Cathepsins S, B and L with aminopeptidases display β-secretase activity associated with the pathogenesis of Alzheimer's disease

beta-site APP-cleaving enzyme (BACE1) cleaves the wild type (WT) beta-site very slowly (k(cat)/K-m: 46.6 M-1 s(-1)). Therefore we searched for additional beta-secretases and identified three cathepsins that split the WT beta-site much faster. Human cathepsin S cleaves the WT beta-site (k(cat)/K-m: 54 700 M-1 s(-1)) 1170-fold faster than BACE1 and cathepsins B and L are 440- and 74-fold faster than BACE1, respectively. These cathepsins split two bonds flanking the WT beta-site (K-MDA), where the K-M bond (85%) is cleaved more efficiently than the D-A bond (15%). Cleavage at the major K-M bond yields A beta (amyloid beta-peptide) extended by N-terminal Met that should be removed to generate A beta initiated by Asp1. The activity of cytosol and microsomal aminopeptidases on relevant peptides revealed rapid removal of N-terminal Met but not N-terminal Asp. Brain aminopeptidases showed similar specificity. Thus, aminopeptidases would convert A beta extended by Met into regular A beta (Asp1) found in amyloid plaques. Earlier studies indicate that A beta is likely produced in the endosome and lysosome system where cathepsins S, B and L are localized and cysteine cathepsin inhibitors reduce the level of A beta in cells and animals. Taken together, cathepsins S, B and L deserve further evaluation as therapeutic targets to develop disease modifying drugs to treat Alzheimer's disease.