Full‐Length and Truncated Alzheimer Amyloid Precursors in Chromaffin Granules: Solubilization of Membrane Amyloid Precursor Is Mediated by an Enzymatic Mechanism

: The amyloid β peptide (Aβ) of Alzheimer disease is derived from the proteolytic processing of the amyloid precursor proteins (APPs), which are considered type I transmembrane proteins. Here we report that the soluble fraction of isolated adrenal medullary chromaffin granules (CG), a model neuronal secretory vesicle system, contains an antigen that immunochemically and on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis was indistinguishable from full‐length APP. A truncated APP fragment with intact Aβ sequence was also detected in the soluble fraction of CG. In vitro experiments showed that full‐length APP was solubilized from CG membranes at 37°C as a function of pH, with a peak of activity between pH 8.5 and pH 9.0. Solubilization of full‐length APP was inhibited by several protease inhibitors, including aprotinin, cystatin, and iodoacetamide, by the divalent cations Ca2+ and Zn2+, and by preheating of the membranes. These results are consistent with and suggest the involvement of an enzymatic mechanism in the solubilization of potentially amyloidogenic full‐length APP. Production of Aβ from a transmembrane APP predicts a proteolytic cleavage within the lipid bilayer, a site relatively inaccessible to proteases. Thus, the detected soluble, potentially amyloidogenic, full‐length APP may be a substrate for the proteases producing Aβ. The detection of soluble APP with intact Aβ sequence in secretory vesicles is consistent with the extracellular topology of amyloid depositions.