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.