Assessment of Kynurenine Pathway Enzyme Activity in Ocular Diseases: Associations with Cataract, Diabetes, Glaucoma, and Pseudoexfoliation Syndrome

Purpose: To investigate the role of the kynurenine pathway (KP) in ocular diseases by evaluating the activity of key enzymes-kynurenine aminotransferase (KAT) and kynurenine monooxygenase (KMO)-and the 3-hydroxykynurenine to kynurenic acid (3-HK/KYNA) ratio in relation to cataract severity, diabetes, glaucoma, and pseudoexfoliation syndrome (PEXS). Methods: Tryptophan metabolite levels were measured in patients undergoing cataract surgery and stratified by SPONCS grading and comorbid conditions. KAT and KMO activities were estimated using metabolite ratios (KYNA/KYN and 3-HK/KYN, respectively). Statistical analyses included Kruskal-Wallis tests with post hoc comparisons and Mann-Whitney U tests. Results: KAT activity declined significantly with increasing SPONCS grade ( p = 0.014), suggesting a progressive loss of KYNA production and antioxidative capacity in advanced cataracts. Diabetic patients exhibited higher KMO activity ( p = 0.039) and elevated 3-HK/KYNA ratios ( p = 0.013), indicating a metabolic shift toward oxidative stress and neurotoxicity. Similarly, glaucoma patients had significantly increased KMO activity ( p = 0.032), consistent with enhanced 3-HK-mediated retinal ganglion cell damage. In contrast, PEXS showed no significant alterations in KP markers. Conclusions: The kynurenine pathway is differentially modulated in ocular diseases. A decline in KAT activity correlates with cataract severity, while upregulation of KMO is prominent in diabetes and glaucoma, revealing disease-specific metabolic dysregulation. Targeting KMO to reduce toxic metabolite accumulation or enhancing KYNA synthesis may offer novel therapeutic avenues. These findings also support the potential of KP metabolites as biomarkers for disease monitoring and progression.