Heat stability of prion rods and recombinant prion protein in water, lipid and lipid-water mixtures

Prion rods, i.e. insoluble infectious aggregates of the N-terminally truncated form of the prion protein, PrP 27-30, and the corresponding recombinant protein, rPrP(90-231), were autoclaved in water, bovine lipid or lipid-water mixtures for 20 min at temperatures from 100 to 170 degreesC. A protocol was developed for the quantitative precipitation of small amounts of protein from large excesses of lipid. PrP remaining undegraded after autoclaving was quantified by Western blot and degradation factors were calculated. The Arrhenius plot of the rate of degradation vs temperature yielded linear relationships for prion rods in water or lipid-water as well as for rPrP(90-231) in lipid-water. The presence of lipids increased the heat stability of prion rods, especially at lower temperatures. Prion rods had a much higher thermal stability compared to rPrP, Autoclaving of prion rods in pure lipid gave different results - not simple degradation but bands indicative of covalently linked dimers, tetramers and higher aggregates. The heat stability of prion rods in pure lipid exceeded that in lipid-water mixtures. Degradation factors larger than 10(4) were reached at 170 degreesC in the presence of lipids and at 150 degreesC in the absence of lipids. The linear correlation of the data allows cautious extrapolation to conditions not tested, i.e. temperatures higher than 170 degreesC. A factual basis for assessing the biological safety of industrial processes utilizing potentially BSE- or scrapie-contaminated animal fat is provided.