EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 305 



contradictions are, howe\'er, so great that it is difficult to come to a 

 definite conclusion from the large amount of published work. For 

 example, Laszlo and Fleischmann (1938) reported that the respiration of 

 nucleated red cells was reversibly inhibited by X rays, Wels (1924) found 

 no effect, and Frankenthal and Back (1944), on irradiation with X-ray 

 doses varying from 500,000 to 1,000,000 r found an increase in respiration. 

 Some of these discrepancies may be due to the effect of environment; 

 others, to the state of the health of the irradiated animals. The tem- 

 perature of the tissues during irradiation was found by Crabtree (1935) to 

 determine the type of metabolic changes produced on irradiation. 



A large number of papers reporting on the effect of X irradiation on 

 tissue metabolism have been limited to the measurement of over-all 

 reactions, oxygen uptake, and lactic acid formation. The oxygen uptake, 

 however, is a function of a large number of oxidations, any one of which 

 may be damaged without alteration in the oxygen uptake unless this 

 oxygen uptake is oriented toward the oxidative reaction catalyzed by the 

 altered enzyme. In fact, Lyman and Barron (1940) found that when 

 nephritis was produced in rats with propylene glycol, the oxygen uptake 

 in the absence of added substrate remained normal whereas it was 

 definitely diminished in the presence of pyruvate, alanine, and lactate. 



Of all the metabolic reactions so far studied, the synthesis of nucleic 

 acids seems to be the most sensitive to the action of ionizing radiations, an 

 inhibition discovered by Hevesy (1945) and confirmed by Holmes (1947, 

 1949). This inhibition had previously been suggested by Marshak (1941) 

 from experiments on the rate of incorporation of P^^. In rats bearing 

 Jensen sarcoma, X irradiation with a few hundred roentgens reduced the 

 rate of formation of desoxyribonucleic acid to one-half. Holmes (1949) 

 found that irradiation of such rats with 2000 r reduced by 75 per cent the 

 rate of incorporation of P^^ into the desoxyribonucleic acid of the sarcoma. 

 Ionizing radiations block the turnover of desoxyribonucleic and ribo- 

 nucleic acids not only in growing tissues but also in normal organs of 

 adult animals (Hevesy, 1948). Considerable quantitative variation in 

 individual experiments results from the known difficulty of removing P^^ 

 contamination from the isolated tissue fractions. Abrams (1951) has 

 repeated these experiments with glycine labeled in the carboxyl group 

 with C^l The rates of synthesis of desoxy and ribonucleic acids is 

 markedly reduced by X irradiation. In marked contrast with this 

 inhibition the synthesis of proteins is not affected. Similarly, glycolysis 

 and respiration of the bone marrow of the rabbit is not affected 6 hours 

 after X irradiation (Lutwak-Mann and Gunz, 1949) . Inhibition of nucleic 

 acid synthesis is not due wholly to direct collision between the enzymes 

 responsible for this process and the ionizing radiations. In animals 

 bearing two tumors, irradiation of one while the second is shielded causes 

 inhibition of nucleic acid synthesis in both tumors (Ahlstrom et al., 1945). 



