Characterization of the glycolipid associated with Alzheimer paired helical filaments.

Abstract

In the present study, analytical techniques including gas chromatography/mass spectrometry (GC/MS)-assisted carbohydrate linkage-analysis, one- and two-dimensional NMR, and matrix-assisted laser desorption/ionization time of flight mass spectroscopy (MALDI-MS) have been used to characterize the structure of the glycolipid associated with the paired helical filaments (PHF) isolated from the neurofibrillary tangles of Alzheimer's diseased brain. The 1H NMR spectrum of acid-hydrolyzed protein-resistant core PHF (prcPHF) displays resonances that can be assigned to fatty acid and glucose. There are no resonances present that would indicate the presence of protein, amino acids, or a sphingosine base. Using two-dimensional homonuclear correlated spectroscopy, homonuclear Hartmann-Hahn, and heteronuclear multiple quantum coherence experiments, resonances in the 1H and 13C NMR spectrum of native PHF were assigned to a nonreducing terminal alpha-1,6-glycosidically linked glucose, an internal alpha-1,6-linked glucose, and an alpha-1,2,6-linked glucose. The narrow line-widths observed for these residues suggest that they arise from glucose residues undergoing rapid segmental motion. The carbohydrate portion of the PHF-associated glycolipid was analyzed using GC/MS linkage analysis and confirmed the presence of terminal and internal alpha-1,6-linked glucose and alpha-1,2,6-linked glucose in a molar ratio of 2:1:1. Three components of the PHF-associated glycolipid fraction having masses 2,416, 2,325, and 2,237 Da were observed using MALDI-MS. The least abundant, heavier mass component (2,416 Da) was best fit to a structure with a tridecamer of glucose having a single esterified C20 fatty acid (Glc13 + C20 or Glc13 + C20:1), whereas the more abundant, lower mass components were best fit to noncovalently associated glycolipid dimers, each with a glucose pentamer or hexamer having two C14, C16, or C18 esterified fatty acids {D[(Glc5 + C18) + (Glc6 + C16)] or D[(Glc5 + C14) + (Glc6 + C14)]}. The ratio of glucose to fatty acid calculated from these best-fit structures of the more abundant mass components (5.5 +/- 1.1:1.0) is in reasonable agreement with the same ratio calculated from peak integrations in the NMR spectra of acid-hydrolyzed prcPHF (6.2 +/- 1.6). Structural similarities between PHF-associated glycolipid and other glycolipid amphiphiles known to form PHF-like filaments indirectly suggest that this unique glycolipid may be an integral component of the PHF suprastructure.