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T.B. Redens, T.C. Welbourne, M.P. Langford; Gamma-Glutamyl Transpeptidase Activity in Serum and Cataractous Lens . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3498.
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Purpose: Gamma-glutamyl transpeptidase (GGT) is usually elevated in serum of cataract patients, but GGT activity (glutaminase) is decreased in cataractous lens. GGT is a large molecule with glutaminase and transferase activity (i.e., converts glutamine to glutamate and catalyses the conversion of glutathione to glutamate). The GGT activities of serum and cataractous lens were assessed to investigate the function of GGT in cataractous lens. Methods: Human serum, cadaver and cataractous lens were analyzed. GGT activities in serum and lens were assessed by conversion of gamma-glutamyl-p-nitroanaline (PNA) in the presence of absence of glycyl-glycine (gly-gly) and with GGT inhibition (acivicin and glutathione). Results: Normal serum and cadaver lens GGT activity was 2-4-fold higher in PNA with gly-gly than in PNA without gly-gly. Acivicin inhibited GGT activity in serum and lens in PNA with gly-gly by greater than 60% while acivicin inhibited GGT activity in serum and lens in PNA without gly-gly by less than 30% consistent with inhibition of GGT glutaminase activity. Concomitantly, GSH reduced GGT activity in serum and cadaver lens in PNA with or without gly-gly by 15-35%. Interestingly, the GGT activity in cataractous and cadaver lens was 52% higher in PNA without gly-gly than in PNA with gly-gly. Glutamate levels were increased by lens GGT with time of incubation in PNA with or without gly-gly. Conclusion: The results suggest that the loss of GGT activity in cataractous lens was more closely associated with loss of glutaminase activity and to a lesser degree the transpeptidase activity. Thus, these results support the metabolic activity of GGT in maintenance of lenticular glutamate and degradation of lenticular GSH levels in cataractous lens.
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