Action Spectra of Photo–Oxidation for Bleached and Dark–Adapted Photoreceptor Outer Segments

Abstract

Abstract: : Purpose: Exposure of the retina to high fluxes of visible light results in peroxidation of photoreceptor outer segments (POS), and the susceptibility to photodamage is increased in dark–adapted retinas. The purpose of this study was to determine the action spectra of susceptibility of bleached and dark–adapted POS to photooxidation in order to identify the chromophore(s) responsible. Methods:POS were isolated either from dark–adapted or bleached bovine retinas. The retinas were allowed to convert free retinal to retinol, what was confirmed by HPLC. Electron spin resonance oximetry was used to determine the rates of photo–induced oxygen uptake during irradiation with narrow–band light from the range 320 – 550 nm of the suspensions of POS. To obtain the action spectra, the rates were normalized to the equal number of incident photons. Results: Susceptibility of bleached POS to photooxidation exhibited a steep increase with decreasing irradiation wavelengths, and the action spectrum was similar to the action spectra of photooxidation determined for peroxidized docosahexaenoic acid. Susceptibility of dark–adapted POS was maximal when irradiated with wavelengths about 370 nm. The rate of photo–induced oxygen uptake at 370 nm was significantly greater than for bleached POS. Pre–irradiation of dark–adapted POS with green light further increased the rate of photooxidation induced by ultraviolet light by up to 30% when all 60 µM rhodopsin was bleached. Conclusions: Products of peroxidation of docosahexaenoic acid are likely to be responsible for photooxidation of bleached POS without retinal. Photooxidation in dark adapted POS seems to primarily mediated by retinal released from rhodopsin.

Keywords: oxidation/oxidative or free radical damage • photoreceptors • retinoids/retinoid binding proteins