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RH Nagaraj, S Padival, P Sarkar, PS Padayatti; Pyridoxamine Inhibits Alpha-dicarbonyl-mediated Modifications of Lens Proteins in Diabetic Rats . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2382.
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Purpose: The alpha-dicarbonyl compounds, methylglyoxal (MG) and glyoxal react with proteins and produce protein-protein crosslinking structures and fluorescent products. We have previously shown the presence of imidazolium protein crosslinking structures and a fluorescent product, argpyrimidine in human lens proteins. Our studies have also shown that these products accumulate in relatively large quantities in highly pigmented cataractous lenses (Chellan and Nagaraj, ABB 368: 98, 1999; Padayatti et al., IOVS 42:1299, 2001). In this study, we have determined the effect of pyridoxamine (PM) on the formation of alpha-dicarbonyl-mediated modifications in lens proteins of diabetic rats and studied effects on enzymes that can metabolize alpha-dicarbonyl compounds. Methods: Four groups of Sprague-Dawley male rats (12 in each group, 150 grams) were used. Diabetes was induced in two groups of rats by injecting streptozocin. PM was administered through drinking water at 400 mg/L in diabetic rats and at 800 mg/L in non-diabetic control rats. After five months of treatment, imidazolium crosslinks, GOLD (glyoxal-lysine dimer) and MOLD (methylglyoxal-lysine dimer) and, argpyrimidine and pentosidine were measured in lens proteins by HPLC. Activity of two alpha-dicarbonyl metabolizing enzymes, aldose reductase and glyoxalase I was measured. Glutathione (GSH) was measured in the protein-free extracts of the lens. Results: The GOLD levels were ∼7-fold higher in diabetic rats when compared to non-diabetic controls. PM treatment reduced these levels by nearly 50% in diabetic rats. MOLD levels were ∼2-fold higher in diabetic rats and were almost normalized by the PM treatment. Argpyrimidine and pentosidine levels were higher in diabetic rats and were corrected by the PM treatment. Surprisingly, the pentosidine and argpyrimidine levels were below normal in PM treated non-diabetic rats. Glyoxalase I activity was significantly reduced in diabetes, but this was prevented by PM treatment. Aldose reductase activity was increased 2.3 times in diabetes and PM treatment further increased this activity by 25%. Diabetes lowered GSH levels by 65%, PM had no effect on GSH levels. Conclusion: PM is able to block alpha-dicarbonyl mediated modification of lens proteins. This could be due to inhibition of the reaction of alpha-dicarbonyls with lens proteins and/or activation/retention of enzymes that can metabolize alpha-dicarbonyl compounds.
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