Kinetic properties of cathepsin D and BACE 1 indicate the need to search for additional beta-secretase candidate(s).
Many studies suggest that BACE 1 is the genuine beta-secretase; however, this is not undisputed. The wild-type (WT) beta-site of the amyloid precursor protein (APP) present in the worldwide population is cleaved very slowly (kcat/Km: approx. 50 m(-1) s(-1)), while proteases acting on relevant substrates are much more efficient (kcat/Km: 10(4)-10(6) m(-1) s(-1)). Knock-out of BACE 1 in mouse markedly reduces A beta formation. Nevertheless, studies in other systems show that knock-out experiments in rodents and corresponding genetic defects in human may reveal different phenotypes. Considering these issues, we searched for other beta-secretase candidate(s), identified cathepsin D, and evaluated properties of cathepsin D related to BACE 1 that were not examined previously. The kinetic constants (kcat, Km, kcat/Km) for cleaving peptides with beta-sites of the WT or the mutated Swedish families (SW) APP by human BACE 1 and cathepsin D were determined and found to be similar. Western blots reveal that in human brain cathepsin D is approximately 280-fold more abundant than BACE 1. Furthermore, pepstatin A strongly inhibits the cleavage of SW and WT peptides by both brain extracts and cathepsin D, but not by BACE 1. These findings indicate that beta-secretase activity observed in brain extracts is mainly due to cathepsin D. Nevertheless, as both BACE 1 and cathepsin D show poor activity towards the WT beta-site sequence, it is necessary to continue the search for additional beta-secretase candidate(s).
Schechter
Israel
I
Department of Immunology, The Weizmann Institute of Science, Rehovot, Israel. israel.schechter@weizmann.ac.il
Ziv
Etty
E
D000595:Amino Acid Sequence; D053829:Amyloid Precursor Protein Secretases / Q000378:metabolism*; D000818:Animals; D016282:Aspartic Acid Endopeptidases / Q000378:metabolism*; D001921:Brain / Q000201:enzymology; D002402:Cathepsin D / Q000378:metabolism*; D002417:Cattle; D002850:Chromatography, Gel; D006801:Humans; D007700:Kinetics; D051379:Mice
D053829:Amyloid Precursor Protein Secretases; D016282:Aspartic Acid Endopeptidases; C509280:BACE1 protein, human; D002402:Cathepsin D