Question

The vitamins, which also serve as precursors of hormone
(A) Vitamin A
(B) Vitamin B12
(C) Vitamin D
(D) Vitamin C
Choose the correct answer from the options given below:

• A. (A) and (B) only.
• B. (A) and (C) only.
• C. (A) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Active Vitamin D (calcitriol) and Retinoic acid (from Vitamin A) are unique because they bind to intracellular nuclear receptors, similar to classical thyroid and steroid hormones, to directly control gene transcription.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Some vitamins do not only serve as enzyme cofactors but are metabolically transformed into signaling molecules that bind to nuclear receptors and regulate gene transcription in a hormone-like manner.
Detailed Explanation:
Let us analyze the hormonal precursor roles of the given vitamins:
- Vitamin A (Retinol): It is metabolized inside cells to form all-trans-retinoic acid and 9-cis-retinoic acid.
Retinoic acid binds to nuclear receptors (RAR and RXR) to act as a potent hormone-like regulator of gene expression, controlling cell differentiation and development.
- Vitamin D: It is synthesized in the skin or absorbed in the diet and undergoes two successive hydroxylations (first in the liver, then in the kidneys) to form its active form, $1,25$-dihydroxyvitamin $\text{D}_3$ (calcitriol).
Calcitriol is a steroid-like hormone that binds to the Vitamin D Receptor (VDR) to regulate calcium and phosphorus homeostasis in the body.
- Vitamin B12 and Vitamin C act primarily as water-soluble coenzymes/reducing agents and do not serve as precursors to transcription-regulating hormones.

Step 2: Final Answer:
Vitamins A and C serve as precursors of hormones. This matches option (B).