Tyrosine modifications of insulin-degrading enzyme enable favorable control of substrate specificity for both Alzheimer’s disease and type-2 diabetes mellitus

Insulin-degrading enzyme (IDE) cleaves amyloid beta (Aβ), insulin, and other bioactive peptides. Because Aβ and insulin are closely related to Alzheimer's disease (AD) and type-2 diabetes mellitus (T2DM), respectively, IDE is a candidate drug target for treating both AD and T2DM. However, the activity of IDE has opposing effects, including decreasing AD risk by degrading Aβ and increasing T2DM risk by degrading insulin. The opposed substrate specificity is associated with the exo- and active sites containing Tyr³¹⁴ and Tyr⁸³¹ residues, the plausible modification targets for controlling substrate specificity. In this study, we used a tyrosine-specific modification regent, Cookson reagent (4-phenyl-1,2,4-triazoline-3,5-dione, PTAD), for IDE and examined the degradation activities on Aβ₄₀ and insulin. Fifteen tyrosine residues, including Tyr³¹⁴ and Tyr⁸³¹, were modified by PTAD. After incubation with PTAD-modified IDE for 3 days, insulin remained intact, whereas Aβ₄₀ was completely degraded. This favorable change of substrate specificity was also observed in the mixture of Aβ₄₀ and insulin, suggesting that tyrosine modification of IDE might be a therapeutic strategy for AD and T2DM.