Processing of the pre- beta -amyloid protein by cathepsin D is enhanced by a familial Alzheimer's disease mutation

A major pre- beta -amyloid protein sub(695) (APP sub(695)) processing activity from Alzheimer's disease brain extracts was identified and found to be indistinguishable from the activity of cathepsin D.APP sub(695) processing activity cleaved APP sub(695) into a series of fragments that reacted on immunoblots to a monoclonal antibody (C286.8a) against beta -amyloid-(1-7)-peptide and cleaved N-dansyl-APP-(591-601)-amide at the Glu-Val and Met-Asp bonds. Fragments of 5.5 kDa and 10-12 kDa were formed from the cleavage of APP sub(695) by cathepsin D at the Glu593-Val594 bond, and had the same N-terminus as a minor form of beta -amyloid released by cells. The Lys595 arrow right Asn and Met596 arrow right Leu substitutions found in a pedigree of familial Alzheimer's disease, increased the cathepsin D-catalyzed rate of accumulation of 5.5 kDa and 10-12 kDa C286.8a-reactive fragments 5-10fold. This substitution also increased the rate of N-dansyl-APP-(591-601)-amide cleavage at the Xaa-Asp bond by up to 41-fold. These observations suggest a role of cathepsin D in beta -amyloid formation under certain circumstances.