Solvent-Dependent Precipitation of Prion Protein

The misfolded isoform of the prion protein (PrPSc) possesses many unusual physiochemical properties. Previously, we and others reported on the differential partitioning of PrPSc 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 PrPSc. In a physiological buffer, PrPSc remained in the supernatant after low speed centrifugation. At pH 5, PrPSc precipitation was nearly complete regardless of the salt content. PrPSc 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 PrPSc precipitation trends were fully described as a function of solvent pH, salt, and ethanol concentration. This model consistently predicted PrPSc partitioning during cold ethanol precipitation steps used in plasma protein purification processes, as shown by experimentally determined distributions of PrPSc 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 PrPSc in these processes and may provide a useful tool for assessing the differential partitioning of PrPSc in a given solvent environment.