Environmental effects on the photochemistry of A2-E, a component of human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofus cin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiatio n that is transmitted by the anterior segment of the human eye, photophysic al and photochemical studies were performed to determine if A2-E could phot osensitize potentially damaging reactions. Steady-state fluorescence measur ements indicate that the fluorescence emission maximum and quantum yield ar e very sensitive to the chemical environment and a correlation between thes e two parameters and the solvent dielectric constant is observed. Time-reso lved absorption experiments of A2-E in pure organic solvents showed no form ation of transient species on the timescale of our experiments. However, wh en these measurements were repeated for A2-E in Triton X-100 micelles, a sh ort-lived (tau similar to 14 mus), weak absorption was observed. This speci es is quenched by oxygen (k = 2 x 10(9) M-1 s(-1)) and by the addition of t he antioxidants, cysteine and N,N,N',N'-tetramethylphenylenediamine. Quench ing of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E-derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is n ot consistent with the triplet excited state. Based on these data we tentat ively assign this absorption to a free radical. In the RPE these initial pr ocesses can ultimately lead to damage to the tissue through the formation o f peroxides and other oxidized species.