ALPHA-1-ANTICHYMOTRYPSIN BINDING TO ALZHEIMER A-BETA PEPTIDES IS SEQUENCE-SPECIFIC AND INDUCES FIBRIL DISAGGREGATION INVITRO

The serine protease inhibitor alpha1-antichymotrypsin (ACT) consistent ly colocalizes with amyloid deposits of Alzheimer's disease (AD) and m ay 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 precu rsor protein. Using negative-stain and immunochemical electron microsc opy, we have investigated the binding of ACT to the fibrils formed by four synthetic Abeta analogues corresponding to the wild-type human 1- 40 sequence [H(wt)(1-40)], a 1-40 peptide [H(Du)(1-40)] containing the Glu22 --> Gln mutation found in hereditary cerebral hemorrhage with a myloidosis of the Dutch type, the N-terminal 1-28 residues [beta(1-28) ], and an internal fragment of Abeta containing residues 1;1-28 [beta( 11-28)]. Each of these peptide analogues assembled into 70-90-angstrom -diameter fibrils resembling native amyloid and, except for beta(11-28 ), bound ACT, as indicated by the appearance of 80-1 00-angstrom globu lar particles that adhered to preformed fibrils and that could be deco rated 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 shorte r fragments. The internal fragment (11-28) did not exhibit ACT binding or any structural changes. These results suggest that a specific sequ ence likely contained within the N-terminal 1 0 residues of Abeta is r esponsible for the formation of the ACT-amyloid complex. Although the observed fibril disassembly is surprising in view of the notion that A CT contributes directly to the physical process involved in amyloid fi bril formation, the induced structural changes may expose new domains in Abeta for additional proteolysis or for interactions with cell-surf ace receptors.