Image Based Question-67332

Identify the Enzymes :1 and 2 as shown in the image.

A. Enzyme 1 : 1 alpha hydoxylase

Enzyme 2 : 25 hydroxylase

B. Enzyme 1 : 1 alpha dehydroxylase

Enzyme 2 : 25 dehydroxylase

C. Enzyme 1 : 25 hydroxylase

Enzyme 2 : 1 alpha hydoxylase

D. Enzyme 1 : 1 beta hydroxylase

Enzyme 2 : 25 hydroxylase

Show Answer

Correct Answer » C Explanation

Ans:C.

Enzyme 1 : 25 hydroxylase

Enzyme 2 : 1 alpha hydroxylase.

Synthesis of Vitamin D is shown in the image.

Vitamin D

• Vitamin D is the major vitamin involved in the absorption of calcium.

• Beside dietary intake, cutaneous synthesis is the other and more important source of vitamin D (cholecalciferol) in the body. The keratinocytes of stratum corneum of the epidermis of skin contain 7-dehydrocholesterol which is converted to Vitamin D3(cholecalciferol) by the action of sunlight (UV light).

• This vitamin D3 (Cholecalciferol) is converted to calcidiol (calcifediol or 25 hydroxy cholecalciferol or 25-hydroxy vitamin D3) by 25 hydroxylase in the liver.

• Calcidiol is the major circulating form of vitamin D3.

• Calcidiol is then converted to Calcitriol (1-25 dihydroxy-cholecalciferol or 1-25 dihydroxy vitamin D3) by 1-a hydroxylation in kidney.

• The final hydroxylation in kidney is rate limiting step.

• Calcitriol is the active form of vitamin D.

• Dietary vitamin D is absorbed in ileum and requires bile salts.
  When Ca2 level is high, little calcitriol is produced, and the kidney produce the relatively inactive metabolite 24, 25 dihydroxycholecalciferol instead.