Alterations in the redox status of proteins have been implicated in the pathology of several neurodegenerative diseases including Alzheimer's and Parkinson's. We report that peroxynitrite and H2O2-induced disulfides in the porcine brain microtubule-associated proteins tau and microtubule-associated protein-2 are substrates for the glutaredoxin reductase system composed of glutathione reductase, human or Escherichia coli glutaredoxin, reduced glutathione, and NADPH. Oxidation and reduction of cysteines in tau and microtubule-associated protein-2 were quantitated by monitoring the incorporation of 5-iodoacetamido-fluorescein, a thiol-specific labeling reagent. Reduction of disulfide bonds in the microtubule-associated proteins by the glutaredoxin reductase system restored their ability to promote the assembly of microtubules composed of purified porcine tubulin. Thiol-disulfide exchange between oxidized glutathione and the microtubule-associated proteins was detected by monitoring protein oxidation and was quantitated by measuring reduced glutathione by HPLC. (C) 2004 Elsevier Inc. All rights reserved.