VARIATIONS IN THE AMINO ACIDS SEQUENCE OF AMYLOID-β: AN OPPORTUNITY TO UNDERSTAND ALZHEIMER’S DISEASE

Abstract

The amino acids (AA) sequences of Amyloid-β (Aβ) peptides in human (hu) and rodent (ro) are identical except for three amino acids, which allows huAβ to bind heme tightly but not roAβ. Literature search points to that the AA sequence of roAβ (i.e., not heme-binding Aβ) is shared among species that do not develop Alzheimer Disease (AD) while the AA sequence of huAβ (i.e., heme-binding Aβ) is shared among species that develop AD with age. Our lab is following this lead; however, it is not clear which fraction of the cellular heme may bind with huAβ. Regulatory heme (RH) is a unique fraction of total cellular heme that is not committed to a specific protein. RH plays role in intermediary metabolism e.g., in microRNA processing or gene regulations. We investigated if RH could be the target for huAβ in situ. To accomplish this goal, we embarked on developing, for the first time, an assay to specifically measure intracellular RH. We found that the cellular concentration of RH is 430±150nM (about 6% of total heme). Furthermore, the dissociation constant (Kd) of heme from huAβ is 140nM, which in conjunction with 430nM intracellular RH, suggest that huAβ and RH could tightly complex in situ. Subsequently, we generated evidence that huAβ binds and depletes intracellular RH. Despite that roAβ and huAβ equally form aggregates, only huAβ binds RH (roAβ's Kd=1000 nM). The hydrophobic segment that causes Aβ to aggregate is identical in both huAβ and roAβ, while the hydrophilic segment (which binds RH) differs by three amino acids (which bind RH in huAβ). Furthermore, huAβ-heme complex catalyzes the oxidation of organic molecules using H2O2. The abnormal intermediary metabolism in RH-depleted cells resembles many of the abnormalities of the brain of AD patients. These observations suggest that huAβ's complex with heme may contribute to huAβ neurotoxicity.