A line of mice deficient in vitamin D binding protein (DBP) was generated by targeted mutagenesis to establish a model for analysis of DBP's biological functions in vitamin D metabolism and action. On vitamin D–replete diets, DBP–/– mice had low levels of total serum vitamin D metabolites but were otherwise normal. When maintained on vitamin D–deficient diets for a brief period, the DBP–/–, but not DBP+/+, mice developed secondary hyperparathyroidism and the accompanying bone changes associated with vitamin D deficiency. DBP markedly prolonged the serum half-life of 25(OH)D and less dramatically prolonged the half-life of vitamin D by slowing its hepatic uptake and increasing the efficiency of its conversion to 25(OH)D in the liver. After an overload of vitamin D, DBP–/– mice were unexpectedly less susceptible to hypercalcemia and its toxic effects. Peak steady-state mRNA levels of the vitamin D–dependent calbindin-D9K gene were induced by 1,25(OH)2D more rapidly in the DBP–/– mice. Thus, the role of DBP is to maintain stable serum stores of vitamin D metabolites and modulate the rates of its bioavailability, activation, and end-organ responsiveness. These properties may have evolved to stabilize and maintain serum levels of vitamin D in environments with variable vitamin D availability.
Fayez F. Safadi, Paul Thornton, Holly Magiera, Bruce W. Hollis, Michael Gentile, John G. Haddad, Stephen A. Liebhaber, Nancy E. Cooke
The Editorial Board will only consider comments that are deemed relevant and of interest to readers. The Journal will not post data that have not been subjected to peer review; or a comment that is essentially a reiteration of another comment.