ENZYMATIC GENERATION OF THE AMINO-TERMINUS OF THE BETA-AMYLOID PEPTIDE

The major pathological change in Alzheimer's disease is the deposition of 39-42-amino acid beta-amyloid peptide (BAP) in the brain. Since BA P begins at the aspartate residue (Asp1, or codon 672 of the amyloid p recursor protein (APP)770 transcript), the ability of several protease s to cleave the peptide bond methionine-Asp1 (M/D) was evaluated by us ing peptides and recombinant APP molecules as substrates. Cathepsin G and chymotrypsin cleave the synthetic peptide HSEVKMDAEF at M/D under acidic conditions, whereas cleavage at lysine-methionine (K/M) predomi nates when the pH is alkaline. Trypsin and cathepsins B, D, and L are unable to cleave the synthetic peptide at M/D. Peptide SEVNLDAEF, repr esenting the mutation found in early onset Alzheimer's disease familie s from Sweden, is cleaved by cathepsin G and chymotrypsin at leucine-a spartate (L/D). Incubation of cathepsin G with soluble protease nexin- 2 obtained from recombinant APP (APP-REP) derivatives resulted in prot eolytic cleavage at or near the amino terminus of BAP. Cathepsin G-med iated cleavage was also observed in the domain representing the amino terminus of BAP when mature plasma membrane-associated APP-REP molecul es were used as substrates. Our results strongly suggest the involveme nt of a chymotrypsin-like serine protease in the generation of the ami no terminus of BAP beginning at Asp1.