Fukagawa N K, Ajami A M, Young V R
Clinical Research Center, Rockefeller University, New York, New York 10021, USA.
Am J Physiol. 1996 Feb;270(2 Pt 1):E209-14. doi: 10.1152/ajpendo.1996.270.2.E209.
Glutathione (GSH), a tripeptide (gamma-glutamyl-cysteinyl-glycine), is thought to be both a storage and a transport form of cysteine (Cys). In a previous study (T. Hiramatsu, N.K. Fukagawa, J.S. Marchini, J. Cortiella, Y.-M. Yu, T.E. Chapman, and V.R. Young. Am. J. Clin. Nutr. 60: 525-533, 1994), the direct tracer-derived estimate of Cys flux was considerably higher than that predicted from estimates of protein turnover. To further examine the components of plasma Cys flux, seven normal-weight healthy adult men and women (26 +/- 2 yr) received stable isotope tracer infusions of L-[methyl-2H3;1-13C]methionine, L-[3,3-2H2]cysteine, and L-[methyl-2H3]leucine for 460 min. After a 3-h baseline period, GSH was administered at approximately 32 mumol.kg-1.h-1 until the end of the study. Expired breath and blood samples were obtained at timed intervals and analyzed for isotope enrichment using mass spectrometry. Leucine, alpha-ketoisocaproate, and methionine (carboxyl carbon, methyl moiety, remethylation, and transsulfuration) turnover were reduced during GSH administration (P < 0.01). In the final hour of GSH administration, Cys flux increased by 61% from 55.1 +/- 1.7 to 88.7 +/- 5.2 mumol.kg-1.h-1 (P < 0.01), which was essentially equivalent to the rate of exogenous GSH infusion. These data suggest that GSH breakdown accounts for approximately 50% of tracer-derived Cys flux basally and for all of the increase in measured Cys turnover during exogenous GSH infusion.
谷胱甘肽（GSH）是一种三肽（γ-谷氨酰-半胱氨酰-甘氨酸），被认为是半胱氨酸（Cys）的一种储存和运输形式。在之前的一项研究中（T. Hiramatsu、N.K. Fukagawa、J.S. Marchini、J. Cortiella、Y.-M. Yu、T.E. Chapman和V.R. Young。《美国临床营养学杂志》60: 525 - 533, 1994），通过示踪剂直接得出的Cys通量估计值远高于根据蛋白质周转率估计值所预测的结果。为了进一步研究血浆Cys通量的组成部分，7名体重正常的健康成年男性和女性（26±2岁）接受了L-[甲基-2H3;1-13C]蛋氨酸、L-[3,3-2H2]半胱氨酸和L-[甲基-2H3]亮氨酸的稳定同位素示踪剂输注，持续460分钟。在3小时的基线期后，以约32 μmol·kg-1·h-1的剂量给予GSH，直至研究结束。定时采集呼出气体和血液样本，并用质谱法分析同位素富集情况。在给予GSH期间，亮氨酸、α-酮异己酸和蛋氨酸（羧基碳、甲基部分、再甲基化和转硫作用）的周转率降低（P < 0.01）。在给予GSH的最后一小时，Cys通量从55.1±1.7增加到88.7±5.2 μmol·kg-1·h-1，增幅为61%（P < 0.01），这基本上等同于外源性GSH的输注速率。这些数据表明GSH分解在基础状态下约占示踪剂衍生Cys通量的50%，并且在外源性GSH输注期间所测量的Cys周转率的全部增加均由其导致。
