Identification of a Chymotrypsin‐Like Mast Cell Protease in Rat Brain Capable of Generating the N‐Terminus of the Alzheimer Amyloid β‐Protein

: Cleavage after Met596 of the β‐amyloid precursor protein to generate the N‐terminus of β‐protein indicates the activity of a protease having chymotrypsin‐like specificity. A chymotrypsin‐like protease is further implicated in Alzheimer's disease by the increased synthesis of the protease inhibitor α1‐antichymotrypsin in pathologically affected brain regions and by the presence in the amyloid deposits of inactivated forms of α1‐antichymotrypsin (indicating irreversible binding to a target chymotrypsin‐like protease). In the present report, we have purified from rat brain a chymotrypsin‐like protease that (a) binds with high affinity to human α1‐antichymotrypsin, (b) proteolytically generates a β‐protein‐containing C‐terminal fragment from full‐length recombinant human β‐amyloid precursor protein, and (c) selectively cleaves methoxysuccinyl‐Glu‐Val‐Lys‐Met‐p‐nitroanilide (a substrate modeling the protease recognition domain for the β‐protein N‐terminal cleavage site). Amino acid sequences of tryptic fragments of the purified rat brain chymotrypsin‐like protease indicate an identity with rat mast cell protease I. Moreover, the ontogeny and compartmentalization of rat brain chymotrypsin‐like protease are consistent with those of connective tissue‐type mast cells in the meningeal and intracortical perivasculature. Because these areas in human brain form extensive β‐amyloid deposits in Alzheimer's disease, Down's syndrome, and hereditary cerebral hemorrhage with amyloidosis of Dutch origin, the present findings suggest that a brain mast cell chymotrypsin‐like protease may participate in generating perivascular β‐protein, which ultimately aggregates into β‐amyloid deposits.