Solvent-dependent precipitation of prion protein.

The misfolded isoform of the prion protein (PrP(Sc)) possesses many unusual physiochemical properties. Previously, we and others reported on the differential partitioning of PrP(Sc) from plasma derived therapeutic proteins during their purification processes. To understand the driving force behind these partitioning differences, we investigated the effects of various solvent conditions on the precipitation of PrP(Sc). In a physiological buffer, PrP(Sc) remained in the supernatant after low speed centrifugation. At pH 5, PrP(Sc) precipitation was nearly complete regardless of the salt content. PrP(Sc) could also be precipitated at pH 8 by adding ethanol, but this precipitation was salt dependent. Based on these observations, an empirical mathematical model was constructed in which the PrP(Sc) precipitation trends were fully described as a function of solvent pH, salt, and ethanol concentration. This model consistently predicted PrP(Sc) partitioning during cold ethanol precipitation steps used in plasma protein purification processes, as shown by experimentally determined distributions of PrP(Sc) and transmissible spongiform encephalopathy (TSE) infectivity. These results indicate that pH, salt, and ethanol content are the major solvent factors determining the precipitation of the infectious PrP(Sc) in these processes and may provide a useful tool for assessing the differential partitioning of PrP(Sc) in a given solvent environment.