Aggregation and metal-binding properties of mutant forms of the amyloid Aβ peptide of Alzheimer's disease

The fibrillogenic properties of Alzheimer's A beta peptides corresponding to residues 1-40 of the normal human sequence and to two mutant forms containing the replacement Ala21 to Gly or Glu22 to Gln were compared. At pH 7.4 and 37 degrees C the Gln22 peptide was found to aggregate and precipitate from solution faster than the normal A beta, whereas the Gly21 peptide aggregated much more slowly. Electron microscopy showed that the aggregates all had fibrillar structures. Circular dichroism spectra of these peptides revealed that aggregation of the normal and Gln22 sequences was associated with spectral changes consistent with a transformation from random coil to beta sheet, whereas the spectrum of the Gly21 peptide remained almost unchanged during a period in which little or no aggregation occurred. When immobilised by spotting onto nitrocellulose membranes the peptides bound similar amounts of the radioisotope 65Zn2+. Of several competing metal ions, tested at 20x the concentration of Zn2+, Cu2+ displaced > 95% of the radioactivity from all three peptides and Ni2+ produced >50% displacement in each case. Some other metal ions tested caused lesser displacement, but Fe2+ and Al3+ were without effect. In a saturation binding assay, a value of 3.2 microM was obtained for the binding of Zn2+ to A beta but our data provided no evidence for a reported higher affinity site (107 nM). The results suggest that the neuropathology associated with the Gly21 mutation is not due to enhanced fibrillogenic or different metal-binding properties of the peptide and that the binding of zinc to amyloid peptides is not a specific phenomenon