When PFOs are incubated with lipid vesicles, the APF immunoreactivity increases in a time-dependent fashion, and the A11 immunoreactivity decreases coordinately, suggesting a precursor product relationship between the epitopes recognized by the two antibodies. PFOs. APFs display a conformation-dependent, generic epitope that is distinct from that of PFOs and amyloid fibrils. Incubation of PFOs with phospholipids vesicles results in a loss of PFO immunoreactivity with a corresponding increase in APF immunoreactivity, suggesting that lipid vesicles catalyze the conversion of PFOs into APFs. The annular anti-protofibril antibody also recognizes heptameric -hemolysin HUP2 pores, but not monomers, suggesting that the antibody Heptasaccharide Glc4Xyl3 recognizes an epitope that is specific for a barrel structural motif. Many age-related neurodegenerative diseases are characterized by the accumulation of amyloid deposits derived from a variety of misfolded proteins (1). These diseases typically have both sporadic and inherited forms, and in many cases the mutations associated with the familial forms are Heptasaccharide Glc4Xyl3 in the gene encoding the protein that accumulates or in genes directly related to its production, processing, or accumulation (2). The genetic linkage between the mutant allele and disease Heptasaccharide Glc4Xyl3 is evidence of the causal relationship of amyloid accumulation to pathogenesis, and many of the mutations either destabilize the natively folded state, produce more amyloidogenic protein, or they increase its propensity to aggregate (3). Although fibrillar amyloid deposits are among the most obvious pathognomonic features of disease, their role in pathogenesis is not clear. The extent of fibrillar amyloid plaque deposition does not correlate well with Alzheimer’s disease pathogenesis, and there are Heptasaccharide Glc4Xyl3 a significant number of non-demented individuals that have equivalent amounts of amyloid plaques as disease patients (4). Pathological changes are observed in transgenic animals before the onset of amyloid plaque accumulation (5, 6), and it has been reported that soluble A oligomers correlate better with dementia than insoluble, fibrillar deposits (7, 8), suggesting that oligomeric forms of A may represent the primary toxic species. Soluble oligomers have been implicated as the primary toxic species in many degenerative diseases where the accumulation of large fibrillar deposits Heptasaccharide Glc4Xyl3 may be either inert, protective, or pathological by a different mechanism (for review, see Refs. 9 and 10). A aggregates have been described ranging in size from dimers up to particles of one million daltons or larger (11C16). In the atomic force microscope prefibrillar oligomers (PFOs)3 appear as spherical particles of 3C10 nm. PFOs appear at early times of incubation and disappear as mature fibrils appear (16C18). At longer times of incubation PFOs appear to coalesce to form curvilinear beaded strings that have been called protofibrils and ring-shaped, pore-like structures referred to as annular protofibrils (APFs) (17). APFs appear to be formed from the circularization of PFO subunits. A similar spectrum of PFOs and APFs has been observed for many types of amyloids, such as -synuclein (19), islet amyloid (20), and non-disease associated neoamyloids (21). Although PFOs, APFs, and fibrils have been observed for many different types of amyloidogenic proteins and peptides (22), their structures, interrelationships, and contributions to disease pathogenesis are not entirely clear. Insoluble fibrils and small soluble pieces of fibrils known as fibrillar oligomers appear to have a distinct and mutually exclusive underlying structure than PFOs because they display generic epitopes that are recognized by distinct conformation-dependent monoclonal antibodies (23, 24) and antisera (25, 26). It is not yet known whether APFs represent a unique conformation or whether they are structurally related to PFOs or fibrils. So far APFs have only been defined morphologically as pore-like structures and have been observed in preparations of PFOs and in fibril-containing preparations (27C29). Familial mutations associated with inherited forms of Parkinson and Alzheimer diseases increase the formation of.