Prudova Anna, Bauman Zachary, Braun Aaron, Vitvitsky Victor, Lu Shelly C, Banerjee Ruma
Redox Biology Center and Biochemistry Department, University of Nebraska, Lincoln, NE 68588-0664, USA.
Proc Natl Acad Sci U S A. 2006 Apr 25;103(17):6489-94. doi: 10.1073/pnas.0509531103. Epub 2006 Apr 13.
The transsulfuration pathway converts homocysteine to cysteine and represents the metabolic link between antioxidant and methylation metabolism. The first and committing step in this pathway is catalyzed by cystathionine beta-synthase (CBS), which is subject to complex regulation, including allosteric activation by the methyl donor, S-adenosylmethionine (AdoMet). In this study, we demonstrate that methionine restriction leads to a >10-fold decrease in CBS protein levels, and pulse proteolysis studies reveal that binding of AdoMet stabilizes the protein against degradation by approximately 12 kcal/mol. These observations predict that under pathological conditions where AdoMet levels are diminished, CBS, and therefore glutathione levels, will be reduced. Indeed, we demonstrate this to be the case in a mouse model for spontaneous steatohepatitis in which the gene for the MAT1A isoenzyme encoding AdoMet synthetase has been disrupted, and in human hepatocellular carcinoma, where MAT1A is silenced. Furthermore, diminished CBS levels are associated with reduced cell viability in hepatoma cells challenged with tert-butyl hydroperoxide. This study uncovers a mechanism by which CBS is allosterically activated by AdoMet under normal conditions but is destabilized under pathological conditions, for redirecting the metabolic flux toward methionine conservation. A mechanistic basis for the coordinate changes in redox and methylation metabolism that are a hallmark of several complex diseases is explained by these observations.
转硫途径将同型半胱氨酸转化为半胱氨酸，代表了抗氧化和甲基化代谢之间的代谢联系。该途径的第一步也是关键步骤由胱硫醚β-合酶（CBS）催化，其受到复杂的调控，包括甲基供体S-腺苷甲硫氨酸（AdoMet）的变构激活。在本研究中，我们证明蛋氨酸限制导致CBS蛋白水平下降超过10倍，脉冲蛋白水解研究表明，AdoMet的结合使该蛋白稳定，抵抗降解的能力提高约12千卡/摩尔。这些观察结果预测，在AdoMet水平降低的病理条件下，CBS以及因此谷胱甘肽水平将会降低。事实上，我们在自发脂肪性肝炎小鼠模型中证实了这一点，该模型中编码AdoMet合成酶的MAT1A同工酶基因已被破坏，在人类肝细胞癌中也是如此，其中MAT1A沉默。此外，CBS水平降低与用叔丁基过氧化氢攻击的肝癌细胞中细胞活力降低有关。本研究揭示了一种机制，在正常条件下CBS被AdoMet变构激活，但在病理条件下不稳定，从而将代谢通量重定向至蛋氨酸守恒。这些观察结果解释了氧化还原和甲基化代谢协同变化的机制基础，这是几种复杂疾病的标志。
