The photochemistry of the retinoids as studied by steady-state and pulsed methods

The retina and retinal pigment epithelium contain a number of retinoids in a metabolic pathway that eventually forms the visual pigments. This study investigates the photochemistry of those retinoids that may contribute to light-induced damage to the retina. These include retinal (RAL), retinol (ROL), retinylpalmitate (ROLpal) and the protonated Schiff-base of retinal (RALsb). Their photochemistry was followed by both EPR spin-trapping techniques and the direct detection of singlet oxygen via its luminescence at 1270 nm. Irradiation (> 300 nm) of RAL, ROL in methanol (MeOH) or RALpal in dimethylformamide, produces free radicals from both solvents. Illumination of RALsb in MeOH containing NADH with light above 400 nm (and even above 455 nm) generates the superoxide radical. We also determined that the quantum yields for singlet oxygen sensitization by RAL, ROL or RALpal in MeOH are 0.05, 0.03 and < 0.01, respectively. These values are at least 75% less than those previously found using chemical methods. These observations indicate that a major photochemical process for these retinoids may be an electron (or hydrogen) process that will lead to radical products, and that the singlet oxygen mechanism is of relatively minor importance in protic solvents. These results may explain the action spectra obtained from light-induced damage to the retina.