ABSTRACT
I. 1,25-DIHYDROXYVITAMIN D3 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] is the biologically active form of the secosteroid vitamin D3 and was originally found to be important for bone and mineral homeostasis. Vitamin D3 can be derived from nutrients (e.g., fortified diary products, fatty fish, and fish liver oils) but is mainly derived from photosynthesis in the skin [1]. The first activation step in the synthesis of active 1,25(OH)2D3 (reviewed in Ref. 2), the 25-hydroxylation of vitamin D3 to 25(OH)D3, occurs predominantly in the liver. This 25-hydroxylated form of vitamin D3 is quite stable (half-life of 3 weeks), and it is mainly in this form that the hormone is transported, bound to vitamin D3-binding protein (DBP), in the serum. Further hydroxylation by the enzyme 25(OH)D3-1-hydroxylase is required to obtain the biologically active form of vitamin D3, 1,25(OH)2D3. This second hydroxylation step occurs mainly in the convoluted tubule cells of the kidney.
