The catalytic domain of insulin-degrading enzyme forms a denaturant-resistant complex with amyloid β peptide -: Implications for Alzheimer disease pathogenesis

Insulin-degrading enzyme (IDE) is central to the turnover of insulin and degrades amyloid beta(A beta) in the mammalian brain. Biochemical and genetic data support the notion that IDE may play a role in late onset Alzheimer disease (AD), and recent studies suggest an association between AD and diabetes mellitus type 2. Here we show that a natively folded recombinant IDE was capable of forming a stable complex with A beta that resisted dissociation after treatment with strong denaturants. This interaction was also observed with rat brain IDE and detected in an SDS-soluble fraction from AD cortical tissue. A beta sequence 17-27, known to be crucial in amyloid assembly, was sufficient to form a stable complex with IDE. Monomeric as opposed to aggregated A beta was competent to associate irreversibly with IDE following a very slow kinetics (t(1/2) similar to 45 min). Partial denaturation of IDE as well as preincubation with a 10-fold molar excess of insulin prevented complex formation, suggesting that the irreversible interaction of A beta takes place with at least part of the substrate binding site of the protease. Limited proteolysis showed that A beta remained bound to a similar to 25-kDa N-terminal fragment of IDE in an SDS-resistant manner. Mass spectrometry after in gel digestion of the IDE.A beta complex showed that peptides derived from the region that includes the catalytic site of IDE were recovered with A beta. Taken together, these results are suggestive of an unprecedented mechanism of conformation-dependent substrate binding that may perturb A beta clearance, insulin turnover, and promote AD pathogenesis.