Retinal

Retinal, or retinaldehyde, is the aldehyde form of vitamin A that functions as the chromophore of vertebrate visual pigments. It exists primarily in two isomeric forms: 11-cis-retinal, which binds to opsin to form the light-sensitive visual pigments in rods (rhodopsin) and cones, and all-trans-retinal, produced after photon absorption and released during photobleaching.

In the phototransduction cascade, light converts 11-cis-retinal to all-trans-retinal, triggering a conformational change in opsin that activates the G protein transducin and a phosphodiesterase cascade. This leads to a drop in intracellular cGMP and the closure of cGMP-gated ion channels, reducing neurotransmitter release and generating a visual signal. All-trans-retinal is then reduced to all-trans-retinol and transported to the retinal pigment epithelium (RPE). In the RPE, all-trans-retinol is esterified by LRAT, converted by RPE65 to 11-cis-retinol, and ultimately oxidized back to 11-cis-retinal, which is transported back to photoreceptors to regenerate the visual pigment.

Nutritional and physiological context: Retinal is derived from dietary vitamin A and is recycled through the visual cycle to sustain vision. Vitamin A deficiency can cause night blindness and other eye disorders, while excessive vitamin A intake can be toxic. Retinal derivatives, collectively known as retinoids, have additional medical applications in dermatology and oncology, but must be used with care due to potential adverse effects.