The structure and mechanism of formation of human calcitonin fibrils.

Turbidity measurements of the kinetics of human calcitonin (hCT) fibrillation showed a linear dependence of the logarithm of fibrillation time (the time the sample is not fibrillated) and the logarithm of hCT concentration. This ln/ln plot linearity and electron microscope observations of fibrils indicate that the fibrillation process can be explained by the double nucleation mechanism that was proposed for the gelation of sickle cell hemoglobin (Ferrone, F. A., Hofrichter, J., Sunshine, H. R., and Eaton, W. A. (1980) Biophys. J. 32, 361-380). Circular dichroism, fluorescence, and infrared spectroscopy studies of fibrils showed that hCT molecules have alpha-helical and beta-sheet secondary structure components. A model for the structure of hCT molecules in fibrils is proposed.