Biochemically, the RPE is a dynamic and complex cell. It must meet demands for its own active metabolism, its extraordinary phagocytic function, and its role as a biological filter for the neurosensory retina. These processes impose a very high energy requirement on the RPE; not surprisingly, RPE cells contain all the enzymes of the 3 major biochemical pathways: glycolysis, the Krebs cycle, and the pentose phosphate pathway. Glucose is the primary carbon source used for energy metabolism and for conversion to protein. Although the RPE does make a minor contribution to the glycosaminoglycan- and proteoglycan-containing interphotoreceptor matrix, glucose is not converted to glycogen in the RPE. Glucosamine, fucose, galactose, and mannose are all metabolized to some extent in the RPE, although mannose seems to be passed on almost directly to the photoreceptors.
More than 80% of the wet weight of the RPE is contributed by water. Proteins, lipids, and nucleic acids contribute most of the remaining weight.
Nearly 850 proteins have been identified in the RPE. Many proteins found in other cells are also present in the RPE. These include hydrolytic enzymes such as glutathione, peroxidase, catalase, and superoxide dismutase, which are important for detoxification. The cytoskeletal proteins actin, myosin, α-actinin, fodrin, and vinculin are also present in both the RPE and other cells.
Some proteins found in the RPE and other cells are localized differently in the RPE. A well-known example of such a protein is Na+,K+-ATPase (also called sodium-potassium pump), which has a unique location in RPE cells. In most polarized epithelial cells, Na+,K+-ATPase is localized to the basolateral membrane, but in the RPE this enzyme is found on the apical membrane. The sodium-potassium pump uses energy derived from adenosine triphosphate (ATP) hydrolysis to transport sodium (Na+) and potassium (K+) against their electrochemical gradients. It is thought that the apical location of Na+,K+-ATPase in the RPE maintains the balance of Na+ and K+ in the subretinal space. RPE cells also contain proteins whose polarity has been shown to be reversed compared with the polarity of other epithelial cells; examples include an isoform of neural cell adhesion molecule (NCAM-140) and folate receptor α.
Some proteins are expressed only in the RPE. One such protein, RPE-specific protein 65 kDa (RPE65), is an obligate component of the isomerization and hydrolysis of vitamin A, which is required for regeneration of visual pigment (described later in Vitamin A Regeneration).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.