Quantitative changes in the amyloid beta A4 precursor protein in Alzheimer cerebrospinal fluid.

The major three secretory isoforms of Alzheimer beta A4 amyloid precursor protein (APP) were quantified in cerebrospinal fluid (CSF) using (1) a newly developed enzyme-linked immunosorbent assay (ELISA) and (2) densitometric analysis of CSF Western blots. The protease inhibitor-containing APP751/770 isoforms represented an average of 10.5% of total APP in CSF of patients with Alzheimer's disease (AD, n = 22), multi-infarct dementia (MID, n = 5) and normal controls (n = 10). APP levels in CSF did not depend on total CSF protein. Both findings are inconsistent with a hematogeneous origin of APP in CSF and suggest an intracerebral source. Total APP, APP695 and APP751/770 were significantly decreased in the AD and in the MID groups, but were not correlated to the ages of patients or controls.