Diagram of the proposed mechanism of protection by astrocytes in preventing cysteine toxicity catalyzed by copper. Diagram of the proposed mechanism of protection by astrocytes in preventing cysteine toxicity catalyzed by copper. Astrocytes release glutathione and indirectly produce cysteine in the extracellular fluid of the CNS. Cysteine, as well as glutathione or other thiols, will be oxidized to disulfide under the catalysis of protein-unbound or loosely bound copper. Molecular oxygen, as the oxidant, accepts electrons step by step to produce superoxide radicals, hydrogen peroxide, and hydroxyl radicals. The latter is the major damaging free radical to the cells. In parallel, astrocytes also release pyruvate, which can react with hydrogen peroxide, preventing the formation of hydroxyl radicals. Xue Feng Wang, and Max S. Cynader J. Neurosci. 2001;21:3322-3331 ©2001 by Society for Neuroscience