Dihydroxycholecalciferol

Dihydroxycholecalciferol

Dihydroxycholecalciferol, also known as calcitriol, is the active form of vitamin D in the human body. It plays a crucial role in maintaining calcium and phosphate homeostasis, which is essential for healthy bone formation and maintenance.

Biochemistry

Dihydroxycholecalciferol is synthesized in the body from cholecalciferol (vitamin D3) through a series of hydroxylation reactions. The first hydroxylation occurs in the liver, converting cholecalciferol to 25-hydroxycholecalciferol (calcidiol). The second hydroxylation takes place in the kidneys, where calcidiol is converted to calcitriol.

Physiological Role

Calcitriol functions as a hormone that regulates the levels of calcium and phosphate in the bloodstream. It enhances the absorption of calcium and phosphate from the intestine, promotes the reabsorption of calcium in the kidneys, and mobilizes calcium from the bones when necessary.

Calcium Homeostasis

Calcitriol increases the expression of calcium-binding proteins in the intestinal epithelium, facilitating the efficient absorption of dietary calcium. It also acts on the renal tubules to reduce the excretion of calcium, thus conserving it in the body.

Bone Health

By maintaining adequate levels of calcium and phosphate, calcitriol ensures proper mineralization of the bone matrix. It works in concert with parathyroid hormone (PTH) to regulate bone remodeling and turnover.

Clinical Significance

Deficiency

A deficiency in calcitriol can lead to rickets in children and osteomalacia in adults, conditions characterized by softening and weakening of the bones. This deficiency is often due to inadequate dietary intake of vitamin D, insufficient sunlight exposure, or disorders affecting the liver or kidneys.

Excess

Excessive levels of calcitriol can result in hypercalcemia, a condition marked by elevated calcium levels in the blood. Symptoms of hypercalcemia include nausea, vomiting, weakness, and in severe cases, cardiac arrhythmias and kidney stones.

Therapeutic Use

Calcitriol is used therapeutically to treat conditions such as hypocalcemia in patients with chronic kidney disease, hypoparathyroidism, and certain types of psoriasis. It is available in oral and intravenous formulations.

Synthesis and Metabolism

The synthesis of calcitriol is tightly regulated by the body. Factors such as parathyroid hormone levels, serum calcium, and phosphate concentrations influence the activity of the enzyme 1-alpha-hydroxylase in the kidneys, which is responsible for the final step in calcitriol synthesis.

See Also

• Vitamin D
• Calcium metabolism
• Bone mineral density