Visual cycle

The "bleach and recycle" process a process utilized within the retina to ensure that the chromophore, 11-cis retinal, is present within opsin molecules in sufficient quantities to allow phototrasduction to occur. To understand the process, we must look first at where the retina gets its vitamin A (retinol) derivatives from.

Vitamin A must be consumed in the diet, as it is not synthesisable by the body. It may, however, be consumed indirectly, in the form of carotenoids such as beta carotene, which can be cleaved to form two retinol molecules. It is absorbed in the gut and is transported around the body via two pathways. In the first, and most predominant, it is esterified with a fatty acid to form a retinyl ester, and packaged into a chylomicron. In the second minor pathway, it is bound to Retinol Binding Protein (RBP) and Transthyretin, which prevents its filtration in the glomeruli. It is via this RBP-Transthyretin pathway that the retina recieves most of its retinoids.

As in transport via the RBP-Transthyretin pathway, retinoids must always be bound to chaperone molecules, for several reasons. Retinoids are toxic, insoluble in aqueous solutions, and prone to oxidation, and as such they must be bound and protected when within the body. The body utilizes a variety of chaperones, particularily in the retina, to transport retinoids.