Received on January 12, 2006
Accepted on March 17, 2006

General Clinical Chemistry

Detection of a Soluble Form of BACE-1 in Human Cerebrospinal Fluid by a Sensitive Activity Assay

¹ TNO Quality of Life, Biomedical Research, Leiden, The Netherlands
² Novartis Institutes for BioMedical Research, Neuroscience Discovery, Novartis Pharma AG, Basel, Switzerland
³ Department of Clinical Chemistry, VU Medical Center, Amsterdam, The Netherlands
⁴ Department of Vascular Surgery, Leiden University Medical Center, Leiden, The Netherlands
⁵ Department of Neurology, Leiden University Medical Center, Leiden, The Netherlands

^* To whom correspondence should be addressed. E-mail: JH.Verheijen{at}pg.tno.nl.

Background: The formation of deposits of the insoluble amyloid -peptide is believed to be causally related with neurodegeneration in Alzheimer disease (AD). The -peptide originates from a larger amyloid precursor protein (APP) by the action of proteolytic enzymes. The first proteolytic event leading to amyloid formation is the cleavage of APP by the membrane-bound aspartyl protease BACE-1, also known as memapsin-2. Inhibition of BACE-1 is thought to be a therapeutic approach to AD. Measuring BACE-1 activity in biological samples would be useful to elucidate the mechanism of AD and for development of AD drugs.

Methods: We developed a sensitive and specific activity assay for BACE-1. The assay is based on a genetically engineered proenzyme that is specifically activated by BACE-1. The resulting active enzyme is measured with a chromogenic substrate. The use of 2 coupled reactions produces a detection limit as low as 0.4 pmol/L.

Results: The assay detected BACE-1 activity in extracts of human brain tissue as well as, unexpectedly, in human cerebrospinal fluid (CSF). Gel electrophoresis and Western blotting identified the BACE-1 present in CSF as a truncated soluble form of the originally membrane-bound BACE-1.

Conclusion: Detection of the soluble form of BACE-1 in CSF, a relatively easily accessible biological fluid, may be useful for monitoring the effects of drug candidates in vivo or may have diagnostic or prognostic applications.