Repair of peroxynitrite damage to tubulin by the thioredoxin reductase system

Cumulative oxidative damage to proteins coupled with a decrease in repair has been implicated in the pathology of several neurodegenerative diseases. Herein we report that peroxynitrite-induced disulfides in porcine brain tubulin are repaired by the thioredoxin reductase system composed of rat liver thioredoxin reductase, human or Escherichia coli thioredoxin, and NADPH. Disulfide bonds between the α-tubulin and the β-tubulin subunits were repaired by thioredoxin reductase as determined by Western blot under nonreducing conditions. Total disulfide repair by thioredoxin reductase was assessed using a sulfhydryl-specific labeling reagent, 5-iodoacetamido-fluorescein. Treatment of tubulin with 1.0 mM peroxynitrite anion decreased 5-iodoacetamido-fluorescein labeling by 48%; repair of peroxynitrite-damaged tubulin with thioredoxin reductase restored sulfhydryl labeling to control levels. Tubulin disulfide reduction by thioredoxin reductase restored tubulin polymerization activity that was lost after peroxynitrite was added. The extent of activity restored by thioredoxin reductase and by the nonspecific disulfide-reducing agent tris(2-carboxyethyl)phosphine hydrochloride was identical; however, activity was not restored to control levels. Tyrosine nitration of tubulin was detected at all concentrations of peroxynitrite tested; thus, tubulin nitration may be responsible for the fraction of activity that could not be restored. Thiol-disulfide exchange between tubulin and thioredoxin was detected by Western blot, thereby providing further support for our observations that optimal repair of tubulin disulfides required thioredoxin.