Deposition of beta-amyloid (Aβ) is considered an important early event in the pathogenesis of Alzheimer's disease (AD). Clearance of Aβ thus represents a potential therapeutic approach. Antibody-mediated clearance of Aβ by vaccination inhibited and cleared Aβ deposition in animal models; however, inflammatory side effects were observed in humans. An alternative potentially noninflammatory approach to facilitate clearance is to proteolytically cleave Aβ. We screened 12 proteolytic recombinant antibody fragments for potential α-secretase activity, a naturally occurring enzyme that cleaves between the Lys 16 and Leu 17 residues of the amyloid precursor protein (APP). We utilized the synthetic α-secretase substrate, benzyloxycarbonyl-L-lysine o-nitrophenyl ester (Z-lys-o-Np) as a preliminary screen for α-secretase activity. Two antibody light chain fragments that hydrolyzed Z-lys-o-Np were identified. Aβ hydrolysis was studied using mass spectrometry to identify the cleavage patterns of the antibodies. The recombinant antibody light chain antibody fragment, c23.5, showed α-secretase-like activity, producing the 1-16 and 17-40 amino acid fragments of Aβ. The second light chain antibody fragment, hk14, demonstrated carboxypeptidase-like activity, cleaving sequentially from the carboxyl terminal of Aβ. These antibody light chains provide a novel route toward engineering efficient therapeutic antibodies capable of cleaving Aβ in vivo.
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