Paired helical ifiaments (PHFs) are prominent components of Alzheimer disease (AD) neurofibrillary tangles (NFTs). Rather than isolating NFTs, we selected for PHF populations that can be extracted from AD brain homogenates. About 50% of PHF immunoreactivity can be obtained in 27,200 x g supernatants following homogenization in buffers containing 0.8 M NaCI. We further enriched for PHFs by taking advantage of their insolubility in the presence of zwitterionic detergents and 2-mercaptoethanol, removal of aggregates by ifitration through 0.45-pum filters, and sucrose density centrifugation. PHF-enriched fractions contained two to five proteins of 57-68 kDa that displayed the same antigenic properties as PHFs. Since the 57-to 68-kDa PHF proteins are antigenically related to T proteins, they are similar to the X proteins previously observed in NFTs. However, further analysis revealed that PHF-associated X can be distinguished from normal, soluble X by PHF antibodies that do not recognize human adult 7 and by one-and two-dimensional PAGE.

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