Preparative and biosynthetic insights into pdA2E and isopdA2E, retinal-derived fluorophores of retinal pigment epithelial lipofuscin

Abstract

Purpose: Retinal-derived fluorophores that accumulate as RPE lipofuscin are implicated in pathological mechanisms of AMD. One component of RPE lipofuscin has been characterized as pdA2E, a pyridinium adduct derived from all-trans-retinal and excess ethanolamine. One-step preparation and biosynthetic studies of pdA2E and its novel isomer called isopdA2E are reported.

Methods: Biosynthetic reaction mixtures, RPE/choroids and neural retinas dissected from bovines, eyes harvested from Abca4(-/-)Rdh8(-/-) mice, irradiated samples, and enzyme-treated solutions were analyzed by HPLC, mass spectrometry, nuclear magnetic resonance spectroscopy, fluorescence spectrophotometry, and density functional theory (DFT).

Results: Optimization of the in vitro synthesis of pdA2E resulted in a biomimetic preparation of this pigment in a yield of 15%; this protocol also allowed the identification of isopdA2E, a double-bond isomer of pdA2E at the C13C14 position in bovine RPE lipofuscin. Interconversion between these two molecules occurs when either pdA2E or isopdA2E is exposed to light. A phospholipase D-based assay demonstrated the possibility of pdA2-PE being formed in neural retina and served as a precursor of pdA2E in the biosynthetic pathway. DFT calculations revealed that the 492-nm absorbance was assigned to the long arm of pdA2E/isopdA2E and the 340/342-nm absorbance to the short arm. Fluorescence efficiency of pdA2E and isopdA2E is very similar, but is much weaker in comparison with A2E, isoA2E, and iisoA2E.

Conclusions: Our results facilitate the understanding of compositions and biosynthetic pathways of adverse RPE lipofuscin.

Keywords: biomarker; biomimetic synthesis; biosynthetic pathway; electronic transition; lipofuscin; retinal pigment epithelium.