INACTIVATION OF ENZYMES 195 



Stripping film autoradiographs were made of squashes and sectioned 

 material from the meristem and from proximal segments where cell di- 

 vision is rare. The observation of Howard and Pelc that no nucleus 

 in division, not even those in early prophase, showed an autoradiograph 

 at 6 hr, but all did at 24 hr, is interpreted to indicate that the incorpo- 

 ration of P^^ into nuclear compounds stops some time before visible pro- 

 phase, and that there is no synthesis during the actual division. Accord- 

 ing to Caspersson's observations, however, a certain increase in nucleic 

 acid formation during the very earliest prophase does take place (99). 



Cells when sufficiently irradiated even in an early interphase fail to 

 divide, or else divide abnormally (29). This fact suggests that irradia- 

 tion must interfere with the mechanism involved in the synthesis of 

 nuclear constituents of cells, which are present even in that early stage 

 of their development. 



That irradiation depresses the formation of desoxyribonucleic acid is 

 clearly brought out by the various observations discussed above. Cell 

 division is a result of intense accumulation of pertinent cellular con- 

 stituents, and if the accumulation is obstructed by irradiation, cell di- 

 vision will not occur. That the accumulation of free purines and nucle- 

 tide takes place during the growth lag, their conversion into nucleotides 

 and nucleic acids starting towards the end of the lag, is suggested by 

 results obtained in the study of ultraviolet absorption (118) and of phos- 

 phorus uptake (120) by Bad. lactis aerogenes. 



Although irradiation interferes with the formation of desoxyribo- 

 nucleic acid, it should be pointed out that doses which produce hemolysis 

 of blood corpuscles and thus make the nucleic acid content of the 

 nucleated blood corpuscles available to the effect of degrading enzymes 

 will lead to a removal of much of the preformed nucleic acid. In a 

 similar way doses which produce far-reaching changes in the bone 

 marrow, making it semifluid, may degrade much of the nucleic acid (123). 



Inactivation of Enzymes 



As previously stated, the synthesis of desoxyribonucleic acid necessi- 

 tates the participation of numerous agencies. The absence of one 

 or more of these may interfere with the formation of desoxyribonucleic 

 acid molecules. For example, the inactivation of enzymes involved in 

 carbohydrate metabolism could interfere with the formation of desoxy- 

 ribonucleic acid. In numerous experiments diluted solutions of different 

 types of enzymes were irradiated with restricted doses of ionizing radi- 

 ation. Frequently an inactivation of the enzymes of different types was 

 observed. 



