262 RADIATION BIOLOGY 



gave a distorted view of their real radiosensitivity. Enzymes, which are 

 one important group of substances, were claimed to be too radioresistant 

 to account for any effects of radiation since enormous doses were needed 

 to cause any effect (Scott, 1937). As long as such experiments were con- 

 ducted with impure extracts from tissues (in unawareness of the protec- 

 tion effect), and with too concentrated solutions (in unawareness of the 

 dilution effect), the true radiosensitivity of enzymes was masked. The 

 use of crystalline enzymes (Dale, 1940, 1942; Forssberg, 1946, 1947; 

 Barron and Dickman, 1949; Barron, Dickman, et al., 1949) showed that 

 quite small X-ray doses of the order of a few hundred roentgens could, 

 under suitable conditions, suffice to inactivate them to a great extent. 

 Such experiments paved the way for further progress. In 1943 this 

 author stated that "It would appear that any biological substances 

 sufficiently active in high dilutions, as, for instance, toxins, antitoxins, 

 hormones, vitamins, etc., can take the place of an enzyme for indicating 

 radiation effects, though the suitability of enzymes due to their nature as 

 catalysts can hardly be equaled." The validity of the dilution and pro- 

 tection effects has been confirmed by many investigators on widely 

 differing materials, e.g., bacteria, sperm, spores, hormones, and drugs. 



Particular examples are the work on bacteriophage (Luria and Exner, 

 1941) and on viruses (Lea et al., 1944). 



PROTECTION EFFECT USED FOR DETERMINING REACTIVITY OF 



SOLUTES WITH RADICALS 



The protection effect is more than an interesting phenomenon; it is of 

 practical importance since it can be used to assess quantitatively the pro- 

 tective power, i.e., the relative ability of various solutes to react with 

 radicals. In such experiments one solute is employed as the indicator 

 and substance of reference, and any protective substance to be compared 

 is added in suitable amounts as the second solute. 



Fricke, Hart, and Smith (1938) have studied the protective efTect occur- 

 ring in mixtures of various simple organic compounds and Dale (1942, 

 1947) and Dale, Davies, and Meredith (1949) have developed this aspect 

 further, using enzymes as particularly suitable indicators for a quantita- 

 tive analysis. It has been found convenient to express the protective 

 power of a given substance as 



^ Dp+e - De ^ weight of e 

 De weight of p 



where Dp+e and De are, respectively, the doses required to produce the 

 same degree of inactivation of the indicator E (usually to 37 per cent) in 

 the presence and absence of the protector P. Q is thus the ratio of the 

 dose received by the protector to that received by the indicator per unit 

 weight of each. 



