Abnormal tau proteins from Alzheimer's disease brains. Purification and amino acid analysis.

Abnormal tau proteins (PHF-tau) were isolated from Alzheimer's disease brains by treatment of paired helical filament enriched-fractions with perchloric acid and boiling of the acid precipitable fraction with beta-mercaptoethanol. These proteins were purified further by a second perchloric acid treatment. The purified PHF-tau proteins were soluble in buffers devoid of sodium dodecyl sulfate. However, they were similar to the abnormal tau extracted from paired helical filaments with sodium dodecyl sulfate, also named A68, in molecular mass (68, 64, and 60 kDa), isoelectric point (pI 5.5-6.5), reactivity with anti-tau antibodies, and in requirement for alkaline phosphatase treatment to bind the Tau-1 antibody. Compared to normal tau, the soluble PHF-tau contained 100% more glycine and 35% less lysine residue. The results suggest that besides phosphorylation other types of modification may be involved in differentiating PHF-tau from normal tau.