Purified recombinant insulin-degrading enzyme degrades amyloid β-protein but does not promote its oligomerization

Amyloid β-protein (Aβ) has been implicated as an early and essential factor in the pathogenesis of Alzheimer's disease. Although its cellular production has been studied extensively, little is known about Aβ clearance. Recently, insulin-degrading enzyme (IDE), a 110-kDa metalloendopeptidase, was found to degrade both endogenously secreted and synthetic Aβ peptides. Surprisingly, IDE-mediated proteolysis of [¹²⁵I]Aβ(1-40) in microglial cell-culture media was accompanied by the formation of ¹²⁵I-labelled peptides with higher apparent molecular masses, raising the possibility that the degradation products act as ‘seeds’ for Aβ oligomerization. To directly address the role of IDE in Aβ degradation and oligomerization, we investigated the action of purified recombinant wild-type and catalytically inactive IDEs. Our data demonstrate that (i) IDE alone is sufficient to cleave purified Aβ that is either unlabelled, iodinated or ³⁵S-labelled; (ii) the initial cleavage sites are His¹⁴–Gln¹⁵, Phe¹⁹–Phe²⁰ and Phe²⁰–Ala²¹; and (iii) incubation of IDE with [¹²⁵I]Aβ, but not with [³⁵S]-Aβ, leads to the formation of slower migrating species on gels. Since iodination labels N-terminal fragments of Aβ, and ³⁵S labels C-terminal products, we analysed unlabelled synthetic fragments of Aβ and determined that only the N-terminal fragments migrate with anomalously high molecular mass. These results indicate that IDE alone is sufficient to degrade Aβ at specific sites, and that its degradation products do not promote oligomerization of the intact Aβ peptide.