MASS AND PHYSICAL DIMENSIONS OF 2 DISTINCT POPULATIONS OF PAIRED HELICAL FILAMENTS

We studied the ultrastructure of two fractions of paired helical filam ents (PHF) from Alzheimer brains separated on sucrose density gradient . Fraction A2 (1M sucrose) contained filaments which were short in len gth and did not aggregate while those in fraction AL2 (1/1.5 M sucrose interface) were mostly aggregated. By scanning transmission electron microscopy, PHF in fraction A2 had significantly more mass per nm leng th of filament (107-120 kD/nm) than those in fraction AL2 (79-85 kD/nm ), and they were also wider in their maximum and minimum widths but di d not differ in their periodicity. Differences in mass and dimensions between two morphologically distinct populations of PHF suggest that a partial proteolysis may be involved in the generation of the aggregat ed population of PHF. The results suggest that a similar process may b e active in the formation of neurofibrillary tangles.