Note: Use of a Distribution of Frequencies Model to Interpret the Tailed Heat Inactivation Curves of Prions

Inactivation of prions responsible of bovine spongiform encephalytis (BSE) by heat is a major concern for the canned food industry in Europe. Experimental data of heat inactivation of prions obtained from the scientific literature have been modelled. Two models were applied to analyse the inactivation curves, the Weibull distribution model and a two-parameter empirical model. Statistical analysis of available data indicated that the Weibull frequency distribution provided the best description of non-linear survival curves. The effect of the temperature on Weibull model parameters ( a and b) was also studied. The shape parameter, b, indicates deviations from linearity in the inactivation curves, and the scale parameter, a, can be considered as a kinetic constant rate for the inactivation. The values of the b parameter were on the order of 0.15-0.2 which meant that these curves had a strong upper concavity. The a values obtained ranged from 1.8 × 10−3 to 2.3 × 10−6 for temperatures between 100 and 160°C. These results indicate that temperature has a relevant effect on the inactivation of prions, although it remains a considerable fraction of prions with infective capacity. The Weibull frequency distribution model appears as a useful and convenient model because the a parameter could allow a quantitative comparison of the inactivation of prions at different temperatures or under different conditions.

High pressure application for food biopolymers

High hydrostatic pressure constitutes an efficient physical tool to modify food biopolymers, such as proteins or starches. This review presents data on the effects of high hydrostatic pressure in combination with temperature on protein stability, enzymatic activity and starch gelatinization. Attention is given to the protein thermodynamics in response to combined pressure and temperature treatments specifically on the pressure-temperature-isokineticity phase diagrams of selected enzymes, prions and starches relevant in food processing and biotechnology.

A single mutation in the activation site of bovine trypsinogen enhances its accumulation in the fermentation broth of the yeast Pichia pastoris

We produced bovine trypsinogen in the yeast Pichia pastoris. Little or no trypsinogen was detected when the gene with its native leader sequence was expressed under the control of the strong aox1 promoter, suggesting that expression of the wild-type bovine trypsinogen was toxic to the cells. We altered the trypsinogen native propeptide sequence by replacing the lysine at position 6 with an aspartic acid, thus destroying the site in the propeptide cleaved by enterokinase and by trypsin. This mutant accumulated up to 10 mg of trypsinogen per liter in shake flask cultures and about 40 mg/liter in 6-liter fermentors. Trypsinogen could be activated in vitro with a dipeptidyl-aminopeptidase, which selectively removed the modified trypsinogen propeptide; the resulting trypsin was fully active and showed evidence of glycosylation. Thus, we have developed a novel protein production scheme that can be used for the expression of proteins, such as proteases, that are deleterious to the producing organism. This system relies on the expression of a zymogen that cannot be activated in vivo coupled with its in vitro purification and activation.

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 degrees C. 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 degrees C in the presence of lipids and at 150 degrees C in the absence of lipids. The linear correlation of the data allows cautious extrapolation to conditions not tested, i.e. temperatures higher than 170 degrees C. A factual basis for assessing the biological safety of industrial processes utilizing potentially BSE-or scrapie-contaminated animal fat is provided.