SPECIES-DIFFERENCES IN THE BINDING-KINETICS OF 25-HYDROXYVITAMIN-D3 TO VITAMIN-D BINDING-PROTEIN

The specific binding of 25-hydroxyvitamin D3 to its binding protein was studied in serum of the human, rhesus monkey, cow, horse, and rat. The free fraction of 25-hydroxyvitamin D3 in the rat was 0.34 ± 0.15 pmol free/nmol total (±SD) and this was lower than in any of the other species (p < 0.01). In the human, the free fraction was 1.5 ± 0.32 pmol free/nmol total, which was higher than in any of the other species (p < 0.001). The differences in the free fraction were mainly due to differences in dissociation constant. The relative levels of free 25-hydroxyvitamin D should be taken into account when extrapolating findings about vitamin D metabolism in animals to the human. A technical outcome of this study is that of the species tested, vitamin D binding protein from rat serum is the most suitable as a reagent component for methods used to measure total 25-hydroxyvitamin D by competitive protein binding assay.