The beta amyloid (A beta), the major protein component of brain senile plaques in Alzheimer's disease, is known to be directly responsible for the production of free radicals toxic to brain tissue and the redox state of Met-35 residue seems to play a particular and critical role in peptide's neurotoxic actions. In this study, we investigated, in human neuroblastoma cells (IMR-32), the relationship between the oxidative state of methionine, and both neurotoxic and pro-apoptotic actions induced by A beta-peptide, comparing the effects of native peptide, in which the Met-35 is present in the reduced state, with those of a modified peptide with oxidized Met-35 (A beta(1-42)(35Met-ox)), as well as an A beta-derivative with Met-35 substituted with norleucine (A beta(1-42)(35NIe)). The obtained results show that A beta induces a time-dependent decrease in cell viability; A beta(1-42)(35Met-ox) was significantly less potent, though inducing a remarkable decrease in cell viability compared to control. On the contrary, no toxic effects were observed after treatment with A beta(1-42)(35NIe). A beta-peptide as well as the amyloid modified peptide with oxidized Met-35 induced the pro-apoptotic gene bax over-expression after 24 h, whereas A beta(1-42)(35NIe) had no effect. Conversely, bcl-2, an anti-apoptotic gene, became highly down-regulated by A beta peptide treatment, in contrast to that evidenced by the A beta(1-42)(35Met-ox) peptide. Finally, A beta caused an increase in caspase-3 activity to be higher with respect to that shown by A beta(1-42)(35Met-ox) while A beta(1-42)(35NIe) had no effect. These results support the hypothesis that A beta-induced neurotoxicity occurs via bax overexpression, bcl-2 down-regulation, and caspase-3 activation, first indicating that methionine 35 redox state may alter this cell death pathway. (c) 2006 Elsevier Inc. All rights reserved.