Aβ amyloid fibrils possess a core structure highly resistant to hydrogen exchange

^*Graduate School of Medicine, University of Tennessee Medical Center, Knoxville, TN 37920; and ^†Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600

Edited by S. Walter Englander, University of Pennsylvania School of Medicine, Swarthmore, PA, and approved October 3, 2000 (received for review June 22, 2000)

Abstract

We describe here experiments designed to characterize the secondary structure of amyloid fibrils of the Alzheimer's amyloid plaque peptide Aβ, using hydrogen-deuterium exchange measurements evaluated by mass spectrometry. The results show that ≈50% of the amide protons of the polypeptide backbone of Aβ(1–40) resist exchange in aqueous, neutral pH buffer even after more than 1,000 h of incubation at room temperature. We attribute this extensive, strong protection to H-bonding by residues in core regions of β-sheet structure within the fibril. The backbone amide hydrogens exchange at variable rates, suggesting different degrees of protection within the fibril. These data suggest that it is unlikely that the entire Aβ sequence is involved in H-bonded secondary structure within the amyloid fibril. Future studies using the methods described here should reveal further details of Aβ fibril structure and assembly. These methods also should be amenable to studies of other amyloid fibrils and protein aggregates.

Footnotes

↵ ‡ Present address: Covance Laboratories, Inc., P.O. Box 7545, Madison, WI 53707-7545.
↵ § To whom reprint requests should be addressed. E-mail: rwetzel{at}mc.utmck.edu.
This paper was submitted directly (Track II) to the PNAS office.
Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.250288897.
Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.250288897
↵ ¶ In the MS of samples from the fibril exchange reaction (Fig. 3), we consistently observe a shoulder at a mass-to-charge ratio coinciding with that of the partially deuterated monomer (Fig. 1 d) and growing relatively larger at long incubation times. One possible source is monomeric (and hence unprotected) Aβ in equilibrium with the fibrils (30), which would exchange at the rate and to the extent of the monomer. Further work is required to fully characterize the source of this peak.
Abbreviation:

HX,

hydrogen deuterium exchange