Insulin degrading enzyme (IDE), a zinc metalloprotease, can specifically recognize and degrade insulin, as well as several amyloidogenic peptides such as amyloid P (AD) and amylin. The disruption of IDE function in rodents leads to glucose intolerance and cerebral A accumulation, hallmarks of type 2 diabetes and Alzheimer's disease, respectively. Using limited proteolysis, we found that human IDE (113 kDa) can be subdivided into two roughly equal sized domains, IDE-N and 1DE-C. Oligomerization plays a key role in the activity of IDE. Size-exclusion chromatography and sedimentation velocity experiments indicate that IDE-N is a monomer and IDE-C serves to oligomerize IDE-N. IDE-C alone does not have catalytic activity. It is IDE-N that contains the crucial catalytic residues, however IDE-N alone has only 2% of the catalytic activity of wild type I DE. By complexing I DE-C with IDE-N, the activity of IDE-N can be restored to similar to 30% that of wild type IDE. Fluorescence polarization assays using labeled insulin reveal that IDE-N has reduced affinity to insulin relative to wild type IDE. Together, our data reveal the modular nature of IDE. IDE-N is the catalytic domain and IDE-C facilitates substrate recognition as well as plays a key role in the oligomerization of IDE. (c) 2006 Elsevier Inc. All rights reserved.