THE ROLE OF VITAMIN A IN VISION

 

Ismail Ibrahim

Vitamins are Noncaloric essential Organic compounds needed in small quantities to perform specific biological functions for normal maintenance of optimum growth and good health. Vitamin A is one of the most studied apocarotenoids in mammals and human. Vitamin A is a 20C molecule that contains a Beta-ionone ring at one end (Karrer et al., 1931; Holmes and Corbet 1937). Mammals are unable to synthesize vitamin precursors. In their first month, mammals get vitamin A as preformed vitamin A from breast milk. In the adult stage, vitamin A is obtained through the diet as a preformed vitamin A in the form of retinol, retinal, retinoic acid when consuming products of animal origin such as meat, milk, fish, egg, or as a provitamin A when consuming vegetables that contain Betacarotene, Alpha-carotene, and Beta-crytoxanthin (Arscott,2013). Vitamin A plays a vital role in visual cycle and cell differentiation.

Visual Cycle

In the retina, vitamin A in the form of II-cis retinal binds to a protein called opsin to make rhodopsin (in rod cells). The activated rhodopsin, following exposure to light converts 11-cis-retinal component of rhodopsin into all trans-retinal through various stages. The all trans-retinal formed is soon separated from the opsin. This process of separation is called photodecomposition and the rhodopsin is said to be bleached by the action of light. The all trans-retinal isomerases to 11-cis-retinal then reunite with opsin in the rod outer segment to form the rhodopsin. This cascade of complex biochemical reactions ultimately results in the generation of receptor potential in the photoreceptors. In this way, the light energy is converted into electrical energy which is further processed and transmitted via visual pathway (A.K kurhana 2007). conclusively vitamin A plays an indispensable role in various biochemical processes especially in the faculty of sight such as visual cycle and color vision and its deficiency can lead to debilitating vision impairment and blindness.