Detection of d-aspartate in tau proteins associated with Alzheimer paired helical filaments

Paired helical filaments (PHF) characteristic of Alzheimer neurofibrillary lesions are known to contain a modified form of microtubule associated protein tau. These proteins, PHF-tau, differ from normal tau in the extent and the site of phosphorylation. To determine whether PHF-tau, tau proteins from normal adult brains (N-tau), tau proteins from Alzheimer brains not associated with PHF (A-tau), and tau proteins from fetal brains (F-tau) differ in racemization, these proteins were compared for their d-aspartate content. The results demonstrated that PHF-tau contain more d-aspartate than N-tau, A-tau and F-tau. The average percentage d-aspartate for these proteins, after a correction for background, are 4.9%, 2.8%, 1.6%, and 1% for PHF-tau, N-tau, A-tau and F-tau, respectively. It remains to be determined if the increase in d-aspartate is a consequence of PHF formation. It is also unknown if the change in d-aspartate content in PHF-tau is associated with phosphorylation, which alters the susceptibility of tau to proteolysis.