IRON-INDUCED ACCUMULATION OF LIPOFUSCIN-LIKE FLUORESCENT PIGMENT IN THE RETINAL-PIGMENT EPITHELIUM

Purpose. One of the most prominent changes that occurs in the retinal pigment epithelium during senescence is the progressive accumulation o f the autofluorescent pigment lipofuscin. Experiments were conducted t o evaluate the role of nonenzymatic oxidation of photoreceptor outer s egments in retinal pigment epithelium lipofuscin formation. Methods. A lbino Fischer rats were given intravitreal injections of ferrous sulfa te, a catalyst that promotes nonenzymatic lipid oxidation. At 2 hours, 24 hours, and 7 days after ferrous sulfate administration, the retina s were examined with fluorescence microscopy to assess the formation o f fluorescent products. At these same time intervals, organic solvent extracts of the retinas and retinal pigment epithelium-choroid complex es were prepared. The extracts were analyzed with thin layer chromatog raphy to assay for the presence of soluble fluorophores. The ultrastru ctural appearances of the retinas were examined at the same time point s. Results. At both 2 hours and 24 hours after the ferrous sulfate tre atment, the photoreceptor outer segments displayed a yellow-green fluo rescence emission that was not present in untreated eyes. Associated w ith this in situ fluorescence were a number of blue-green emitting flu orophores in organic solvent extracts that did not correspond to any o f the fluorophores extracted from the retinal pigment epithelium of ol d animals. One week after the ferrous sulfate treatment, the photorece ptor cells had degenerated and the retinal pigment epithelium containe d large amounts of an autofluorescent pigment with a golden-yellow emi ssion typical of lipofuscin. The iron-induced fluorophores could not b e extracted from this pigment into either chloroform or dichloromethan e. Conclusions. The initial fluorophores that were formed as a result of nonenzymatic oxidation of outer segment components did not appear t o be the same as those responsible for retinal pigment epithelium lipo fuscin fluorescence. However, after the oxidized outer segments were p hagocytosed by the retinal pigment epithelium, the latter cells became filled with a yellow-emitting fluorescent pigment that was similar in its fluorescence properties to lipofuscin. These observations suggest that lipofuscin fluorophores are not direct products of nonenzymatic lipid oxidation. However, some of these oxidation products may be modi fied after uptake by the retinal pigment epithelium to form insoluble lipofuscin fluorophores.