Thiol-Disulfide homeostasis in tear fluid: Alterations associated with diabetic retinopathy

Abstract

Background

This study aimed to evaluate dynamic thiol/disulfide homeostasis in the tear fluid of patients with type 2 diabetes and to investigate its relationship with the presence of diabetic retinopathy (DR), considering it as a potential biomarker of oxidative stress.

Methods

A total of 96 individuals were included in the study. Participants were divided into three groups: 32 patients with diabetic retinopathy (Group 1, DR (+)), 31 patients with diabetes but without retinopathy (Group 2, DR (–)), and 33 healthy volunteers (Group 3). All participants underwent a comprehensive ophthalmological examination, including best-corrected visual acuity, slit-lamp biomicroscopy, and fundus evaluation. Tear samples were collected using Schirmer strips and stored in phosphate-buffered Eppendorf tubes at −80 °C until analysis. Total thiol, native thiol, and disulfide levels were analyzed using a spectrophotometric method. Patients with a history of intraocular surgery, trauma, glaucoma, or systemic diseases other than diabetes were excluded.

Results

There were statistically significant differences between the groups in terms of total thiol, disulfide, disulfide/total thiol, and disulfide/native thiol ratios (p < 0.001). Total thiol levels were significantly lower, while disulfide levels and ratios were significantly higher in the DR (+) group. No significant difference was found in native thiol levels (p = 0.194). HbA1c and fasting blood glucose levels were significantly higher in the DR (+) group compared to the other groups (p < 0.001). Central macular thickness was also significantly greater in the DR (+) group (p < 0.001). Correlation analysis revealed a positive and significant relationship between HbA1c and fasting blood glucose (r = 0.551; p < 0.01), whereas no significant correlation was found between central macular thickness and glycemic parameters.

Conclusion

The increased disulfide levels and disulfide ratios in the tear fluid of patients with diabetic retinopathy suggest a shift in thiol/disulfide homeostasis toward oxidative stress. These tear-based biomarkers may serve as valuable tools for the early diagnosis and monitoring of diabetic retinopathy.