Secretory form of Alzheimer amyloid precursor protein 695 in human brain lacks beta/A4 amyloid immunoreactivity

It is not clear how Alzheimer amyloid precursor proteins (APP) are metabolized in the brain itself. Secretory forms of APP in a phosphate buffer-soluble fraction were purified from post-mortem human brain by heparin-affinity and ion-exchange chromatography and analyzed by N-terminal amino acid sequencing and SDS polyacrylamide gel electrophoresis/immunoblotting. We found apparently similar multi-isoforms of secretory APP (at 93-97, 105-112 and 123 KDa) to those that we have described recently in cerebrospinal fluid. Antisera to the initial part of the beta/A4 sequence labelled only those bands that were found to react with antiserum to the Kunitz-type inhibitor insert of APP, suggesting that beta/A4 amyloid may be generated specifically from APP-695.