Characterization of endogenous APP processing in a cell-free system

We have developed a simple in vitro assay using tissue homogenates that allows detection and characterization of several endogenous proteolytic activities which convert Alzheimer's amyloid precursor protein (APP) to the smaller, carboxy-terminal fragments, postulated to be intermediates in the formation of β-amyloid peptide (Aβ). Incubation at 37°C results in the degradation of transmembrane APP and formation of a mixture of carboxy-terminal containing peptides with mass values of 9-12 kDa. Epitope mapping and electrophoretic comparison with a truncated APP standard showed one of these peptides to contain the entire Aβ sequence. Analysis of pH dependence shows that formation of this carboxy-terminal product as well as another fragment, that is the likely product of 'secretase' activity, requires acidic pH. This suggests that cleavage of full-length APP to secreted forms may take place in an acidic intracellular compartment.