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Abnormal homocysteine metabolism and glutathione depletion in yolk sacs and embryos of diabetic rats

      Objective

      Exposure to maternal hyperglycemia induces both an increase in oxidative stress and a decrease in glutathione (GSH) in the developing embryo. We hypothesized that the resulting pro-oxidant microenvironment during vulnerable stages of organogenesis may contribute to hyperglycemia-induced malformations.

      Study design

      Diabetes was induced in 8-week-old female rats by IV streptozotocin injection. Glucose was monitored until diabetic levels of hyperglycemia (>250 mg/dL) were achieved. A sustained-release insulin pellet was then inserted subcutaneously. Rats were mated after normal and stable glucose levels (80-150 mg/dL) had been attained. On gestational day (GD) 4, the implants were withdrawn. Experimental rats were sacrificed on GD12 and embryos and yolk sacs were examined morphologically and stored at −80°C until analysis. Intracellular levels of oxidized (GSSG) and reduced glutathione (GSH) and the metabolic precursors, homocysteine (Hcy), methionine, cysteine (reduced), and cystine (oxidized) were analyzed by HPLC with electrochemical detection.

      Results

      A 2-fold decrease in the redox ratio (GSH/GSSG) was observed in the malformed compared to normal embryos. In the yolk sac, a 3-fold decrease in GSH/GSSG ratio was observed in malformed embryos, indicating a significant increase in intracellular oxidative stress. A decrease in the glutathione precursors, methionine and cysteine, was associated with decreased GSH levels. The increase in Hcy and cysteine is consistent with hyperglycemia-induced inhibition of γ-glutamylcysteine synthetase, the rate-limiting enzyme for glutathione synthesis.

      Conclusion

      These data provide strong evidence that glutathione-mediated antioxidant defense mechanisms are compromised in embryos and yolk sacs subjected to maternal hyperglycemia during critical stages of organogenesis. The resulting pro-oxidant intracellular environment is considered critical in the genesis of diabetic embryopathy.