VIRUSES — STANLEY 269 



It is obvious that studies on these split products should reveal in- 

 formation concerning the nature of the components making up the 

 virus and perhaps furnish a clue as to the mode of synthesis of the 

 virus. Materials similar in structure to urea, such as guanidine, as 

 vrell as apparently unrelated substances, such as sodium dodecyl 

 sulfate, also cause disintegration of the virus. Enzymes have been 

 found to cause the break-up of certain viruses. Neither tobacco mo- 

 saic nor bushy stunt virus is split by trypsin, but this enzyme causes 

 the rapid hydrolysis of alfalfa mosaic virus. Tobacco mosaic virus 

 appears to be digested slowly by pepsin, although the rate of hydroly- 

 sis is much slower than might have been anticipated. All viruses 

 appear to be denatured by heat, and the temperature at which de- 

 naturation occurs depends upon the virus and to some extent upon 

 conditions such as the hydrogen-ion concentration and the kind of 

 salts present. 



In the heat and other types of denaturation reactions that have 

 just been described, there is a more or less complete disintegration of 

 the virus structure and the products which are formed do not have 

 the properties which characterized the intact virus. They are of 

 low molecular weight, do not react serologically with antiserum to 

 virus, and do not induce the formation of antibodies which neutral- 

 ize virus. It has been found possible, however, to inactivate viruses 

 without such a complete disintegration of structure. When tobacco 

 mosaic virus is treated with mild oxidizing agents, form.aldehyde, 

 nitrous acid, or ultraviolet light, the properties of the resulting ma- 

 terials are, with the exception of virus activity, very similar to those 

 of the intact virus. For example, the size and shape are not meas- 

 urably affected, the materials give a precipitin reaction with anti- 

 serum to active virus, and, perhaps most important, the injection of 

 the inactive materials gives rise to the production of antibodies 

 which will specifically neutralize tobacco mosaic virus activity. It 

 appears that these treatments cause no great change in the general 

 topography of the virus structure, but bring about changes that are 

 very small with respect to the structure as a whole but which are 

 nevertheless sufficiently definite to cause the loss of virus activity. 

 This fact may be quite important in connection with efforts to pro- 

 tect ourselves against the devastating effects of virus diseases. As 

 you may know, in the three general methods of protection which 

 are now employed, active virus plus immune serum, active virus 

 usually of a strain that will cause a less severe disease, or inactivated 

 virus is used to secure immunization. The second method is used 

 extensively and successfully in the protection against smallpox, 

 yellow fever, and certain other virus diseases, and the third method, 

 which has proved less satisfactory, has been used with claims for 

 success for many viruses such as hog cholera, rinderpest, dog dis- 



