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

Amyloid beta -protein (A beta) has been implicated as an early and essentia l factor in the pathogenesis of Alzheimer's disease. Although its cellular production has been studied extensively, little is known about A beta clear ance. Recently, insulin-degrading enzyme (IDE), a 110-kDa metalloendopeptid ase, was found to degrade both endogenously secreted and synthetic A beta p eptides. Surprisingly, IDE-mediated proteolysis of [I-125]A beta>(*) over b ar * (1-40) in microglial cell-culture media was accompanied by the formati on of I-125-labelled peptides with higher apparent molecular masses, raisin g the possibility that the degradation products act as 'seeds' for A beta o ligomerization. To directly address the role of IDE in AP degradation and o ligomerization, we investigated the action of purified recombinant wild-typ e and catalytically inactive IDEs. Our data demonstrate that (i) IDE alone is sufficient to cleave purified A beta that is either unlabelled, iodinate d or S-35- Labelled; (ii) the initial cleavage sites are His(14)-Gln(15), P he(19)-Phe(20) and Phe(20)-Ala(21); and (iii) incubation of IDE with [125I] AB, but not with [S-35]-A beta, leads to the formation of slower migrating species on gels. Since iodination labels N-terminal fragments of A beta, an d 35S labels C-terminal products, we analysed unlabelled synthetic fragment s of A beta and determined that only the N-terminal fragments migrate with anomalously high molecular mass. These results indicate that IDE alone is s ufficient to degrade A beta at specific sites, and that its degradation pro ducts do not promote oligomerization of the intact A beta peptide.