AGGREGATION AND METAL-BINDING PROPERTIES OF MUTANT FORMS OF THE AMYLOID A-BETA PEPTIDE OF ALZHEIMERS-DISEASE

The fibrillogenic properties of Alzheimer's A beta peptides correspond ing to residues 1-40 of the normal human sequence and to two mutant fo rms containing the replacement Ala(21) to Gly or Glu(22) to Gin were c ompared. At pH 7.4 and 37 degrees C the Gln(22) peptide was found to a ggregate and precipitate from solution faster than the normal A beta, whereas the Gly(21) peptide aggregated much more slowly. Electron micr oscopy showed that the aggregates all had fibrillar structures. Circul ar dichroism spectra of these peptides revealed that aggregation of th e normal and Gln(22) sequences was associated with spectral changes co nsistent with a transformation from random coil to beta sheet, whereas the spectrum of the Gly(21) 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 a mounts of the radioisotope Zn-65(2+). Of several competing metal ions, tested at 20x the concentration of Zn2+, Cu2+ displaced >95% of the r adioactivity from all three peptides and Ni2+ produced >50% displaceme nt in each case. Some other metal ions tested caused lesser displaceme nt, but Fe2+ and Al3+ were without effect. In a saturation binding ass ay, a value of 3.2 mu m was obtained for the binding of Zn2+ to AP but our data provided no evidence for a reported higher affinity site (10 7 nM). The results suggest that the neuropathology associated with the Gly(21) mutation is not due to enhanced fibrillogenic or different me tal-binding properties of the peptide and that the binding of zinc to amyloid peptides is not a specific phenomenon.