Diabetic and non-diabetic human cornea and tear γ-glutamyl transpeptidase activity

Abstract

Background: Diabetes-related eye disease is due in part to oxidative stress. Gamma-glutamyl transpeptidase (GGT) is a γ-glutamyl cycle enzyme that protects against oxidative stress via glutathione recapture. This study investigates corneal and Schirmer tears GGT activity in diabetic and non-diabetic adults aged 50 to 83 years old.

Methods: GGT activity was determined by colorimetric assay on 50 corneas from 14 diabetic (without keratopathy) and 20 non-diabetic donors and on Schirmer type 1 test strips (no anesthesia) of 14 diabetic and 14 non-diabetic subjects.

Results: Type 1 (T1) diabetic cornea GGT activity was 40% lower than Type 2 (T2) diabetic cornea GGT activity (P = 0.04), but GGT activity was similar for corneas (without keratopathy) from diabetic and non-diabetic donors (P ≥ 0.44 for all). The number of endothelial cells/unit of GGT activity in diabetic corneas was 22% higher (P = 0.1) than in non-diabetic corneas. GGT activity per Schirmer strip and GGT activity per mm of tears were 36% and 50% higher (P ≤ 0.008 for all) for non-diabetic (tear volume dependent) than diabetic donors (tear volume independent), respectively. GGT activity per mm was 50% lower in T1 than T2 diabetics (P = 0.02). Higher tear GGT activity in non-diabetic than diabetic females (P ≤ 0.05) was due to higher GGT activity in the African American females.

Conclusion: GGT activity was less in T1 than T2 diabetics, but comparable to non-diabetic corneas. Schirmer tear GGT activity in diabetic eyes was tear volume independent, less in T1 than T2, lower than in tear volume dependent, non-diabetic female eyes. Low cornea and tear GGT activity suggests loss of antioxidant potential and supports ocular antioxidant therapy for diabetic patients.

Keywords: cornea; diabetes; endothelium; epithelium; eye; oxidative stress; tear; γ-glutamyl transpeptidase.

Figure 1

COD for the diabetic and non-diabetic corneal donors. Abbreviation: COD, cause of death.

Figure 2

Clinical evaluation of diabetic and non-diabetic tear donors. Abbreviations: T1, type 1 diabetes; T2, type 2 diabetes; DR, diabetic retinopathy.

Figure 3

Non-diabetic and diabetic corneal GGT activity. (A) Mean ± SEM for corneal, endothelial and epithelial GGT activity within non-DM and DM donor groups. (B) Comparison of GGT activity in T1 and T2 diabetic corneas. Abbreviations: GGT, γ-glutamyl transpeptidase; SEM, standard error of the mean; DM, diabetes mellitus; T1, type 1 diabetes; T2, type 2 diabetes.

Figure 4

Endothelial cell density and GGT activity. (A) Mean (±SEM) corneal endothelial cell density in 50 to 83 year old non-diabetic and/or diabetic donor groups. (B) Mean (±SEM) number of corneal endothelial cells/unit of GGT activity in the non-diabetic and/or diabetic donor groups. Abbreviations: GGT, γ-glutamyl transpeptidase; SEM, standard error of the mean; DM, diabetes mellitus.

Figure 5

(A) Scatter graph plot of the bilateral Schirmer tear production of all adult donors versus age showing a general decline in tear production (left and right trend lines) with donor age. (B) Scatter graph plot with trend line (linear regression) of tear volume (mm) per Schirmer test strip versus GGT activity per Schirmer test strip for non-diabetic and diabetic tear donors. (C) Distribution of GGT (red) on DAPI-stained (blue nuclei) human conjunctival epithelial cells (×400). (D) GGT reaction substrate (yellow color) was concentrated on the Schirmer’s test strip in the conjunctival contact area (bracket) and decreased toward the tear meniscus (arrow). Abbreviation: GGT, γ-glutamyl transpeptidase.