Regulation of serine protease activity by aluminum: implications for Alzheimer disease

The brain of Alzheimer disease patients contains plaques that are diagnostic for the disease. The plaques also contain beta-amyloid peptide, alpha 1-antichymotrypsin, and the element aluminum. We present indirect evidence that can relate all three components of plaques to each other in such a way as to suggest their involvement in the etiology of the disease. The beta-amyloid peptide is derived by proteolytic processing from beta-amyloid precursor proteins and some of these proteins contain a domain that is highly homologous to bovine pancreatic trypsin inhibitor. Bovine pancreatic trypsin inhibitor also inhibits alpha-chymotrypsin and we show that aluminum affects both the activity and the inhibition of this enzyme. At pH 6.5, in the presence of aluminum, the enzyme activity is doubled, and the inhibitor is only 1% as effective as in the absence of the metal ion. The inhibition by BX-9, a protease inhibitor prepared from protein components of amyloid plaques, is also reduced by aluminum; so too is that by alpha 1-antichymotrypsin but to a lesser degree. In the Alzheimer brain, we propose that aluminum may accelerate proteolytic processing of the beta-amyloid precursor protein by suppression of the inhibitor domain. Thus, the beta-amyloid peptide may accumulate and initiate plaque formation.