人類APP695及BACE1-454蛋白質之純化與BACE抑制劑的活性評估

Abstract

阿滋海默症(Alzheimer’s disease, AD)是一種慢性神經退化疾病，在罹患阿滋海默症的病人腦中常可看到老年斑(senile plaque，SP)和神經纖維糾結(neurofibrillary tangles，NFT)這兩種病徵，特別以海馬迴(hippocampus)和杏仁核(amygdala)中最多。老年斑是阿滋海默症患者腦中常見的病徵，老年斑主要是由β-amyloid peptide (Aβ)大小約4 kDa的蛋白質所組成，Aβ是由amyloid precursor protein (APP)經由β-secretase及γ-secretase依序切割而形成。在之前的研究結果指出β-site APP cleaving enzyme (BACE)所反應為Aβ形成的速率決定步驟，所以BACE被認為是治療AD的一個重要的標的物。為了要成功建立一個研究in vitro酵素動力學系統及篩選具有BACE抑制活性的化合物，進而探討化合物對於BACE的抑制機轉，我們必須大量純化出BACE及BACE的受質APP695，於是建構了分別可以表現出具有glutathione-S-transferase (GST) APP融合蛋白及不具有transmembrane domain的BACE1-454與GST之融合蛋白的質體，將這些質體放入大腸桿菌中，利用IPTG (isopropyl-beta-D-thiogalactopyranoside)誘導的方式大量表現GST-APP695及GST-BACE1-454融合蛋白質，以便用親和性層析法純化這些蛋白質。由於經過誘導而大量表現的GST-BACE1-454融合蛋白質，在大腸桿菌內會以inclusion bodies的形式存在，於是我們運用denaturating及refolding的動作得到具有酵素活性之BACE1-454。同時，並利用實驗室之前利用FRET原理所建立篩選系統對一系列化合物之BACE抑制活性進行評估。

β-site APP cleaving enzyme (BACE) participates in the rate-limiting step about Aβ production. It is an important target for BACE to treat AD. In order to characterize BACE catalyzed reaction by in vitro enzyme kinetic studies, establish a screening system to identity potential BACE inhibitors, and determine the inhibitory mechanism of potential BACE inhibitors, it is necessary to obtain large amounts of BACE and its substrate, APP695. Plasmids encoding the glutathione-S-transferase (GST)-APP fusion protein and GST-BACE (without the transmembrane domain) fusion protein were constructed and transformed into specific Escherichia coli strains to induce overexpression of the above fusion proteins. Protein purification was accomplished by affinity chromatography. Since the GST-BACE1-454 fusion protein overexpressed in Escherichia coli was found to form inclusion bodies, a denaturating/refolding approach was applied to obtain active BACE1-454. In the mean while, we used the fluorescence resonance energy transfer (FRET) assay system previously established in our laboratory to screen a series of compounds for potential BACE inhibitors.