ANIMAL VIRUSES 479 



function. Labeling with radioactive isotopes can be of great value, but tliis 

 procedure has not yet been fully exploited in the animal virus field. 



IV. Chemical Composition of Infective Particles and Other Virus- 

 Specific Products 



The value of the results of chemical analyses depends mainly on the purity 

 of the preparations under study. This particular point brings us to one of the 

 great difficulties in carrying out chemical studies on animal viruses. 



Impurities, such as normal components of the host cell or virus-specific 

 products of a type other than the particles in question, are often j^resent. 

 They are sometimes adsorbed to the latter and, therefore, are difficult to 

 remove. In other cases, they are not bound to the particles in question, but 

 are so similar to them in their physical and chemical properties that tliey 

 cannot be separated, and are recognized only with difficulty. 



AVith this situation in imnd, the following discussion will include a brief 

 summary of the particular purification procedure and tests for purity and 

 identification employed, in order to facilitate a critical judgment of the 

 available data. 



A. Smaller Viruses 



1. Poliomyelitis Virus 



The purest animal virus preparation now available is obtained with the 

 infective particles of poliovirus. These particles are uniform and spherical, 

 approximately 27 m/it in diameter, with a particle weight of ---' 6.7 X 10^ 

 (Schwerdt et al.., 1954; Schwerdt, 1957a). 



The purification (Schwerdt and SchaiTer, 1956; Schwerdt, 1957a) procedure 

 involves the use of the supernatant fluid of infected tissue cultures. Only the 

 most important steps in the procedure are given here: precipitation of the 

 virus with methanol at pH 4 and elution of the precipitate in molar NaCl at 

 pH 9. This is followed by two extractions with w-butanol, then precipitation 

 and elution as previously, but without the methanol. Further treatment 

 mcludes one cycle of high- and and low-speed centrifugation, treatment with 

 ribonuclease (RNAase) and desoxj^ibonuclease (DNAase), followed by a 

 second cycle of high- and low-speed centrifugation. In this manner, one can 

 obtain approximately 0.2 mg. protein from one liter of tissue culture fluid. 

 Impurities still present are chiefly removed by sedimentation in a sucrose 

 density gradient, by which four fractions, sedimenting with dillerent 

 velocities, could be separated. The virus contamed m the fastest fraction, is 

 now so pure that it can be crystallized into small tetragonal prisms with pyra- 

 midal ends. Recrystallization leads to no loss in infectivity. Ultracentrifugal 

 studies and electron microscopy, as well as serological tests with antiserum to 

 normal host-cell antigens, further confirmed the jJurity of the preparations 

 vol. 1—32 



