Abstractβ-site APP-cleaving enzyme (BACE1) cleaves the wild type (WT) β-site very slowly (kcat/Km: 46.6 m-1s-1). Therefore we searched for additional β-secretases and identified three cathepsins that split the WT β-site much faster. Human cathepsin S cleaves the WT β-site (kcat/Km: 54 700 m-1s-1) 1170-fold faster than BACE1 and cathepsins B and L are 440- and 74-fold faster than BACE1, respectively. These cathepsins split two bonds flanking the WT β-site (K-MD-A), where the K-M bond (85%) is cleaved more efficiently than the D-A bond (15%). Cleavage at the major K-M bond yields Aβ (amyloid β-peptide) extended by N-terminal Met that should be removed to generate Aβ initiated by Asp1. The activity of cytosol and microsomal aminopeptidases on relevant peptides revealed rapid removal of N-terminal Met but not N-terminal Asp. Brain aminopeptidases showed similar specificity. Thus, aminopeptidases would convert Aβ extended by Met into regular Aβ (Asp1) found in amyloid plaques. Earlier studies indicate that Aβ is likely produced in the endosome and lysosome system where cathepsins S, B and L are localized and cysteine cathepsin inhibitors reduce the level of Aβ in cells and animals. Taken together, cathepsins S, B and L deserve further evaluation as therapeutic targets to develop disease modifying drugs to treat Alzheimer’s disease.
摘要β位点APP裂解酶(BACE1)裂解野生型(WT)β位点的速度非常慢(kcat/Km:46.6 m-1s-1)。因此,我们寻找其他的 β 分泌酶,发现了三种能更快地裂解 WT β 位点的酪蛋白。人类胰蛋白酶 S 分裂 WT β 位点的速度(kcat/Km:54 700 m-1s-1)比 BACE1 快 1170 倍,胰蛋白酶 B 和 L 分别比 BACE1 快 440 倍和 74 倍。这些酪蛋白能裂解 WT β 位点侧翼的两个键(K-MD-A),其中 K-M 键(85%)的裂解效率高于 D-A 键(15%)。主要 K-M 键的裂解会产生由 N 端 Met 延伸的 Aβ(淀粉样β肽),该 N 端 Met 应被移除,以产生由 Asp1 启动的 Aβ。细胞质和微粒体氨肽酶对相关肽的活性表明,N-末端 Met 可被快速去除,但 N-末端 Asp 却不能。脑氨肽酶也表现出类似的特异性。因此,氨肽酶会将由 Met 延伸的 Aβ 转化为淀粉样斑块中的普通 Aβ(Asp1)。早先的研究表明,Aβ很可能是在内质体和溶酶体系统中产生的,而酪蛋白酶 S、B 和 L 就位于内质体和溶酶体系统中,半胱氨酸酪蛋白酶抑制剂会降低细胞和动物中 Aβ 的水平。总之,作为开发治疗阿尔茨海默病的疾病调节药物的治疗靶点,梓蛋白 S、B 和 L 值得进一步评估。