THE PHOTOCHEMISTRY OF THE RETINOIDS AS STUDIED BY STEADY-STATE AND PULSED METHODS

The retina and retinal pigment epithelium contain a number of retinoid s in a metabolic pathway that eventually forms the visual pigments. Th is study investigates the photochemistry of those retinoids that may c ontribute to light-induced damage to the retina. These include retinal (RAL), retinol (ROL), retinylpalmitate (ROLpal) and the protonated Sc hiff-base of retinal (RAL(sb)). Their photochemistry was followed by b oth EPR spin-trapping techniques and the direct detection of singlet o xygen via its luminescence at 1270 nm. Irradiation (>300 nm) of RAL, R OL in methanol (MeOH) or RALpal in dimethylformamide, produces free ra dicals from both solvents. Illumination of RAL(sb) in MeOH containing NADH with light above 400 nm (and even above 455 nm) generates the sup eroxide radical. We also determined that the quantum yields for single t oxygen sensitization by RAL, ROL or RALpal in MeOH are 0.05, 0.03 an d <0.01, respectively. These values are at least 75% less than those p reviously found using chemical methods. These observations indicate th at a major photochemical process for these retinoids may be an electro n (or hydrogen) process that will lead to radical products, and that t he singlet oxygen mechanism is of relatively minor importance in proti c solvents. These results may explain the action spectra obtained from light-induced damage to the retina.