Identification of Retinal Proteins that are Nitrated in Elevated Glucose Concentration

Abstract

Abstract: : Purpose: Inhibition of diabetic retinopathy in animals by aminoguanidine is accompanied by a reduction in retinal protein nitration. We sought the identity of retinal proteins that are nitrated in diabetes as an approach to better understanding the pathogenic mechanisms of this retinopathy. Methods: In vivo studies utilized retinas collected from STZ-diabetic rats (2 month duration), and in vitro studies used a transformed Muller cell line (rMC-1) incubated in normal (5mM) and high (25 mM) glucose. Retinas or cells were homogenized and subjected to 2D Western analysis using an anti-nitrotyrosine antibody. Immunoreactive spots were excised from the 2D gel, digested in situ with trypsin and analyzed by MALDI-TOF MS and/or LC MS/MS. Results: Nitrotyrosine immunoreactivity in Western blots of retinas from diabetic rats was greater than that from nondiabetic rats or from diabetic rats treated with aminoguanidine. Likewise, rMC-1 cells incubated in 25 mM glucose exhibited greater nitrotyrosine immunoreactivity in Western blots than cells incubated in 5 mM glucose or in 25 mM glucose + aminoguanidine. The same nitrotyrosine immunoreactive proteins were identified from retinal homogenates and cultured rMC-1 cells, and include alpha enolase, glyceraldehyde 3-phosphate dehydrogenase, voltage-dependent anion-selective channel protein, aldolase, triosephosphate isomerase. Many of the identified nitrotyrosine immunoreactive proteins appear to be glycolytic enzymes. Alpha enolase exhibited the most dramatic change in apparent nitrotyrosine content. Conclusions: Hyperglycemia appears to increase nitration of glycolytic enzymes in retina and retinal cells. NO or nitration can alter enzyme activities of proteins. The present findings justify further consideration of nitration and nitric oxide as possible mediators in the mechanisms of diabetic retinopathy.

Keywords: diabetic retinopathy • nitric oxide • protein modifications-post translational