstract

25-Hydroxyvitamin D-binding capacity and affinity were studied in human cord, adult, and maternal sera, and in sera from women receiving oral contraceptives, by in vitro satuaration analyses employing dextran-coated charcoal to adsorb unbound sterol. 25-Hydroxyergocalciferol and 25-hydroxycholecalciferol were equipotent in their ability to displace 3H 25-hydroxycholecalciferol from human serum binding sites. At 0C, the apparent dissociation constant for the serum binding of 25-hydroxycholecalciferol was low (Kd=8x 10-10M). Cord and adult sera had a similar 25-hydroxycholecalciferol binding capacity (1.8 x 10-6M), but the binding capacity of maternal sera and the sera from women receiving oral contraceptives was significantly higher. At physiological serum concentrations of 25-hydroxyvitamin D (5 x 10-8M), only 2-3% of human serum 25-hydroxyvitamin D-bindig sites are occupied.

PIP: Human serum binding capacity and affinity for 25-hydroxyergocalciferol and 25-hydroxycholecalciferol by human serum as well as the binding capacity and affinity of cord, adult, and maternal sera for 25-hydroxycholecalciferol were investigated by in vitro saturation analyses employing dextran-coated charcoal to absorb unbound sterol. The compounds were equipotent in their ability to displace tritiated 25-hydroxycholecalciferol from human serum binding sites. The dissociation constant at 0 degrees C for serum binding of 25-hydroxycholecalciferol was 8 X 10 (-10)M. Cord and adult sera had a similar 25-hydroxycholecalciferol binding capacity (1.8 X 10(-6)M) but the binding capacity of maternal sera and the sera from women receiving oral contraceptives was significantly higher (p less than .001). At physiological serum concentrations of 25-hydroxyvitamin D (5 X 10 (-8)M) only 2-3% of human serum 25-hydroxyvitamin D-binding sites were occupied.