PURIFICATION OF A TRYPSIN-TYPE PROTEASE FROM HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS WHICH IS HIGHLY SENSITIVE TO THE KUNITZ INHIBITOR DOMAIN PEPTIDE OF ALZHEIMERS-DISEASE AMYLOID PROTEIN-PRECURSOR

A trypsin-type protease was purified to enzymatic homogeneity from hum an umbilical vein endothelial cells by sequential affinity chromatogra phies. The enzyme specifically hydrolyzed dibasic substrates with leuc ine at the P3 positions, but scarcely hydrolyzed the other substrates tested. The enzyme was strongly inhibited by the Kunitz inhibitor doma in peptide of Alzheimer's disease amyloid protein precursor (Ki value, 0.35 nM) and by the microbial inhibitors leupeptin and antipain. Thes e results, together with a previous finding of a significant increase in the expression of Alzheimer's amyloid protein precursors (betaAPPs) with the Kunitz inhibitor domain in Alzheimer's disease, suggest that the activity of the trypsin-type protease is suppressed by an increas e of betaAPPs with inhibitor activity in Alzheimer's disease, resultin g in aberrant intracellular protein catabolism including degradation o f betaAPPs and beta-protein deposition.