All of the following statements regarding vitamin A metabolism and function are correct EXCEPT:

Explanation

## Analysis of Vitamin A Metabolism and Storage ### Correct Statements (Options 0, 1, 2) **Option 0 — RBP Transport (CORRECT)** **Key Point:** Retinol-binding protein (RBP) is the primary plasma carrier for retinol. It binds retinol in a 1:1 molar ratio and delivers it to target tissues via RBP receptors (STRA6). This is essential for vitamin A homeostasis. **Option 1 — Beta-Carotene Cleavage (CORRECT)** **High-Yield:** Beta-carotene 15,15'-oxygenase (BCDO1) catalyzes the central cleavage of β-carotene at the 15,15'-double bond, yielding two molecules of retinal (vitamin A aldehyde). This is the major pathway for dietary carotenoid conversion. **Option 2 — Retinoic Acid as Nuclear Ligand (CORRECT)** **Key Point:** Retinoic acid (RA) is the active metabolite that binds retinoic acid receptors (RAR: α, β, γ) and retinoid X receptors (RXR: α, β, γ). These heterodimers bind response elements in DNA and regulate transcription of genes involved in differentiation, growth, and immunity. ### Incorrect Statement (Option 3) — THE ANSWER **Warning:** This is the trap. Vitamin A is stored **primarily in the liver as retinyl esters**, but mobilization is **NOT rapid** — it takes **days to weeks**, not hours. The liver maintains a large reserve pool (100–300 mg in a well-nourished adult), but: - Retinyl esters must first be hydrolyzed to retinol by hepatic retinyl ester hydrolase. - Retinol is then bound to RBP (which requires adequate protein status and zinc for RBP synthesis). - Only then is the complex released into plasma. This slow mobilization is why vitamin A deficiency develops over **months**, not days, even with zero dietary intake. **Clinical Pearl:** In acute liver disease or severe malnutrition, RBP synthesis is impaired, leading to low plasma retinol despite adequate hepatic stores — a classic dissociation seen in protein-energy malnutrition. ### Summary Table | Aspect | Correct | Incorrect | | --- | --- | --- | | **RBP function** | Essential plasma carrier | — | | **β-carotene cleavage** | Central 15,15' bond → 2 retinal | — | | **Retinoic acid action** | Nuclear receptor ligand | — | | **Liver storage mobilization** | — | Hours (should be days–weeks) |