Aqueous Humor Ascorbate and Total Reactive Antioxidant Potential: Correlation with Introcular pO₂ in vivo

Abstract

Purpose: Increased oxygen (pO₂) in the anterior segment correlates with risk factors for open angle glaucoma (African-American heritage, thinner central cornea, post-vitrectomy). Previous studies have noted that aqueous humor (AH) antioxidant levels are decreased in glaucoma patients. Our cross-sectional study tested the hypothesis that antioxidant protection is lower in patients with elevated pO₂ in AH.

Methods: 230 patients undergoing keratoplasty, cataract and/or glaucoma surgery were included. Intraocular pO₂ measurements in 5 locations were obtained with the Oxylab^TM probe as previously described. AH specimens were stored in liquid nitrogen tank until analysis. Total reactive antioxidant potential (TRAP) was measured by chemiluminescence and ascorbate (AsA) by colorimetry. Ascorbate oxidase determined TRAP contribution by AsA. Subjects were categorized in 4 groups: Reference (REF)- no prior lens/vitreous surgery, Post-vitrectomy (VIT), Pseudophakic (IOL), and Corneal decompensation (CD). Multivariate regression, ANOVA, and student's t-test were used for data analysis.

Results: There was a significant decrease in TRAP, AsA levels (p<0.01) and contribution to TRAP by AsA in the VIT (p<0.001) and CD (p<0.001, 0.0001, 0.01) compared to REF group. Compared to REF, there was also significant increase in pO₂ in the IOL group at the lens and anterior chamber (AC) center (p<0.0001, 0.01), and pO₂ was increased in VIT in the posterior chamber (PC), AC angle and lens surface (p<0.001, 0.01, 0.01). The CD group had significantly increased pO₂ under the cornea, in AC center and lens surface (p<0.0001, 0.01, 0.01). No differences were noted in REF vs. IOL. Race, glaucoma, diabetes, age, and corneal thickness were not correlated with TRAP or AsA levels.

Conclusions: Ascorbate is the main component of TRAP in AH. Residual TRAP not contributed by AsA with elevated pO₂ (corneal decompensation, post-vitrectomy status) increased compared to REF indicating unidentified antioxidant(s). In contrast to prior studies, decreased TRAP or AsA levels were not detected in glaucoma patients. The significant decrease in TRAP and AsA in cases of increased pO₂ suggest oxygen degrades antioxoidant protective molecules in AH. If AH has decreased ability to quench free radicals, then this could explain higher risk of oxidative damage to cells bathed by AH (e.g. trabecular meshwork) and increased risk of glaucoma development.