ABSTRACT
While the etiology of Alzheimer's disease (AD) is still unknown, an increased formation of amyloid-ß (Aß) peptide and oxidative processes are major pathological mechanism of the disease. The interaction of Aß with free heme leads to the formation of peroxidase-active Aß-heme complexes. However, enzyme-kinetic data and systematic mutational studies are still missing. These aspects were addressed in this study to evaluate the role of Aß-heme complexes in AD. The enzyme-kinetic measurements showed peroxidase-specific pH- and H2O2-dependencies. In addition, the enzymatic activity of Aß-heme complexes constantly increased at higher peptide excess. Moreover, the role of the Aß sequence for the named enzymatic activity was tested, depicting human-specific R5, Y10, and H13 as essential amino acids. Also by studying Y10 as an endogenous peroxidase substrate for Aß-heme complexes, ratio-specific effects were observed, showing an optimal dityrosine formation at an about 40-fold peptide excess. As dityrosine formation promotes Aß fibrillation while free heme disturbs protein aggregation, we also investigated the effect of Aß-heme complex-derived peroxidase activity on the formation of Aß fibrils. The fluorescence measurements showed a different fibrillation behavior at strong peroxidase activity, leading also to altered fibril morphologies. The latter was detected by electron microscopy. As illustrated by selected in vivo measurements on a mouse model of AD, the disease is also characterized by Aß-derived microvessel destructions and hemolytic processes. Thus, thrombo-hemorrhagic events are discussed as a source for free heme in brain tissue. In summary, we suggest the formation and enzymatic activity of Aß-heme complexes as pathological key features of AD.
