X-ray probe microanalysis of Alzheimer disease soluble and insoluble paired helical filaments

The paired helical filaments of Alzheimer disease, which have been shown to consist of both soluble and insoluble forms, were examined by X-ray probe microanalysis in order to determine if there existed differences in their elemental composition. The soluble paired helical filaments contained both sulfur and phosphorus, supporting their composition being enriched in a phosphorylated protein. The insoluble paired helical filament core structures, which retained their morphology after extensive protease digestion, contained only a small amount of sulfur over background, which suggests that they are not composed entirely of protein. This significant difference in sulfur and phosphorus indicates a difference in composition between the soluble and insoluble paired helical filaments, and that the paired helical filament core structures may attribute their insolubility to their being predominately non-proteinaceous.