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Metabolism of Vitamin D

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Vitamin D

There are two types of vitamins. First one is water soluble and the second one is fat soluble. Vitamin D is one of the fat-soluble vitamins. Fat-soluble vitamins need a fat medium to store in the body. Vitamin D is one of the fat-soluble vitamins.

Types of Vitamin D

Vitamin D₂
Vitamin D₃

Vitamin D₂ is called as ergocalciferol and produced from the irradiation of ergosterol, which was separated from ergot, a plant source.

Vitamin D₃ is known as cholecalciferol. Vitamin D₃ is of animal origin and is produced in the Malpighian layer of the epidermis. To convert pro vitamin D₃ into cholecalciferol ultraviolet rays are passed through the body. Cholecalciferol is the active form and is present in the skin.

SOURCES OF VITAMIN D

The foods which contain Vitamin D are

  • Carrots
  • Papaya
  • Squash
  • Fish
  • Meat
  • Mangoes
  • Egg Yolk
  • Cheese
  • Sweet potatoes
  • Peppers
  • Melon
  • Peaches
  • Apricots
  • Fortified Cereals
  • Butter
  • Tofu
  • Salmon
  • Sunlight
  • Mushrooms
  • Cod liver oil

ABSORPTION, STORAGE, AND EXCRETION OF VITAMIN D

Bile salts are important for its absorption. The absorbed Vitamin just appears in lymph as a part of chylomicrons. Vitamin D is a fat-soluble Vitamin so it is stored in fat. It is transported in the plasma in combination with a carrier protein called Vitamin d binding protein. It is mainly discharged in bile. Some vitamin D is deactivated in the liver by the hepatic microsomal drug metabolizing system.

METABOLISM OF VITAMIN D

Vitamin D is not acting as such but is converted into an active form in the body. Vitamin D is just transformed to 25-hydroxy Vitamin D in the mitochondria and microsomes of the liver cells. 25-hydroxylase, a mixed function oxidase involving cytochrome P450 catalyze this reaction.

25-hydroxy vitamin D is still inactive and it is then converted into an active form by 1- hydroxylase enzyme. 25- Dihydroxy vitamin D is the active form. 1α-hydroxylase is also a mixed function oxidase enzyme, it occurs in the mitochondria of cells lining renal proximal convoluted tubules. Mg⁺², NADPH ˖ H⁺ and molecular oxygen cytochrome P450, a flavoprotein and ferredoxin are also required for this reaction and are components of the enzyme system.

An active form of vitamin D can also produce by the tissues of the placenta, bone, macrophages and skin keratinocytes from its precursor. Another dihydroxy derivative which is known as 24, 25- dihydroxy vitamin. Vitamin D is normally produced in less amount and is less active than 1, 25-dihydroxy vitamin D. It is produced in certain diseases and in response to a high plasma level of calcium and inorganic phosphate.

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