Diabetic and Non–Diabetic Vitreous Levels of –Glutamyltranspeptidase, Glutamate (Glu), Glutamine (Gln) and Glutathione (GSH)

Abstract

Abstract: : Purpose: To determine the relationship between the levels of γ–glutamyltranspeptidase (GGT) activity and GGT substrates (Gln and GSH) and products (Glu) in diabetic and non–diabetic vitreous humor (VH) samples. Methods: Twelve intra–operative VH samples were collected from consenting individuals and analyzed for GGT activity (i.e., one unit equals the conversion of 1 nanomole of L–γ–glutamyl–p–nitroanalide to p–nitroanaline/h/ml). The VH amino acid concentration was determined by HPLC. The VH protein concentration was determined by BioRad dye–binding. VH protein profiles were compared by Coomassie Brilliant Blue staining after separation on 10% SDS–PAGE gels. Results: The VH GGT activity in 7 diabetic samples was significantly higher (p=0.009) than in 5 non–diabetic samples (8±3.5 versus 2.1±0.9 U/ml). The concentrations of Gln, Glu, and alanine (Ala) in diabetic VH samples were higher (p’s<0.01) than the Gln (790±345 vs. 329±167 nanomoles/ml), Glu (9.5±4.4 vs. 2±2 nanomoles/ml), and Ala (287±70 vs. 111±24 nanomoles/ml) concentrations in non–diabetic VH samples. GSH concentration was below detection (<2 nanomoles/ml). The VH protein concentration tended to be higher (p=0.12) in the diabetic than in the non–diabetic VH samples (2.8±2.7 vs. 0.44±0.6 µg/ml). No differences were apparent between the protein profiles of diabetic and non–diabetic VH. Conclusions: GGT activity and the concentrations of Gln, Glu, and Ala are higher in diabetic than in non–diabetic VH possibly due to increased permeability of the blood–ocular barrier (BOB). The absence of GSH and elevated Gln suggests GGT generates an increase in VH Glu in situ and the Glu could in turn modulate the permeability of the BOB accounting for the tendency for elevated protein in diabetic VH.

Keywords: vitreous • proliferative vitreoretinopathy • diabetes