The precursor protein of the non-A beta component of Alzheimer's disease amyloid (NACP) is a presynaptic protein of the central nervous system. The physiological function of NACP is not known, but the localization of NACP at presynaptic nerve terminals suggests that it may be involved in the neuronal function. To better understand the physiological function of NACP and its role in the pathogenesis of Alzheimer's disease (AD), the biochemical and biophysical properties fo NACP were investigated. The NACP behaves abnormally on FPLC gel-filtration chromatography with the apparent molecular mass of 70 kDa, and the results from chemical cross-linking, limited proteolysis and CD experiments suggest that significant parts of NACP may be unfolded. The abnormal hydrodynamic behavior and extreme protease sensitivity of NACP could result from the extended structure which may be related to the physiological function and pathological role of this protein in the development of AD.