Glycine therapy inhibits the progression of cataract in streptozotocin-induced diabetic rats

PURPOSE: The purpose of this paper was to investigate the effect of the oral administration of L-glycine (Gly) on the development of diabetic cataract induced by streptozotocin (STZ) in rats. METHODS: Two groups of male Wistar rats were intraperitoneally injected with a single dose of STZ (65 mg/kg bodyweight). Then, one group of diabetic rats and a control group were administered with 1% of Gly in drinking water for three months, ad libitum. Cataract development was monitored biweekly through ophthalmoscope inspection and was classified into four stages. At the end of 12 weeks, the animals were sacrificed and some biochemical parameters were determined in their lenses. The parameters include advanced glycation end products (AGEs), glycated proteins, total and soluble protein, glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), aldose reductase (AR), and sorbitol dehydrogenase (SDH). Some parameters were also determined in the serum and blood of the rats. RESULTS: Diabetic cataract gradually progressed in the STZ-administered group with no other treatment. Consequently, up to the end of the experiment, 2/3 of the animals in this group reached to the last stage of the cataract (mature cataract). The progress of this process was much slower in the diabetic group that was treated with Gly. At the end of the study, the visual cataract score was significantly lower in the diabetic group treated with Gly compared to those administered with STZ. Some lens parameters, including glycated proteins, AGEs, SOD, and AR activities, were increased while some others, including soluble and total protein, GSH level, and CAT activity, were decreased due to diabetes induction. After Gly treatment, all the above-named parameters had reverse changes except for the CAT activity. The SDH activity in the lenses had no changes due to diabetes or treatment. In addition, this treatment significantly decreased the amount of serum glucose (Glc), serum AGEs, and glycated hemoglobin (HbA1c) in the diabetic rats. Gly also increased the ferric reducing antioxidant power (FRAP) in the serum of diabetic rats. However, the decreased bodyweight of animals due to diabetes induction was not compensated by Gly administration. It is important to note that Gly had no effect on normal rat parameters. CONCLUSIONS: The results indicated that the oral administration of Gly significantly delayed the onset and the progression of diabetic cataract in rats. These effects were due to its antiglycating action and to a lesser extent, due to the inhibition of oxidative stress and polyol pathway.