Chargé S B, de Koning E J, Clark A
Diabetes Research Laboratories, University of Oxford, Radcliffe Infirmary, U.K.
Biochemistry. 1995 Nov 7;34(44):14588-93. doi: 10.1021/bi00044a038.
Islet amyloid polypeptide (IAPP) is the constituent of amyloid deposits in pancreatic islets of type 2 diabetes in man, monkeys, and cats. This 37 amino acid peptide aggregates in vitro to form beta-pleated sheet fibrils. Rodent IAPP has a different amino acid sequence and does not form amyloid either in vitro or in vivo. Fibrillogenic properties of human IAPP (hIAPP) were determined in vitro. The effect of pH and time course of fibril formation was studied by light scattering spectroscopy. Aggregation of hIAPP1-37NH2 and hIAPPTyr20-29 (0.25 mg/mL) was maximal at neutral/basic and acidic pH, respectively. The ultrastructure of hIAPP1-37NH2 fibrils (0.2 mg/mL) was examined using negative staining for electron microscopy. Short fibrils composed of 2 or more filaments were observed at pH 3-9 after 30 min incubation. At pH 7-9, IAPP fibrils formed a gel. After 6 months at pH 3, large sheets of parallel fibrils were seen. Specific binding of 125I-hIAPP1-37NH2 to preformed IAPP fibrils detected by quantitative autoradiography and radioassay was maximal at pH 3. Binding was enhanced by insulin (3.7 nmol/L) and unaffected by glucose, calcium, glucagon, and apolipoprotein E. 125I-hIAPP1-37NH2 bound specifically to islet amyloid in pancreatic tissue sections from type 2 diabetic patients.
Binding to preformed IAPP fibrils is maximal at acid pH when hIAPP is largely in soluble form. IAPP is secreted together with insulin from the acidic secretory granules (pH 5.5) to the neutral pH of the extracellular space under normal conditions. These changes in pH together with increased accumulation of extracellular hIAPP in diabetes may promote amyloid formation.
胰岛淀粉样多肽（IAPP）是人类、猴子和猫的2型糖尿病胰岛中淀粉样沉积物的组成成分。这种37个氨基酸的肽在体外聚集形成β-折叠片层纤维。啮齿动物的IAPP具有不同的氨基酸序列，在体外或体内均不形成淀粉样物质。在体外测定了人IAPP（hIAPP）的成纤维特性。通过光散射光谱研究了pH值和纤维形成的时间进程的影响。hIAPP1-37NH2和hIAPPTyr20-29（0.25mg/mL）的聚集分别在中性/碱性和酸性pH下最大。使用电子显微镜负染色检查hIAPP1-37NH2纤维（0.2mg/mL）的超微结构。孵育30分钟后，在pH 3-9下观察到由2条或更多条细丝组成的短纤维。在pH 7-9时，IAPP纤维形成凝胶。在pH 3下6个月后，可见大片平行纤维。通过定量放射自显影和放射测定法检测到的¹²⁵I-hIAPP1-37NH2与预先形成的IAPP纤维的特异性结合在pH 3时最大。胰岛素（3.7nmol/L）增强结合，而葡萄糖、钙、胰高血糖素和载脂蛋白E不影响结合。¹²⁵I-hIAPP1-37NH2与2型糖尿病患者胰腺组织切片中的胰岛淀粉样物质特异性结合。
当hIAPP主要以可溶形式存在时，在酸性pH下与预先形成的IAPP纤维的结合最大。在正常条件下，IAPP与胰岛素一起从酸性分泌颗粒（pH值5.5）分泌到细胞外空间的中性pH环境中。pH值的这些变化以及糖尿病患者细胞外hIAPP积累的增加可能促进淀粉样物质的形成。
