Hydrolysis of the amyloid beta-peptide (A beta) 1-40 between Asp23-Val24 produces non-aggregating fragments. An electrospray mass spectrometric study.

The aggregation of full-length (residues 1-40) amyloid beta-peptide (A beta) and fragments corresponding to residues 1-23 and 24-40 was studied by electrospray mass spectrometry, using gramicidin as a non-aggregating reference. Following a lag period, A beta(1-40) at 140 microM concentration aggregates with apparent first-order kinetics. Under acidic conditions A beta(1-40) undergoes spontaneous cleavage between Asp23-Val24 and to a lesser extent also at two other Asp-X motifs. Incubation in acidic H(2)18O showed incorporation of 18O in fragment A beta(1-23), confirming that the Asp23-Val24 peptide bond had been hydrolyzed. Incubation of synthetic A beta(1-23) and A beta(24-40) peptides with A beta(1-40) showed that A beta(24-40) remained in solution for several months, that A beta(1-23) partly disappeared from solution, whereas A beta(1-40) completely disappeared. Further, treatment of sedimentable aggregates formed after co-incubation of the three peptides with hexafluoro-2-propanol or formic acid recovered the intensity of A beta(1-40). These data support previous studies showing that the region of A beta encompassing residues 16-24 is necessary for aggregation into amyloid fibrils.