276 RADIATION BIOLOGY 



in the discussion, some cleavage products, e.g., hydrogen sulfide and 

 ammonia, may have detrimental effects when liberated within the cell. 

 Products like these, or still unknown ones, may have connections with 

 radiomimetic substances or may even be the cause of what appears as 

 chromosome breakage and its consequences. When a normal constituent 

 of a cell is split up after reaction with radicals the fragments are new 

 entities which may or may not be detrimental to the cell. They may 

 also be a new type of radical Avith properties different from the primary 

 radicals which initiated the cleavage. 



Finally consideration will be given to investigations which aim at 

 establishing a change, caused by irradiation of the quantities of con- 

 stituents in biological systems like cells or tissues. In such investiga- 

 tions certain technical difficulties are encountered. It is not always easy 

 to ensure that extraction methods are quantitative, and the percentage 

 changes resulting from radiation have to be sufficiently great to be 

 detectable by ordinary chemical analysis. Furthermore, the changes 

 which occur are often of a transient nature, as has been revealed by the 

 modern tracer techniciue, and, therefore, ordinary chemical methods are 

 not adaptable enough to deal with them adecjuately. One example of a 

 definite change, however, should be mentioned here, i.e., the destruction 

 by X rays of the hormone auxin within living plants (Vicia faba) due to 

 oxidation, possibly by hydrogen peroxide (Skoog, 1935-1936). 



Often, however, it is not so much the total amount of a substance which 

 is of importance but its functional level existing during perhaps quite 

 short periods. The brilliant method, conceived by Hevesy, of using 

 radioactive isotopes to label atoms in a molecule in order to follow the 

 fate of a compound, is much better suited to deal with transient changes, 

 and has the advantage of extremely high sensitivity. 



This method, employing P^^, was used successfully (Euler and Hevesy, 

 1944; Ahlstrom, Euler, and Hevesy, 1945; Holmes, 1947) to prove that 

 desoxyribonucleic acid formation is inhibited by the action of X rays in 

 tumor tissue of rats. Mitchell (1942) came to the same conclusion with 

 regard to irradiated human malignant tumors by examining sections by 

 ultraviolet microscopy. Hevesy (1949a) also proved that X radiation 

 causes a decrease of uptake of C^^ into desoxyribonucleic acid but an 

 increase of uptake of C^^ in the tissue of whole organs (kidney, liver, etc., 

 Hevesy, 1949b). Hevesy emphasizes that, as far as is now known, 

 nucleic acids are the only compounds which are formed at a reduced rate 

 under the effect of X rays. These experiments measure the quantity of 

 newly formed nucleic acid, but, of course, cannot give any indication of 

 the mechanism by which the formation takes place. 



Apart from these radiation-induced metabolic changes of cjuantity of 

 nucleic acids, investigations have been carried out as to the chemical 

 changes of nucleic acids. Here again research is confronted with diffi- 



