EFFECT OF PH AND INSULIN ON FIBRILLOGENESIS OF ISLET AMYLOID POLYPEPTIDE IN-VITRO

Islet amyloid polypeptide (IAPP) is the constituent of amyloid deposit s in pancreatic islets of type 2 diabetes in man, monkeys, and cats. T his 37 amino acid peptide aggregates in vitro to form beta-pleated she et fibrils. Rodent IAPP has a different amino acid sequence and does n ot form amyloid either in vitro or in vivo. Fibrillogenic properties o f human IAPP (hIAPP) were determined in vitro. The effect of pH and ti me course of fibril formation was studied by light scattering spectros copy. Aggregation of hIAPP(1-37NH2) and hlAPP(Tyr20-29) (0.25 mg/mL) w as maximal at neutral/basic and acidic pH, respectively. The ultrastru cture of hIAPP(1-37NH2) fibrils (0.2 mg/mL) was examined using negativ e staining for electron microscopy. Short fibrils composed of 2 or mor e 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 p arallel fibrils were seen. Specific binding of I-125-hlAPP(1-3NH2) to preformed IAPP fibrils detected by quantitative autoradiography and ra dioassay was maximal at pH 3. Binding was enhanced by insulin (3.7 nmo YL) and unaffected by glucose, calcium, glucagon, and apolipoprotein E . I-125-hIAPP(1-37NH2) bound specifically to islet amyloid in pancreat ic tissue sections from type 2 diabetic patients. Conclusions: 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 acid ic secretory granules (pH 5.5) to the neutral pH of the extracellular space under normal conditions. These changes in pH together with incre ased accumulation of extracellular hIAPP in diabetes may promote amylo id formation.