Mackic J B, Kannan R, Kaplowitz N, Zlokovic B V
Department of Neurological Surgery, Childrens Hospital Los Angeles 90027, USA.
Exp Eye Res. 1997 Apr;64(4):615-26. doi: 10.1006/exer.1996.0260.
Transport of circulating sulfur amino acids (SAA) into the lens epithelium and de novo glutathione (GSH) synthesis were studied in the perfused guinea-pig eye. Plasma-to-aqueous transfer of SAA was in their intact form (> or = 98%) and comparable with sucrose (an extracellular marker) within 30 min. The unidirectional transport rates (ml min-1 g-1) of 35S-labeled cystine, cystine and methionine into the epithelium were: 0.0057, 0.0003 and 0.0073 from plasma, and 1.41, 0.005 and 1.69 from aqueous, respectively. The unidirectional epithelial uptake was limited to 1 min for all three [35S]SAA, and the isotopic steady-state ratio was achieved between 1 and 30 min. Cortical uptake was time-dependent and progressive between 1 and 30 min, but undetectable within 1 min. The high performance liquid chromatography (HPLC) analysis of the epithelium revealed that following 1 min of unidirectional [35S]cysteine transport, 3% of the label was incorporated into GSH and > or = 95% was as cysteine. An average incorporation of [35S]cysteine into GSH within the 30 min period was 0.83% min-1 and 1%/min for the epithelium and cortex, respectively. Infusions of [35S]cystine and methionine failed to demonstrate incorporation into GSH. Maximal rates of de novo GSH epithelial synthesis were approximately 3 and 12 pmol g-1 from plasma and aqueous cysteine, respectively. A t1/2 of 5480 hr was estimated if epithelial GSH had to be replaced exclusively by synthesis from aqueous cysteine. Given the limited aqueous and epithelial cysteine pools, and low (from cysteine) or undetectable (from cystine and methionine) incorporation of the label into GSH, we conclude that de novo GSH synthesis from circulating and aqueous SAA can be only a minor source of the millimolar concentration of GSH in the epithelium.
在灌注的豚鼠眼中研究了循环硫氨基酸（SAA）向晶状体上皮的转运以及从头合成谷胱甘肽（GSH）的过程。SAA从血浆到房水的转运形式为完整形式（≥98%），且在30分钟内与蔗糖（一种细胞外标志物）的转运情况相当。35S标记的胱氨酸、半胱氨酸和蛋氨酸向晶状体上皮的单向转运速率（毫升·分钟-1·克-1）分别为：从血浆转运时为0.0057、0.0003和0.0073，从房水转运时为1.41、0.005和1.69。对于所有三种[35S]SAA，上皮细胞的单向摄取在1分钟内达到极限，且在1至30分钟内达到同位素稳态比率。皮质摄取在1至30分钟内呈时间依赖性且逐渐增加，但在1分钟内检测不到。对上皮细胞进行的高效液相色谱（HPLC）分析显示，在单向[35S]半胱氨酸转运1分钟后，3%的标记物掺入GSH，≥95%为半胱氨酸。在30分钟内，[35S]半胱氨酸掺入上皮细胞和皮质GSH的平均速率分别为0.83%/分钟和1%/分钟。输注[35S]胱氨酸和蛋氨酸未能显示出掺入GSH的情况。从血浆和房水半胱氨酸开始，上皮细胞从头合成GSH的最大速率分别约为3和12皮摩尔·克-1。如果上皮细胞GSH必须完全由房水半胱氨酸合成来替代，则估计半衰期为5480小时。鉴于房水和上皮细胞半胱氨酸池有限，且标记物从半胱氨酸（低）或从胱氨酸和蛋氨酸（未检测到）掺入GSH的量很少，我们得出结论，从循环和房水SAA从头合成GSH只能是上皮细胞中毫摩尔浓度GSH的一个次要来源。
