Biochemistry of visual pigment regeneration: the Friedenwald lecture.

Phototransduction and the visual cycle play complementary roles in vertebrate vision. Phototransduction is initiated by the photoisomerization of 11-cis-retinal bound to opsin and ultimately results in a change in the release of neurotransmitter by photoreceptor cells. The visual cycle restores the product of photoisomerization, all-trans-retinal, to the 11-cis configuration and allows the regeneration of bleached visual pigments (Fig. 1). The biochemical mechanism of phototransduction has been extensively studied during the past 2 decades, and as a result, the process serves as the paradigm for understanding G-protein–coupled receptors in general. In contrast, molecular understanding of the visual cycle is poorly developed, and many fundamental questions regarding reactions, enzymes, and control mechanisms remain unanswered. Sequences of cDNAs encoding three visual cycle enzymes have been published; however, molecular information is unavailable for the other three presumed enzymes of the cycle, including retinol isomerase (isomerohydrolase). It has been reported that regeneration of visual pigments is a slow process and that photoisomerization of 11-cis-retinal in rhodopsin is very rapid. In fact, complete dark adaptation in humans requires approximately 40 minutes, and conversion of rhodopsin to photorhodopsin requires only 200 fsec. However, this is not a fair comparison, because it is clear that the photolysis and regeneration rates in the living eye must be equal and opposite in sign at ambient levels of illumination. Any other situation would not be compatible with vision, because visual pigment would dissipate rapidly. Alpern has demonstrated for human rods and cones that a steady state level of bleached visual pigments, in which the bleach and regeneration rates are equal and of opposite sign, is present at different levels of physiologic illumination. In humans the progress curves for the regain of visual threshold and for the regeneration of visual pigment coincide when displayed on a semi–log plot. The molecular explanation for this log–linear relationship is not well understood, and the relationship may be fortuitous, but most agree that a photoproduct is responsible for desensitization of the visual system. Although the molecular identity of the desensitizing intermediate(s) remains a matter of active investigation, it is clear that visual cycle reactions are important in determining the steady state level of bleached visual pigment and thus the sensitivity of the retina. The critical role of the retinal pigment epithelium (RPE) in visual pigment regeneration is apparent from the studies of 19th century investigators who demonstrated that dissected frog retina could regenerate its bleached visual pigment only when in contact with the RPE. The reader is referred to Marmor and Martin for a depiction of some of their insightful experiments. It is fortunate that the early physiologists used frog eyes for their experiments because rodent eyes do not regenerate their visual pigments when removed from the animal, as will be discussed later. Fifty years later, Wald used extraction techniques to show that vitamin A was involved in the visual process and formulated the first modern version of the visual cycle including the participation of the RPE (Fig. 2). The role of the RPE in the visual cycle became more clear after the classic study by Dowling, 22 which demonstrated movement of retinoid out of the neural retina and into RPE during extensive bleaching and return during recovery in the dark. Later studies by other investigators with techniques offering more resolution verified the fundamental observation. Bernstein et al. and Rando provided a molecular explanation for the necessity of the RPE with the demonstration that the critical enzymatic regeneration of the 11-cis configuration occurred within this tissue. The transcellular migration of the retinoids during bleaching and regeneration is all the more remarkable, considering the anatomy of the journey (Fig. 3). The relatively insoluble retinoid must leave the disc membranes, diffuse through a cytosolic compartment to reach the plasma membrane of the rod outer segment, traverse the plasma membrane, diffuse across the subretinal space to reach the plasma membrane of the RPE cell, enter into the reactions of the visual cycle in this cell, and make the return journey! A current working hypothesis for the reactions of the vertebrate rod visual cycle is shown in schematic form in Figure 4. The figure depicts internal and plasma membranes of the RPE and rod photoreceptor cells and the interphotoreceptor matrix space (subretinal space) separating these two cells. The visual cycle enzymes in RPE are all associated with membranes; however, their localization to subcellular compartFrom the Departments of Ophthalmology and Biochemistry, University of Washington School of Medicine, Seattle, Washington. Supported in part by National Institutes of Health Grants RO1 EY02317, EY01730, and EY09339 and by unrestricted awards from Research to Prevent Blindness. JCS is a Senior Scientific Investigator of Research to Prevent Blindness. Submitted for publication August 6, 1999; accepted August 30, 1999. Commercial relationships policy: N. Corresponding author: John C. Saari, Department of Ophthalmology, Box 356485, University of Washington, Seattle, WA 98195-6485. jsaari@u.washington.edu