| Abstract |
Insoluble Abeta1-42 is the main component of the amyloid plaque. We have previously demonstrated that exposure to low pH can confer the molten globule state on soluble Abeta1-42 in vitro [Biochem. J. 361 (2000) 547] and unfolding experiments with guadinine hydrochloride (GdnHCl) have now confirmed this observation. The molten globule state of the protein has many biological properties and understanding the mechanisms of its formation is an important step in devising a therapeutic strategy for Alzheimer's disease (AD). We therefore investigated the ability of a series of synthetic eight-residue peptides derived from Abeta1-42 to inhibit the acid-induced aggregation of Abeta1-42 and identified the potent peptides to be Abeta15-22, Abeta16-23 and Abeta17-24. A1-antichymotrypsin, a member of the serine proteinase inhibitor (serpin) family is another major component of the amyloid plaque. In the present study, we investigated the proteolytic activity of Abeta1-42 against casein at different pHs. Chemical modification of amino acid residues in Abeta1-42 indicated that serine and histidine residues, but not aspartic acid, are necessary for enzymatic activity, suggesting that it is a serine proteinase. Amino acid substitution studies indicate that glutamic acids at positions 11 and 22 participate indirectly in proteolysis and we surmise that amino acid residues 29-42 are required to stabilize the conformer. A study of metal ions suggested that Cu2+ affected the enzymatic activity, but Zn2+ and Fe2+ did not. Interestingly, Abeta14-21 and Abeta15-22 were the only peptides that inhibited the proteolytic activity of Abeta42. Therefore, Abeta15-22 may control both aggregation of Abeta1-42 at acidic pH and its proteolytic activity at neutral pH. Consequently, we suggest that it may be of use in the therapy of Alzheimer's disease.
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