Retinaldehyde functions in the visual system as the prosthetic group of the opsins, while retinoic acid modulates gene expression and tissue differentiation, acting by way of nuclear receptors.
Vitamin A or retinol is the immediate precursor to two important active metabolites: retinal, which plays a critical role in vision, and retinoic acid, which serves as an intracellular messenger that affects transcription of a number of genes. Retinol is converted by the enzyme RPE65 within the retinal pigment epithelium into 11-cis-retinal. Within the eye, 11-cis-retinal is bound to the protein opsin to form rhodopsin in rod cells and iodopsin in cone cells.
When light hits rhodopsin it activates rhodopsin and sets off a series of chemical reactions that create electrical signals. These signals are transmitted to the brain, where they are interpreted as vision.
Iodopsins are used in daylight vision and are analogous to rhodopsin (visual purple) that is used in night vision.
Due to its essential role in the visual system, acute vitamin A deprivation impairs photoreceptor function and causes night blindness (poor vision under dim light conditions), while chronic deprivation results in retinal dystrophies and photoreceptor cell death.
Vitamin A in visual system
