Tear fluid small molecular antioxidants profiling shows lowered glutathione in keratoconus

Keratoconus (KC) is a non-inflammatory disease of the cornea involving structural changes. Oxidative stress is reported to be parts of its pathology, yet the tear antioxidant status contributed by smaller molecule antioxidants can be indicative of the disease. Therefore this study is an attempt to present the status of small molecule antioxidants in KC condition as well as the influence of contact lens wear (CLW) in KC as evaluated in the tear specimen. Tear fluid was collected using schirmer strips from KC with and without CLW (n = 40) with age matched controls (n = 29). Tear fluid antioxidants cysteine, ascorbic acid, glutathione, uric acid and tyrosine were determined by HPLC electrochemical detection. Tear reactive oxygen species was estimated by fluorescence detection using dichlorodihydrofluorescein diacetate (DCF-DA) method. The corneal epithelial mRNA expression of the enzymes, gamma-glutamine cysteinyl synthase (γ-GCS) and gamma-glutamyl transpeptidase (γ-GT) by semiquantitative RT-PCR. Among the five antioxidant molecules estimated, GSH decreased significantly 50.9 ± 9.4 μM in control and 16 ± 5.7 μM in KC (p = 0.015) with increase in tyrosine 13.9 ± 2.6 μM in control, 30 ± 6.4 μM in KC cases (p = 0.022) and uric acid 162 ± 18 μM in control and 210 ± 32 μM (p < 0.00) in KC compared to the controls. The ROS levels were increased significantly, 55.7 ± 16.7 AU in KC and 23.2 ± 5.8 AU in controls (p = 0.023). No significant change in the tear antioxidants studied was observed in KC cases with and without CL wear. However tyrosine levels were increased significantly in CL wear amongst healthy controls compared to controls (p < 0.000). γ-GCS and γ-GT showed no significant change in KC epithelial cells. Though variations were seen in other antioxidants analysed, they had no statistical significance. Tear specimen in KC can indicate the antioxidant status. KC is associated with increased tear levels of tyrosine, uric acid and decrease in GSH apart from increased ROS. Glutathione decreases with increase in oxidative stress and this emphasises the need for antioxidants to balance the redox status in disease management of KC.