Question

Which of the following molecules functions as provitamin $\text{D}_3$?}

• A. 7-Dehydrocholesterol
• B. Ergosterol
• C. Ergocalciferol
• D. 1, 25-Dihydroxy cholecalciferol

Hint:

7-Dehydrocholesterol is converted to Vitamin $\text{D}_3$ (cholecalciferol) in the skin via UV light.
Ergosterol is the fungal precursor converted to Vitamin $\text{D}_2$ (ergocalciferol).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Provitamins are inactive biochemical precursors that can be converted into active vitamins through metabolic pathways or environmental exposures (such as ultraviolet light).

Step 2: Detailed Explanation:
Let us evaluate the functions of the listed compounds:
1. 7-Dehydrocholesterol: This cholesterol derivative is present in the epidermal layers of skin in animals.
Upon exposure to ultraviolet B (UVB) radiation from sunlight, the B-ring of 7-dehydrocholesterol undergoes photolytic cleavage to form previtamin $\text{D}_3$, which spontaneously isomerizes to form vitamin $\text{D}_3$ (cholecalciferol).
Therefore, 7-dehydrocholesterol is the direct precursor and provitamin $\text{D}_3$.
2. Ergosterol: A sterol found in fungal cell membranes that serves as the precursor to vitamin $\text{D}_2$ (ergocalciferol) upon UV exposure, making it provitamin $\text{D}_2$.
3. Ergocalciferol: This is active vitamin $\text{D}_2$, not a provitamin.
4. 1,25-Dihydroxycholecalciferol (Calcitriol): This is the active hormonal form of vitamin $\text{D}_3$ produced through successive hydroxylations in the liver and kidneys.

Step 3: Final Answer:
The molecule that serves as provitamin $\text{D}_3$ is 7-Dehydrocholesterol, which corresponds to Option (A).