Lipoprotein-free amyloidogenic peptides in plasma are elevated in patientswith sporadic Alzheimer's disease and Down's syndrome

About 30% of the soluble amyloid beta (sA beta) that circulates in normal h uman plasma is associated with lipoprotein particles. In sporadic Alzheimer 's disease patients, free sA beta 42 but not sA beta 40 is increased approx imately 2.3-fold compared with age-matched controls, although a more marked elevation (approximately 8-fold for free sA beta 40 and about 20-fold for sA beta 42) is found in Down's syndrome patients. The data suggest that lip oprotein-sA beta dissociation may contribute to the influx of sA beta into the brain as a result of decreased plasma clearance.