Peroxidized Docosahexaenoic Fatty Acid as a Photogenerator of Reactive Oxygen Species in the Retina

Abstract

Abstract: : Purpose: Docosahexaenoic fatty acid (DHA) constitutes about 50% of fatty acids in photoreceptor outer segments (POS) and is present in substantial amounts in the retinal lipofuscin. As excessive exposure to visible light induces lipid peroxidation in POS and lipofuscin, peroxidized DHA is formed. The aim of this study was to determine photosensitizing properties of peroxidized DHA. Methods: Free DHA or 1-palmitoyl-2-docosahexaenoyl-glycero-3-phosphocholine (16:0-22:6 PC) were peroxidized by exposure to the air. Formation of the products absorbing UVA and visible light was monitored by optical absorption spectroscopy. Photoreactivity of DHA was studied in liposomes containing native or peroxidized DHA or their extracts in organic solvents. Suceptibility of liposomes to photooxidation as a function of irradiation wavelength was measured by electron spin resonance (ESR) oximetry. ESR spin trapping was used to detect and identify radicals formed during irradiation. Time-resolved detection of singlet oxygen phosphorescence was used to determine the ability to photogenerate as well as to quench singlet oxygen. Nanosecond laser flash photolysis combined with time resolved absorption spectrometry was used to detect primary intermediates formed upon photoexcitation. Results: The absorption of the products formed due to peroxidation of DHA extended up to 500 nm. Irradiation of peroxidized DHA with UVA or blue light induced further oxidation. Action spectra of the rates of photo-induced oxygen uptake in suspension of liposomes containing peroxidized DHA exhibited a steep increase with decreasing wavelengths. Using DMPO as a spin trap, catalase-dependent photo-induced formation of DMPO-OH adduct was observed. Using POBN as a spin trap, a catalase-independent radical was trapped. Quantum yields of singlet oxygen generation varied between 0.13 - 0.25. A short-lived transient generated by laser flash photolysis of peroxidized DHA was observed with the absorption maximum around 420 nm. This transient, identified as a triplet state, was quenched by oxygen. Conclusions: The products of peroxidation of DHA are photoreactive – they undergo photooxidation and photogenerate reactive oxygen species upon irradiation with UVA or blue light. Photoreactive products of DHA peroxidation may potentiate damaging effects of light to the retina and are likely to be the photosensitizers responsible for lipofuscin photoreactivity.

Keywords: radiation damage: light/UV • oxidation/oxidative or free radical damage • retinal pigment epithelium