The serine protease inhibitor alpha-1-antichymotrypsin (ACT) consistently colocalizes with amyloid deposits of Alzheimer's disease (AD) and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. AD amyloid fibrils are composed primarily of A-beta, which is a proteolytic fragment of the larger beta-amyloid precursor protein. Using negative-stain and immunochemical electron microscopy, we have investigated the binding of ACT to the fibrils formed by four synthetic A-beta analogues corresponding to the wild-type human 1-40 sequence (H-wt(1-40)), a 1-40 peptide (H-Du(1-40)) containing the Glu-22 fwdarw Gln mutation found in hereditary cerebral hemorrhage with amyloidosis of the Dutch type, the N-terminal 1-28 residues (beta(1-28)), and an internal fragment of A-beta containing residues 11-28 (beta(11-28)). Each of these peptide analogues assembled into 70-90- ANG -diameter fibrils resembling native amyloid and, except for beta(11-28), bound ACT, as indicated by the appearance of 80-100- ANG globular particles that adhered to preformed fibrils and that could be decorated with anti-ACT antibodies. Under the conditions used, ACT binding destabilized the in vitro fibrils and produced a gradual dissolution of the macromolecular assemblies into constituent filaments and shorter fragments. The internal fragment (11-28) did not exhibit ACT binding or any structural changes. These results suggest that a specific sequence likely contained within the N-terminal 10 residues of A-beta is responsible for the formation of the ACT-amyloid complex. Although the observed fibril disassembly is surprising in view of the notion that ACT contributes directly to the physical process involved in amyloid fibril formation, the induced structural changes may expose new domains in A-beta for additional proteolysis or for interactions with cell-surface receptors.
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