Degradation of soluble amyloid beta-peptides 1-40, 1-42, and the Dutch variant 1-40Q by insulin degrading enzyme from Alzheimer disease and control brains

Insulin degrading enzyme (IDE) is a metalloprotease that has been involved in amyloid beta peptide (A beta) degradation in the brain. We analyzed the ability of human brain soluble fraction to degrade A beta analogs 1-40, 1-4 2 and the Dutch variant 1-40Q at physiological concentrations (1 nM). The r ate of synthetic I-125-A beta degradation was similar among the A beta anal ogs, as demonstrated by trichloroacetic acid precipitation and SDS-PAGE. A 110 kDa protein, corresponding to the molecular mass of IDE, was affinity l abeled with either I-125-insulin, I-125-A beta 1-40 or I-125-A beta 1-42 an d both A beta degradation and cross-linking were specifically inhibited by an excess of each peptide. Sensitivity to inhibitors was consistent with th e reported inhibitor profile of IDE. Taken together, these results suggeste d that the degradation of A beta analogs was due to IDE or a closely relate d protease. The apparent Km, as determined using partially purified IDE fro m rat liver, were 2.2 +/- 0.4, 2.0 +/- 0.1 and 2.3 +/- 0.3 mu M for A beta 1-40, A beta 1-42 and A beta 1-40Q, respectively. Comparison of IDE activit y from seven AD brain cytosolic fractions and six age-matched controls reve aled a significant decrease in A beta degrading activity in the first group , supporting the hypothesis that a reduced IDE activity may contribute to A beta accumulation in the brain.