PURIFICATION AND SPECTROSCOPIC CHARACTERIZATION OF BETA-AMYLOID PRECURSOR PROTEIN FROM PORCINE BRAINS

Soluble and membrane-bound isoforms of beta-amyloid protein precursor (APP) of Alzheimer's disease were extracted and purified from porcine brains. At least three types of soluble APP and membrane-bound APP wit h different molecular masses, ranging from 86 kDa to 116 kDa, were obt ained. CD and infrared spectroscopies were used to determine the overa ll secondary-structure content of APP. The infrared spectra of soluble and membrane-bound APP (in dry and hydrated states) were similar in t he amide-I and amide-II regions, suggesting that the overall secondary structures of the soluble and membrane isoforms were roughly identica l. The amide-I band is composed of at least five component bands, loca ted at 1694, 1674, 1652, 1637 and 1618 cm(-1) for soluble APP, and loc ated at 1687, 1674: 1652, 1637 and 1614-1606 cm(-1) for membrane-bound APP, as evidenced by their respective second-derivative infrared spec tra. The 1651-1652-cm(-1) band was associated with alpha-helix structu res, while two types of beta-sheet structures are evidenced by two cha racteristic pairs of component bands. The 1674-cm(-1) and 1637-cm(-1) bands for soluble APP and membrane-bound APP were tentatively associat ed to beta-sheet structures. The second pair of bands, located at 1694 cm(-1) and at 1618 cm(-1) for soluble APP and at 1687 cm(-1) and 1614 -1606 cm(-1) for membrane-bound APP, were associated with intermolecul ar beta-sheet structures or aggregated strands, as confirmed by heat d enaturation. CD spectra indicated the presence of alpha-helix structur es in soluble and membrane-bound APP. The secondary-structure content, estimated from CD spectra, was about 40-45% alpha-helix and 15-20% be ta-sheet structures for soluble and membrane-bound APP.