Vitamin D metabolism and mechanisms of calcium transport

Vitamin D₃ undergoes sequential hydroxylations in the liver and kidney to form 1,25-dihydroxyvitamin D₃, the biologically active form of the vitamin. 1,25-dihydroxyvitamin D₃ is metabolized by several processes in various target tissues that decrease the biological activity of the sterol. In addition, 1,25-dihydroxyvitamin D₃ is excreted in the bile as polar metabolites, such as glucuronides and, possibly sulfates and neutral polar steroids. These compounds undergo an enterohepatic recirculation in both man and experimental animals. 1,25-dihydroxyvitamin D₃ increases the absorption of calcium in the intestine and the reabsorption of calcium in the kidney. It induces the synthesis of several proteins, the most notable of which is calcium binding protein that is thought to play a role in the absorption of calcium. The vitamin D-dependent calcium binding proteins and the calcium-magnesium ATPase calcium pump are co-localized in several tissues that play a role in the absorption of calcium.