And finally, retinal glutamine synthetase showed prominent protein carbonyls in ocular hypertensive eyes. Glutamine synthetase is a key enzyme that helps maintain the physiological level of extracellular glutamate, thereby modulating excitotoxicity that results from the impaired glutamate–glutamine cycle. Glutamine synthetase has been shown to be particularly sensitive to inactivation by oxidants, and altered activity of this protein after oxidation has been shown to lead to the accumulation of glutamate resulting in neurotoxicity.
68 In addition to an age-related decline in glutamine synthetase activity in the human brain, oxidatively inactivated glutamine synthetase has been associated with brain injury after ischemia
69 or neurodegenerative diseases.
38 70 Retinal glutamine synthetase, which is located mainly in the Müller cells,
71 72 similarly plays a crucial role in balancing the neurotoxic effect of glutamate on RGCs. Therefore, our observations in ocular hypertensive eyes suggest that after oxidative modification of glutamine synthetase, the resultant failure in the effective conversion of glutamate to glutamine may be associated with the glutamate excitotoxicity to RGCs that has been implicated in the neurodegenerative process of glaucoma. Although multiple efforts did not confirm
73 74 75 76 the elevated glutamate levels detected in the vitreous of glaucomatous eyes,
77 whether glutamate excitotoxicity plays a role in glaucomatous neurodegeneration still remains elusive. What is also conflicting is that if the intravitreal or intraretinal level of glutamate was elevated in glaucomatous eyes, an increase in the expression of glutamine synthetase by retinal Müller cells would be expected as a compensatory mechanism. However, expression of this protein has not been found to be increased in glaucomatous eyes using immunohistochemistry.
78 Similarly, there was no detectable difference between the glutamine synthetase immunolabeling of ocular hypertensive and control retinas in our study. We wonder whether this might partly be due to oxidative modification of glutamine synthetase in ocular hypertensive eyes, which may affect immunoreactivity by interfering with the antibody binding, and/or may increase the degradation of the oxidized protein. For example, another study using immunochemical determination showed that the concentration of glutamine synthetase significantly decreases after oxidation, indicating that the oxidative inactivation leads to the degradation of this enzyme.
79 Thus, although many aspects are yet unclear, alteration in glutamine synthetase activity due to oxidative protein modification appears to be important in controlling the potential neurotoxicity of extracellular glutamate on RGCs.