Regenerable and Simultaneous Surface Plasmon Resonance Detection of Aβ(1-40) and Aβ(1-42) Peptides in Cerebrospinal Fluids with Signal Amplification by Streptavidin Conjugated to an N-Terminus-Specific Antibody

A major constituent in the deposit of the brain in a patient with Alzheimer's disease (AD) is the aggregates/fibrils of amyloid-beta (A beta) peptides containing 39-43 amino acids. The total A beta levels and the concentration ratio between the most abundant A beta(1-40) peptide and the more aggregation-prone A beta(1-42) in body fluids (e.g., cerebrospinal fluid or CSF) have been suggested as possible criteria for early diagnosis of AD. By immobilizing capture antibodies specific to the two peptides in separate fluidic channels, surface plasmon resonance (SPR) has been used to quantify A beta(1-40) and A beta(1-42) present in CSF samples collected from AD patients and healthy donors. With signal amplification by streptavidin conjugated to an antibody that is selective to the common N-terminus of the A beta peptides, concentrations as low as 20 pM can be readily measured. The range of A beta peptide concentrations measurable by this method spans 4 orders of magnitude. The ability of regenerating the sensor surface for repeated measurements not only improves the reproducibility but also enhances the sample throughput. Our data reveal that the ratio of A beta(1-40) concentration versus A beta(1-42) concentration in CSF samples from AD patients is almost twice as high as that from healthy persons. In contrast to the commonly used enzyme-linked immunosorbent assay (ELISA), SPR obviates the need of a more expensive and less stable enzyme conjugate and the use of carcinogenic substrate for the signal detection and allows the binding events to be monitored in real time.