Biochemistry of the Retina & Pigment Epithelium

Saari, John

Abstract

The overall goal of this research project is to provide new understanding of the molecular mechanisms and control of vitamin A metabolism in retina. Two aspects will receive particular attention, visual cycle reactions and retinoic acid production and function. The restorative process that allows regeneration of bleached visual pigment involves generation of 11-cis-from an all-trans-configuration of retinoid. The accompanying reactions, known collectively as the visual cycle, involve metabolic processing of retinoids in at least two cell types of retina (photoreceptor and retinal pigment epithelium), coupled with coordinated intercellular flow of retinoid. While the cycle reactions have been the object of intensive study for decades, our knowledge of their detail and control lags far behind that accumulated for phototransduction. During the course of this project we will carry out a detailed investigation of the chemistry and enzymology of two key reactions of the cycle in retinal pigment epithelium, retinyl ester synthesis, and 11-cis-retinol oxidation. Interaction of cycle enzymes with purported substrate carrier proteins and control of the reactions will be stressed. Although vitamin A in retina has generally been considered as a precursor of the visual pigment chromophores, retinoic acid and its carrier protein CRABP are present in retina in relatively large amounts. Since this retinoid can not be reduced to retinaldehyde or retinol, it is likely to be involved in a function of vitamin A unrelated to the visual cycle. Experiments are proposed that will examine retinoic acid function in retina in more detail, concentrating on providing a molecular description of the enzymes involved in its synthesis and control. At present the relative paucity of detailed information about retinoic acid function in retina, and visual cycle enzymes and their control precludes even asking relevant questions about their possible relationship to retinal disease. The concepts, antibodies and cDNA probes that will be obtained during this project should be applicable in studies designed to explore the relationship of retinoid metabolism to retinal disease.