| Age-related macular degeneration (AMD) is an ocular disease that causes severe visual loss and legal blindness in the elderly population. The pathophysiology of AMD is complex and may include genetic predispositions, accumulation of lipofuscin and drusen, local inflammation and neovascularization. Therefore, specific age-related changes in the retinal pigment epithelium (RPE) and Bruch's membrane have been investigated.;The accumulation of lipofuscin has been shown to precede the death of photoreceptor cells and the deterioration of the RPE. As a result, the determination of the photosensitive components of lipofuscin have been of major interest. One of these components, previously identified as a bis-retinoid pyridinium compound, is referred to as A2E. A2E has been characterized by mass spectrometry and is known to have a mass of 592 Da. The remaining chromaphores in RPE lipofuscin are structurally related to A2E as determined by their fragmentation pattern with losses of M+/- 190, 174 and/or 150 Da. Analysis of lipofuscin from various donors indicates that the extracts consist of as many as fifteen of these hydrophobic components which are also observed to form spontaneously in vitro over extended periods of time.;Previous studies have shown that numerous structural changes are induced in Bruch's membrane with age. These changes may have a harmful effect on Bruch's membrane, resulting in damage to RPE cells and the onset of AMD. Recent research has identified a commonly inherited variant of the complement factor H gene from different groups of AMD patients linking the genetics of the disease to inflammation. During inflammation there is activation of nitric oxide synthase and release of nitric oxide, which could lead to non-enzymatic nitration within extracellular deposits and/or intrinsic extracellular matrix (ECM) protein components of human Bruch's membrane. Two possible biomarkers for non-enzymatic nitration in aged human Bruch's membrane have been identified, which include 3-nitrotyrosine and nitrated A2E. The presence of nitrated A2E could not be detected in RPE extracts, suggesting that nitro-A2E may be a Bruch's membrane specific biomarker. The nonenzymatic glycation and nitration of the basement membrane protein laminin, as a model for aging Bruch's membrane, was also investigated. The results indicated that fragments containing lysine and arginine residues were preferentially modified in the glycated and irradiated samples. However, nitration of laminin fragments was not observed. Instead several of the fragments ending in a lysine residue appeared to bind to other fragments also ending in a lysine residue, indicating a polymerization-type reaction. This study provides evidence that glycation, nitration, and the presence of A2E may be involved in modifications to essential basement membrane proteins leading to deleterious changes within the RPE ECM environment. |
