Photosensitized light-induced damage of IRBP (interphotoreceptor retinoid-binding protein): Effects on binding properties

Abstract

Purpose. To determine if IRBP (interphotoreceptor retinoid-binding protein) is damaged following irradiation by visible light in the presence of bound all- trans retinal. Methods. Following irradiation of the IRBP-all- trans retinal complex, the retinal was removed and damage to IRBP measured as loss of titratable thiol groups, loss of tryptophan fluorescence, and changes in retinol-binding-induced fluorescence. Results. IRBP irradiated by itself showed only minimal loss of tryptophan fluorescence; this loss was substantially in-creased by irradiation in the presence of all- trans retinal. Thiol groups and retinol-binding activity were also shown to be reduced. The damage to IRBP seemed to involve photosen-sitization by the all- trans retinal, which was in turn protected from bleaching by the IRBP. The binding affinity was shown to be reduced ten-fold following irradiation. Conclusion. In the eye, IRBP can stabilise vitamin A and debatably may be responsible for transport of different forms of vitamin A between the photoreceptor cells and pigment epithelium. If this is the case, it would play a key role in rhodopsin regeneration after bleaching. IRBP also appears to be necessary to sustain photoreceptor cells. Light was shown to cause photosensitized damage to IRBP, and thus might impair the regeneration process and photoreceptor viability.