Once photoreceptor outer segments are phagocytosed, inclusive of A2E, they are delivered to retinal pigment epithelial (RPE) cells for disposal. However, A2E has been shown to be resistant to any sort of degradative enzyme thus accumulates in the RPE. The relationship between lipofuscin accumulation and retinal degeneration is illustrated by Stargardt disease type 1 (STGD1, MIM:248200) (Allikmets et al. 1997).

Cytosolic ribulose-5-phosphate-3-epimerase (RPE), using Fe2+ as cofactor, catalyzes the reversible interconversion of D-ribulose 5-phosphate (RU5P) and D-xylulose 5-phosphate (XY5P) (Bose & Pilz 1985, Liang et al. 2011). The electrophoretic properties of RPE activity detected in extracts of mouse-human somatic cell hybrids suggest that the active form of the enzyme is a homodimer (Spencer & Hopkinson 1980). Ribulose-phosphate 3-epimerase-like protein 1 (RPEL1), based on sequence similarity, is suggested to function as RPE.

Although interphotoreceptor retinoid-binding protein (RBP3, IRBP) (Fong & Bridges 1988, Fong et al. 1990) is not required to move all-trans-retinol (atROL) from photoreceptor cells to the retinal pigment epithelium (RPE), it may function to regulate retinoid trafficking and possibly protect retinoids from biochemical damage. RBP3 is secreted by photoreceptor cells into the interphotoreceptor matrix (IPM), where, being a larger protein (135kDa) than the IPM space, becomes trapped (see mini-review Gonzalez-Fernandez & Ghosh 2008). It is through this space that retinoids move between the RPE and photoreceptor outer segments during the retinoid cycle. Once atROL enters the RPE, it binds with RBP1.

All-trans-retinol esters (atREs) serve as substrates for retinoid isomerohydrolase (RPE65), located in retinal pigment epithelium (RPE) cells. RPE65 hydrolyses atREs to 11-cis-retinol (11cROL), thus performing an isomerase activity as well as hydrolysis. RPE65 is membrane-bound, this being dependent on the palmitylation of the residue Cys-112 (Takahashi et al. 2009). RPE65 normally undergoes a light-dependent translocation to become more concentrated in the central region of RPE cells. This translocation requires Unconventional myosin-VIIa (MYO7A or USH1B) (Lopes et al. 2011).

Cytosolic ribulose-5-phosphate-3-epimerase (RPE) catalyzes the reversible interconversion of D-xylulose 5-phosphate and D-ribulose 5-phosphate (Bose and 1985). The electrophoretic properties of RPE activity detected in extracts of mouse-human somatic cell hybrids suggest that the active form of the enzyme is a homodimer (Spencer and Hopkinson 1980).

Using NADP+ as cofactor, several members of the short-chain dehydrogenases/reductases (SDR) family can (reversibly) catalyse the oxidation of 11-cis-retinol (11cROL) to 11-cis-retinal (11cRAL) in retinal pigment epithelium (RPE) cells. Retinol dehydrogenases 10 and 11 (RDH10 and 11) are two such members utilizing the cofactor NADP+ (Wu et al. 2002, Kedishvili et al. 2002 respectively). Cellular retinaldehyde-binding protein (RLBP1), the protein bound to 11cRAL in RPE, is not present in photoreceptor cells.

Lipofuscin is a yellow-brown pigment grain composed mainly of lipids but also sugars and certain metals. Accumulation of lipofuscin is associated with degenerative diseases such as Alzheimer's disease, Parkinson's disease, chronic obstructive pulmonary disease and retinal macular degeneration.

A prominent component of lipofuscin in retinal pigment epithelial (RPE) cells is the bisretinoid A2E (di-retinoid-pyridinium-ethanolamine), the end-product of the condensation of 2 molecules of all-trans-retinal (atRAL) and phosphatidylethanolamine (PE) in photoreceptor outer disc membranes. Once formed, A2E is phagocytosed, together with outer segments (Kevany & Palczewski 2010), to RPE where it accumulates. There is no evidence as yet to indicate that A2E can be catabolised (Sparrow et al. 2012, Sparrow et al. 2010). A simplified biosynthetic pathway for A2E is described here.

Retinol binding protein (RBP4) delivers all-trans-retinol (atROL) from liver stores to peripheral tissues. Defects in RBP4 cause retinol-binding protein deficiency (RBP deficiency, MIM:180250), causing night vision problems and a typical 'xerophthalmic fundus' with progressive atrophy of the retinal pigment epithelium (RPE) (Seeliger et al. 1999, Biesalski et al. 1999).

The retinoid cycle (also referred to as the visual cycle) is the process by which the visual chromophore 11-cis-retinal (11cRAL) is released from light-activated opsins in the form all-trans-retinal and isomerized back to its 11-cis isomer ready for another photoisomerization reaction. This process involves oxidation, reduction and isomerization reactions and take place in the retinal pigment epithelium (RPE) and photoreceptor segments of the eye (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011). This section describes the retinoid cycle in rods during dark/twilight conditions.

Rods and cones share the same mechanism for the phototransduction process but perform functionally different roles. Although cone photoreceptors make up around 5% of all photoreceptor cells and are outnumbered 20 to 1 by rod photoreceptors, they mediate daylight vision in the human eye whereas rods mediate twilight vision. Also, cones are around 100-times less light-sensitive than rods thereby depriving us of colour vision in dark conditions in which cones cannot function. Rod function saturates in even moderate amounts of light whereas cones can adjust to even very bright light conditions, a process called light adaptation. In bright conditions, rods can take up to one hour to regain their sensitivity whereas cones can recover in a few minutes, a process called dark adaptation and which allows us to retain visual perception in changing light conditions.

Cone cells express three types of opsin which allow colour discrimination. Long Wavelength Sensitive Opsin (OPN1LW) detects red , Short Wavelength Sensitive Opsin (OPN1SW) detects blue, and Medium Wavelength Sensitive Opsin (OPN1MW) detects green regions of the light spectrum.

In the canonical retinoid (visual) cycle, the visual chromophore is regenerated in reactions involving the rod outer segments (ROS) and the retinal pigment epithelium (RPE). For cones, chromophore recycling is independent of the RPE and instead involves Muller cells in the retina which supply the chromophore selectively to cones. The molecular steps of the cone retinoid (visual) cycle are outlined in this section. The ability of cones to react to bright and differing light conditions means it has to regenerate the chromophore much quicker than rods. All-trans-retinol (atROL) released from cone outer segments is taken up by Muller cells where it is directly isomerized back to 11-cis-retinol (11cROL) then esterified by LRAT. When required, these 11-cis-retinyl esters can be hydrolysed by 11-cis-RE hydrolases back to 11cROL then oxidised in the cone photoreceptor cell to regenerate 11-cis-retinal (11cRAL), the visual chromophore (see reviews von Lintig 2012, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).