Vitamin A

Structure of Vitamin A

• Vitamin A includes a group of bioactive molecules termed retinoids, which comprise both natural and synthetic forms.
• Retinol: A primary alcohol with a β-ionone ring and unsaturated side chain, found in animal tissues as retinyl esters.
• Retinal: An aldehyde formed by the oxidation of retinol; retinol and retinal can interconvert easily.
• Retinoic Acid: The acid resulting from the oxidation of retinal; cannot be reduced back to retinal or retinol.
• β-Carotene: Found in plant foods, it can be converted to two molecules of retinal in the intestine, though this process in humans is inefficient.

Absorption and Transport of Vitamin A

• Transport to the Liver: Dietary retinyl esters are hydrolyzed in the intestinal mucosa, releasing retinol and fatty acids, which are then re-esterified and secreted in chylomicrons into the lymphatic system. Chylomicron remnants are taken up by the liver for storage.
• Release from the Liver: Retinol is released and transported to tissues by plasma retinol-binding protein (RBP). The retinol-RBP complex attaches to specific receptors, allowing retinol to enter cells, where it is carried to the nucleus by cellular retinol-binding proteins.

Mechanism of Action of Vitamin A

• Retinol is oxidized to retinoic acid, which binds to specific receptor proteins in the nucleus of target tissues.
• The activated retinoic acid-receptor complex interacts with nuclear chromatin, regulating RNA synthesis and protein production, influencing various physiological functions.
• Retinoids control gene expression, such as keratin in epithelial tissues, and are part of a superfamily of transcriptional regulators, similar to steroid hormones.

Functions of Vitamin A

• Visual Cycle: Vitamin A is crucial for visual pigments in rod and cone cells. Rhodopsin, composed of 11-cis retinal and opsin, undergoes isomerization upon light exposure, regenerating through a series of transformations to maintain vision.
• Growth: A deficiency in vitamin A results in decreased growth rates in children and slowed bone development.
• Reproduction: Retinol and retinal are essential for reproduction; they support spermatogenesis in males and fetal maintenance in females. Retinoic acid is not involved in reproduction or vision but aids growth and epithelial differentiation.
• Maintenance of Epithelial Cells: Necessary for the normal differentiation of epithelial tissues and the secretion of mucus.

Distribution of Vitamin A

• Rich sources include liver, kidney, cream, butter, and egg yolk for pre-formed vitamin A.
• Yellow and dark green vegetables and fruits are dietary sources of carotenes, precursors for vitamin A synthesis.