Panayiotidis Mihalis I, Stabler Sally P, Allen Robert H, Pappa Aglaia, White Carl W
School of Community Health Sciences, University of Nevada, Reno, NV 89557, USA.
Mutat Res. 2009 Mar 31;674(1-2):23-30. doi: 10.1016/j.mrgentox.2008.10.006. Epub 2008 Oct 25.
The effects of low, moderate and severe oxidative stress on the steady-state levels of the metabolites involved in the transmethylation/transsulfuration pathway were studied in lung epithelial (A549) cells. When cells were exposed to low (0.1 mM) or moderate (1.0 mM) concentrations of hydrogen peroxide (H(2)O(2)) or tert-butylhydroperoxide (t-butOOH), intracellular levels of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) were significantly decreased, while the SAM/SAH ratio remained the same or elevated. Likewise, extracellular levels of SAM and SAH metabolites remained steady or elevated. Both intracellular and extracellular levels of homocyst(e)ine and cyst(e)ine were decreased. Cell contents of serine, cystathionine and methionine were also decreased. Total intracellular glutathione content was decreased only by moderate t-butOOH exposure. When cells were exposed to high concentrations (10mM) of either of the peroxides, extracellular levels of methionine, cystathionine, and total cyst(e)ine were depleted, mostly due to direct oxidation of sulfur amino acids by peroxides, as indicated by oxidative treatment of culture media alone. Similar to low and moderate oxidative conditions, the levels of SAM, SAH, and sulfur amino acids were decreased, while cell SAM/SAH ratio increased. Paradoxically, under high peroxide exposure, extracellular concentrations of SAM, SAH, and cyst(e)ine were increased, indicating cellular release, despite the severe methionine depletion. Intracellular total glutathione was also decreased. The results indicate that lung epithelial cells release high levels of SAM, probably as an adaptive response to increased oxidative stress, even when the substrate for SAM formation, methionine, is critically depleted.
研究了低、中、重度氧化应激对肺上皮（A549）细胞中参与转甲基化/转硫途径的代谢物稳态水平的影响。当细胞暴露于低浓度（0.1 mM）或中等浓度（1.0 mM）的过氧化氢（H₂O₂）或叔丁基过氧化氢（t-butOOH）时，S-腺苷甲硫氨酸（SAM）和S-腺苷同型半胱氨酸（SAH）的细胞内水平显著降低，而SAM/SAH比值保持不变或升高。同样，SAM和SAH代谢物的细胞外水平保持稳定或升高。同型半胱氨酸和胱氨酸的细胞内和细胞外水平均降低。丝氨酸、胱硫醚和甲硫氨酸的细胞含量也降低。仅在中等浓度的t-butOOH暴露下，细胞内总谷胱甘肽含量才降低。当细胞暴露于高浓度（10 mM）的任何一种过氧化物时，甲硫氨酸、胱硫醚和总胱氨酸的细胞外水平会耗尽，这主要是由于过氧化物对含硫氨基酸的直接氧化，仅对培养基进行氧化处理就表明了这一点。与低和中等氧化条件类似，SAM、SAH和含硫氨基酸的水平降低，而细胞SAM/SAH比值增加。矛盾的是，在高浓度过氧化物暴露下，尽管甲硫氨酸严重耗尽，但SAM、SAH和胱氨酸的细胞外浓度却增加，表明细胞发生了释放。细胞内总谷胱甘肽也降低。结果表明，即使SAM形成的底物甲硫氨酸严重耗尽，肺上皮细胞仍会释放高水平的SAM，这可能是对氧化应激增加的一种适应性反应。
