Elevated 4-Hydroxynonenal in Ventricular Fluid in Alzheimer’s Disease

beta-Amyloid is a 39- to 43-amino-acid neurotoxic peptide that aggregates to form the core of Alzheimer disease-associated senile (amyloid) plaques. No satisfactory hypothesis has yet been proposed to explain the mechanism of beta-amyloid aggregation and toxicity. We present mass spectrometric and electron paramagnetic resonance spin trapping evidence that beta-amyloid, in aqueous solution, fragments and generates free radical peptides. beta-Amyloid fragments, at concentrations that previously have been shown to be neurotoxic to cultured neurons, can inactivate oxidation-sensitive glutamine synthetase and creatine kinase enzymes. Also, salicylate hydroxylation assays indicate that reactive oxygen species are generated by the beta-amyloid-(25-35) fragment during cell-free incubation. These results are formulated into a free radical-based unifying hypothesis for neurotoxicity of beta-amyloid and are discussed with reference to membrane molecular alterations in Alzheimer disease.

Alzheimer disease (AD) is characterized by deposits of an aggregated 42-amino-acid beta-amyloid peptide (beta AP) in the brain and cerebrovasculature. After a concentration-dependent lag period during in vitro incubations, soluble preparations of synthetic beta AP slowly form fibrillar aggregates that resemble natural amyloid and are measurable by sedimentation and thioflavin T-based fluorescence. Aggregation of soluble beta AP in these in vitro assays is enhanced by addition of small amounts of pre-aggregated beta-amyloid "seed" material. We also have prepared these seeds by using a naturally occurring reaction between glucose and protein amino groups resulting in the formation of advanced "glycosylation" end products (AGEs) which chemically crosslink proteins. AGE-modified beta AP-nucleation seeds further accelerated aggregation of soluble beta AP compared to non-modified "seed" material. Over time, nonenzymatic advanced glycation also results in the gradual accumulation of a set of posttranslational covalent adducts on long-lived proteins in vivo. In a standardized competitive ELISA, plaque fractions of AD brains were found to contain about 3-fold more AGE adducts per mg of protein than preparations from healthy, age-matched controls. These results suggest that the in vivo half-life of beta-amyloid is prolonged in AD, resulting in greater accumulation of AGE modifications which in turn may act to promote accumulation of additional amyloid.