Question

The physiologically active form of vitamin D in the body is

• A. 25-OH-\(D_3\)
• B. 25, 26-\((OH)_2D_3\)
• C. 1, 25-\((OH)_2D_3\)
• D. 24, 25-\((OH)_2D_3\)

Hint:

Remember the metabolic pathway sequence: Cholecalciferol (Skin/Diet) \(\rightarrow\) 25-OH-\(D_3\) (Liver) \(\rightarrow\) 1,25-\((OH)_2D_3\) (Kidney, Active form).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Vitamin D obtained from the diet or synthesized in the skin by ultraviolet light is biologically inactive.
It must undergo two successive hydroxylations in the liver and kidneys to become physiologically active.

Step 2: Detailed Explanation:
The inactive vitamin \(D_3\) (cholecalciferol) is transported to the liver.
In the liver, it undergoes the first hydroxylation by the enzyme 25-hydroxylase to form 25-hydroxyvitamin \(D_3\) (also known as calcidiol or 25-OH-\(D_3\)).
25-OH-\(D_3\) is the major circulating form of vitamin D used to determine clinical vitamin D status.
From the liver, 25-OH-\(D_3\) is transported via the blood to the kidneys.
In the proximal renal tubules of the kidneys, it undergoes a second hydroxylation by the enzyme \(1\alpha\)-hydroxylase.
This forms 1,25-dihydroxyvitamin \(D_3\) (also known as calcitriol or 1, 25-\((OH)_2D_3\)).
Calcitriol is the fully active hormone that binds to vitamin D receptors (VDR) to regulate calcium and phosphorus absorption.
The other metabolites like 24,25-\((OH)_2D_3\) and 25,26-\((OH)_2D_3\) are inactive breakdown products.

Step 3: Final Answer:
The physiologically active form of vitamin D is 1, 25-\((OH)_2D_3\).