Abstract: A proteolytic factor has been isolated from the brains of humans with Alzheimer's disease which specifically cleaves the peptide having the sequence His-Ser-Glu-Val-Lys-Met-Asp-Ala-Glu-Phe (SEQ ID NO: 1) between Met and Asp. This peptide is in the cleavage site of the N-terminal domain of the amyloid .beta.-protein precursor.

Abstract: A proteolytic factor is capable of cleaving .beta.-protein precursor at a site near the .beta.-protein N-terminus. Also, a method for treating Alzheimer's disease in a patient includes steps of reducing .beta.-protein precursor proteolysis outside the .beta.-protein domain at a site near the .beta.-protein N-terminus. Also, a method for purifying an enzyme from a sample includes steps of incubating the sample with a labelled substrate of the enzyme or with a labelled fragment of a substrate to which the enzyme binds, treating the sample with a crosslinking agent to crosslink any enzyme-substrate complexes in the sample, and recovering labelled complexes. Also, a method for diagnosis in a subject of a disease characterized by accumulation of amyloid includes determining the level, in a sample of tissue or body fluid from the subject, of an AD proteolytic factor.