Vitamin A : Functions

1. Visual cycle: Vitamin A is a component of the visual pigments of rod and cone cells. Rhodopsin, the visual pigment of the rod cells in the retina, consists of 11-cis retinal bound to the protein opsin ( Fig.1). When rhodopsin, a G protein–coupled receptor, is exposed to light, a series of photochemical isomerizations occurs, which results in the bleaching of rhodopsin and release of all-trans retinal and opsin. This process activates the G protein transducin, triggering a nerve impulse that is transmitted by the optic nerve to the brain. Regeneration of rhodopsin requires isomerization of all-trans retinal back to 11-cis retinal. All-trans retinal is reduced to all-trans retinol, esterified, and isomerized to 11-cis retinol that is oxidized to 11-cis retinal. The latter combines with opsin to form rhodopsin, thus completing the cycle. Similar reactions are responsible for color vision in the cone cells.
2. Epithelial cell maintenance: Vitamin A is essential for normal differentiation of epithelial tissues and mucus secretion and, thus, supports the body’s barrier-based defense against pathogens.

3. Reproduction: Retinol and retinal are essential for normal reproduction, supporting spermatogenesis in the male and preventing fetal resorption in the female. Retinoic acid is inactive in maintaining reproduction and in the visual cycle but promotes growth and differentiation of epithelial cells.

Figure 1: Absorption, transport, and storage of vitamin A and its derivatives. [Note: β-Carotene is a carotenoid, a plant pigment with antioxidant activity.] RBP = retinol binding protein; TTR = transthyretin; RAR = retinoic acid receptor; CoA = coenzyme A; mRNA = messenger RNA.