Table 2. PrP levels in prion-infected brains of C57BL/6 and Tga20, and specific infectivities of PK-resistant PrP.

PrP in 22L-infected mouse brains (arbitrary units/µg protein)
Preparation	C57BL/6 (%)	Tga20 (%)	Tga20/C57BL/6
Total PrP	18471 (100)	45243 (100)	2.4
TL-resistant PrP	2050(11)	1606 (4)	0.8
PK-resistant PrP	4137 (22)	742 (2)	0.18
Ratio TL/PK	1∶2	2.2∶1

The PrP content of brain homogenates was assessed by western blot and quantification of immunoreactive bands in arbitrary pixel units by UVP (BioSpectrum®AC Imaging System). Each measurement was adjusted for gel loading to provide the pixel value per µg of sample protein. All gels have been normalized to each other allowing the comparison of PrP values. TL- and PK-resistant PrP values have been calculated using regression curves of signal intensities obtained on serial dilutions of each sample (Fig. 2 B and C and Fig. S3).

^*The specific infectivity of resPrP^Sc is herein defined as the ratio of the RI measured in LD9 cells (susceptible to all three prion strains) relative to resPrP^Sc (pixels per ng of brain protein, see above).