Protection against amyloid beta peptide and iron/hydrogen peroxide toxicity by alpha lipoic acid

Current evidence supports the role of oxidative stress in the pathogenesis of neuron degeneration in Alzheimer's disease (AD). α-Lipoic acid (LA), an essential cofactor in mitochondrial dehydrogenase reactions, functions as an antioxidant and reduces oxidative stress in aged animals. Here, we describe the effects of LA and its reduced form, dihydrolipoic acid (DHLA), in neuron cultures treated with amyloid β-peptide (Aβ 25-35) and iron/hydrogen peroxide (Fe/H₂O₂). Pretreatment of dissociated primary hippocampal cultures with LA significantly protected against Aβ and Fe/H₂O₂ toxicity. In contrast, concomitant treatment of cultures with LA and Fe/H₂O₂ significantly potentiated the toxicity. Decreased cell survival in cultures treated concomitantly with LA and Fe/H₂O₂ correlated with increased free radical production measured by dichlorofluorescein fluorescence. Treatment of cortical neurons with DHLA significantly protected glucose-transport against Fe/H ₂O₂ or Aβ-mediated decreases although treatment with LA did not provide protection. These data suggest that DHLA, the reduced form of LA, significantly protects against both Aβ and Fe/H₂O ₂ mediated toxicity. The data also suggest that concomitant exposure to LA and Fe/H₂O₂ significantly potentiates the oxidative stress. Overall, these data suggest that the oxidation state of LA is critical to its function and that in the absence of studies of LA/DHLA equilibria in human brain the use of LA as an antioxidant in disorders where there is increased Fe such as AD is of questionable efficacy.