Mitton K P, Dzialoszynski T, Sanford S E, Trevithick J R
Laboratory for Mechanisms of Ocular Diseases, National Eye Institute, Bethesda, Maryland, USA.
Curr Eye Res. 1997 Jun;16(6):564-71. doi: 10.1076/ceyr.16.6.564.5078.
Glutathione (GSH) loss precedes vacuole formation in the diabetic rat lens, but the cause of this loss is not known. Cysteine availability is a rate limiting factor to glutathione biosynthesis in rat and human lenses but its concentration is not known; therefore free cysteine was measured prior to lens hydration in the diabetic rat lens. GSH can regenerate ascorbate from dehydroascorbate within the lens and potentially modulate the ascorbate pool; therefore ascorbate loss is also a possibility that has not been examined previously.
Diabetes was induced in Wistar rats to provide a slowly progressing model of cortical cataract. Age-matched control rats were injected with buffer vehicle only. Lens condition was monitored by binocular slit-lamp microscope after pupil dilation. Lens cysteine and glutathione were measured in the same lens, while ascorbate and total ascorbate (ascorbate + dehydroascorbate) of the contralateral lens were quantified by high performance liquid chromatography electrochemical detection. The 1- and 2-week periods of diabetes were chosen as they both preceded lens hydration changes and Na+/K+ changes, to avoid leakage due to ruptured cell membranes.
Lens weights were not significantly different compared to controls at either the 1- or 2-week periods, and lenses were completely free of initial vacuole formation. Lens GSH concentration was diminished by 72% compared with controls after 1 week of diabetes and 74% after 2 weeks of diabetes. Lens free cysteine was decreased by 62% and 78% compared with controls after 1 and 2 weeks of diabetes, respectively. Total lens ascorbate concentration was decreased by 34% after 1 week of diabetes and 48% after 2 weeks of diabetes. Dehydroascorbate levels represented less than 10% of the total lens ascorbate pool in all experimental groups. GSH and ascorbate concentration were correlated after 1 week of diabetes (p < 0.005) and after 2 weeks of diabetes (p < 0.001). GSH and cysteine concentration were also correlated after 1 week of diabetes (p < 0.001) and after 2 weeks of diabetes (p < 0.001).
Decreased free cysteine, in the diabetic rat lens, precedes hydration changes and vacuole formation, contributing to decreased glutathione content. While cysteine was not abundant in the lens, its concentration is greater than previously supposed. The lens ascorbate pool was also diminished prior to lens hydration.
在糖尿病大鼠晶状体中，谷胱甘肽（GSH）的损失先于液泡形成，但其损失原因尚不清楚。半胱氨酸的可用性是大鼠和人类晶状体中谷胱甘肽生物合成的限速因素，但其浓度未知；因此，在糖尿病大鼠晶状体水化之前测量了游离半胱氨酸。GSH可在晶状体内从脱氢抗坏血酸再生抗坏血酸，并可能调节抗坏血酸池；因此，抗坏血酸的损失也是一种可能性，此前尚未进行过研究。
在Wistar大鼠中诱导糖尿病，以提供皮质性白内障的缓慢进展模型。年龄匹配的对照大鼠仅注射缓冲液载体。瞳孔散大后，用双目裂隙灯显微镜监测晶状体状况。在同一晶状体中测量晶状体半胱氨酸和谷胱甘肽，而对侧晶状体的抗坏血酸和总抗坏血酸（抗坏血酸+脱氢抗坏血酸）通过高效液相色谱电化学检测进行定量。选择糖尿病1周和2周的时间段，因为它们都先于晶状体水化变化和Na+/K+变化，以避免因细胞膜破裂而导致的渗漏。
在糖尿病1周或2周时，晶状体重量与对照组相比无显著差异，且晶状体完全没有初始液泡形成。糖尿病1周后，晶状体GSH浓度比对照组降低了72%，糖尿病2周后降低了74%。糖尿病1周和2周后，晶状体游离半胱氨酸分别比对照组降低了62%和78%。糖尿病1周后，晶状体总抗坏血酸浓度降低了34%，糖尿病2周后降低了48%。在所有实验组中，脱氢抗坏血酸水平占晶状体总抗坏血酸池的比例不到10%。糖尿病1周后（p<0.005）和糖尿病2周后（p<0.001），GSH和抗坏血酸浓度呈正相关。糖尿病1周后（p<0.001）和糖尿病2周后（p<0.001），GSH和半胱氨酸浓度也呈正相关。
糖尿病大鼠晶状体中游离半胱氨酸的减少先于水化变化和液泡形成，导致谷胱甘肽含量降低。虽然半胱氨酸在晶状体中并不丰富，但其浓度比之前认为的要高。在晶状体水化之前，晶状体抗坏血酸池也减少了。
