Comparative studies for amyloid beta degradation: “Neprilysin vs insulysin”, “monomeric vs aggregate”, and “whole Aβ(40) vs its peptide fragments”

Amyloid beta (Aβ) proteins are produced from amyloid precursor protein cleaved by β- and γ-secretases, and are the main components of senile plaques pathologically found in Alzheimer's disease (AD) patient brains. Therefore, the relationship between AD and Aβs has been well studied for both therapeutic and diagnostic purposes. Several enzymes have been reported to degrade Aβs in vivo, with neprilysin (NEP) and insulysin (insulin-degrading enzyme, IDE) being the most prominent. In this article, we describe the mass spectrometric characterization of peptide fragments generated using NEP and IDE, and clarify the differences in digestion specificities between these two enzymes for non-aggregated Aβ(40), aggregated Aβ(40), and Aβ(40) peptide fragments, including Aβ(16). Our results allowed identification of all the peptide fragments from non-aggregated Aβ(40): NEP, 23 peptide fragments consisting of 2–11 amino-acid residues, 17 cleavage sites; IDE, 23 peptide fragments consisting of 6–33 amino-acid residues, 15 cleavage sites. Also, we confirmed that IDE can digest only whole Aβ(40), whereas NEP can digest both Aβ(40) and partial structures such as Aβ(16) and peptide fragments generated by the digestion of Aβ(40) by IDE. Furthermore, we confirmed that IDE and NEP are unable to digest aggregated Aβ(40).