Effects of different methods of purification on aggregation of scrapie infectivity.

High levels of scrapie infectivity were found in detergent-insoluble residues of hamster brain purified by either repeated pelleting in 10% NaCl or by separation in Nycodenz gradients. Titres determined by the method of incubation interval assay were 100-fold higher than titres measured by endpoint dilution assay. The protein profiles and end-labelled RNA examined by one-dimensional polyacrylamide gel electrophoresis were not different from samples prepared from uninfected brain. Preparations produced by repeated pelleting were treated with RNase A and/or 7 M-urea with no loss of scrapie infectivity. However, the infectivity of samples prepared by gradient centrifugation in Nycodenz were reduced by 2 to 3 log10 LD50 by treatment with RNase A alone but not in combination with SDS. These results suggest that the scrapie agent may be aggregated by methods of purification employing pelleting in high concentrations of salt, or by adding polycations to disaggregated samples.