EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 301 



sions) is strongly inhibited by 20,000 r of X-ray irradiation (66 per cent). 

 Inhibition of respiration can be observed even on irradiation with 500 r. 

 Whether this inhibition is reversible or not is difficult to determine in 

 dilute suspensions of sperm because under such conditions the respiration 

 of these cells is maintained at a constant rate for only 3 hours (Barron, 

 Gasvoda, and Flood, 1949). Inhibition of respiration of grasshopper 

 eggs with small doses of X rays is reversed 5 hours after irradiation 

 (Tahmisian and Barron, 1947). The extent of inhibition of respiration 

 by irradiation of sea urchin sperm increases when the oxygen uptake is 

 measured in the presence of succinate or acetate, substances that require 

 sulfhydryl enzymes for their metabolism. Sea urchin eggs are more 

 resistant to the action of X rays, yet inhibition of respiration can be 

 observed on irradiation with X rays down to 1000 r. Fertilization power 

 of sperm, as well as cell division of eggs, is more sensitive to the action of 

 ionizing radiations (Henshaw, 1932, 1940; Evans et at., 1942). 



Bacterial cells and yeast have been used extensively for the study of 

 the mechanism of action of ionizing radiations. These studies have been 

 limited, however, to the viability of the irradiated cells, i.e., the power of 

 the irradiated cells to multiply when transferred in dilute solutions to 

 culture media. It was found on plotting the logarithm of the "surviving 

 fraction" against the irradiation that a more or less straight line fitted 

 the data. The exponential "survival" curve was considered a necessary 

 consequence of the target theory. Lacassagne (1928) and Holweck 

 (1929) calculated the dimensions of this "target" or "zone sensible," 

 and Lea et at. (1941) suggested that the target was the gene of the bacteria. 

 In all these studies the authors confined themselves to only one of the 

 multiple properties of the cell — the loss of the multiplication power of the 

 microorganisms. Bacterial cells and yeast are very complicated organ- 

 isms, in which are found, among other things, a large number of enzymes 

 possessing the same complexity as those found in mammalian tissues. 

 Furthermore, they are covered by a semipermeable membrane which 

 plays an important role in their metabolism. Ionizing radiations, like 

 many other agents, will affect bacteria in any of these multiple structures. 

 The complexity of the cell thus renders the "target" theory too simple, 

 and even the process of inhibition of cell multiplication by ionizing radia- 

 tions has been found by HoUaender et at. (1951) to be decreased in the 

 absence of dissolved oxygen, evidence that inhibition is not due entirely 

 to direct impingement of the ionizing radiation on some "sensitive spot." 

 X-ray irradiation of Corynebacterium creatinovorans (suspensions of 

 10-hour cultures) produces inhibition of cell multiplication and of cell 

 respiration. Whereas multiplication proceeds undisturbed when the 

 X-ray dose is less than 1000 r, cell respiration is slightly inhibited (5 per 

 cent) . On irradiation with 7000 r there are 50 per cent inhibition of cell 



